Author: Pam Knox

Have you noticed that this winter has been full of ups and downs in temperature, with warmth and drought in some areas and impactful winter storms in others? The western United States has been mostly warmer and drier than normal with little precipitation except in the highest peaks, and even then, it has been scarce. The eastern United States has been hit by several significant winter precipitation events and a crippling freeze that reached down into central Florida. In this blog post we will discuss some of these storms and how the polar vortex repeatedly brought frigid air from the North Pole into the eastern half of the country, damaging agriculture, trees, and outdoor plants.

What winter events occurred this season?

While December was relatively mild for most of the country, things changed in January, when several waves of cold air moved out of Canada into the eastern United States. In January, a massive ice storm stretched across the southern US from New Mexico to New Jersey from January 23 to 26. It brought incredible damage and power outages to parts of Mississippi and surrounding states due to the heavy ice which brought down trees and power lines. Just a week later, a snowstorm brought snow to the Southeast. I received 2.6 inches on February 1, but areas to the east in the Carolinas received much more. In both of these storms, frigid air moved all the way south into Florida, causing an estimated combined agricultural loss of $3.17 billion, significantly impacting sugar cane, strawberries, sweet corn, citrus, and vegetables. In one of the outbreaks, freezing temperatures reached all the way to Cuba, the first time that has ever happened. We discussed how different types of precipitation affect gardens in a GP post last year.

More recently, a massive snowstorm brought feet of snow to parts of New England as a very strong low-pressure center formed over off the East Coast pushed humid air counterclockwise into areas from Delaware and Washington D. C. to Maine. When the humid air hit the cold air from Canada, it helped form incredible amounts of snow along the coast and inland. Some areas of Rhode Island, Massachusetts, New York, and New Jersey received over 30 inches of snow from the most recent storm, which occurred just a few days ago. Meanwhile, the western United States has been in a snow drought, which means that this summer is likely to be a bad one for wildfires as so much of the region is much drier than usual already. Note that it is not the only place that has had a snow drought this year—Iceland has also been unusually free of snow this year.

What caused the surges of cold air that helped form the storms this winter?

This year’s cold outbreaks are related to changes in a feature known as the “polar vortex” which forms over the polar regions in the winter. It is known as a vortex because it is a region of low pressure with air spinning around it that occurs high above the surface of the earth. The name sounds dangerous and scary because of social media, but it is really just a natural part of the earth’s atmospheric circulation that occurs on a regular basis. Underneath a strong polar vortex, the frigid air that forms in the dark winter days is trapped near the pole by the polar jet stream, which corrals the air and keeps it from surging to the mid-latitudes. But in some years, the polar vortex weakens or can even split into two different circulations. When this happens, the weaker jet stream allows arctic air to escape from near the poles and flow south.

The flow does not always move in the same direction. You might remember the Texas freeze of 2021, which brought frigid air into Texas, straining their power supplies and leading to widespread power outages during very cold conditions. This year, the southward surges of the coldest air have headed towards the eastern United States, providing the source of freezing air that has contributed to the development of the ice storms and snow. The type of winter precipitation that forms depends on the vertical temperature structure of the atmosphere where the moisture is falling, which can be different in each storm.

What causes the polar vortex to send air into the mid-latitudes?

The polar vortex expands every year as winter occurs due to the tilt of the earth’s pole away from the sun in, allowing cold air to develop across the Arctic. We naturally get some cold weather each winter as the vortex grows. But sometimes the flow around the vortex is disrupted by unusual warming high in the atmosphere over the poles. This is called a “Sudden Stratospheric Warning” or SSW and can result in the rapid increase in air temperature in the stratosphere high above the earth’s surface. This allows the location of the polar vortex to shift and sometimes to split into two different vortices, allowing the coldest air to bulge southward towards areas that don’t get really cold weather every year.

How will this affect our gardens this spring?

A study published today indicates that another SSW event appears to be underway, most likely the last one of this winter’s season (they don’t generally occur in the summer because the poles are much warmer due to the long summer days and 24-hour sunlight). It usually takes a few weeks for the impacts of a SSW to be felt as the polar vortex distorts, so that could mean another burst of cold air could come to the United States in mid- to late March, potentially bringing a later than usual freeze to areas that are usually well into their growing season by then. Of course, we don’t know yet where the bulge of coldest air will go. It could be the eastern U. S. again, or it could be farther west, or it could be into Europe. At this point, it is just something to watch. But if you are considering putting out frost-sensitive plants in your gardens this year, you might want to consider holding off for at least a couple of weeks to see whether another cold spell comes towards your location. Meanwhile, enjoy the change of the seasons (this includes our readers in the Southern Hemisphere too!) and watch your gardens as they transform!

 Everyone has their favorite season. Mine is spring, because of the pop of early flowers, the hundreds of shades of green that appear as bushes and trees leaf out, and the warmer temperatures that come with the march towards summer. Winter is my least favorite because of the lack of color after the deciduous trees drop their leaves, leaving stark black branches against white clouds or snow.  I admit I don’t like the cold either! Most of us talk about the seasons a lot, but what are they really? Today’s topic is how seasons are defined and how that relates to gardening.

Definitions of seasons

Seasons can be defined in several ways. The most common definition of a season is the astronomical season. This is related to the movement of the earth around the sun. Since the earth’s axis of rotation is tilted by 23.5 degrees relative to the plane of the earth’s orbit, the amount of sunlight anywhere on earth receives changes depending on whether the pole is tilted towards the sun or away from it. In the Northern Hemisphere, summer begins at the summer solstice, when the North Pole is pointed most directly at the sun. This is when the sun appears highest in the sky at noon, usually around June 21. The winter solstice occurs when the earth is halfway around the sun six months later and the North Pole is pointed most directly away from the sun. This means less energy is reaching that hemisphere, resulting in fewer hours of daylight and less incoming sunlight, resulting in cooler temperatures. Between the two solstices are the vernal (spring) and autumnal (fall) equinoxes, the dates on which the lengths of day and night are basically equal. They fall on roughly March 21 and September 21. As the earth progresses on its orbit around the sun, we cycle through these geometric changes, resulting in the variations of incoming energy that drive the rise and fall of average temperature over the year.

Climatologists and meteorologists define the seasons by date instead of by geometry. Northern Hemisphere winter is considered December, January, and February, spring as March through May, summer as June through August, and fall as September through November. In part, these calendar months were chosen for ease in calculating averages for monthly and seasonal climate reports in the days before calculators were available and all averages were calculated by hand. But it turns out that the calendar months are a better measure of when the warmest and coolest three-month periods occur (climatological summer and winter) than astronomical seasons are. So the seasons change for climatologists several weeks before the astronomical change of the seasons occurs. If you want to learn a lot more about different ways to define seasons climatologically, then I recommend Brian Brettschneider’s detailed article on Defining the Seasons.

Defining seasons by phenology

In addition to astronomical and climatological seasons, there are other ways of defining the seasons as well. One method of measuring the start of a season is through the use of phenology, the study of when a particular environmental event is observed to occur. This could be the first leaf on a honeysuckle bush, the first cherry blossom, or the first date a northern lake is frozen over in winter. Gardeners’ observations are critical in marking the change of seasons using phenology because they carefully watch the plants and animals in their local landscapes over time. The National Phenological Network in the United States is a citizen science effort underway for many years that catalogs the changing seasons by monitoring and recording a variety of phenological events as reported by gardeners and others. Many other countries have similar phenological societies that catalog the first blooms of different trees and flowers as well as changes in rivers and lakes or the appearance of migratory birds like robins for the first time each spring. Maybe some of you contribute to this effort in your own community or country. Some countries like Japan have kept records of first cherry blossoms on particular species of trees for hundreds of years and have seen how this has changed over time. This allows us to see long-term trends in climate based on based on how changes in temperature and precipitation are affecting the way plants grow.

A new phenological index: the Late Bloom Index

While many phenological studies keep track of “first” occurrences like the first cherry blossom or the first leaf on a particular type of tree, the National Phenological Network has recently introduced a new index called the “Late Bloom Index”. This index records the last occurrence in a year of a phenological event such as the last time a bloom is observed on a particular plant species. Keeping track of both the first and last bloom of a plant species gives us a deeper understanding of a plant’s life cycle as well as allowing us to track climate variations through phenology.

The seasons are changing

Use of phenological data has allowed us to study changes in climate in a new way. A recent study of seasons defined by the phenology of different native plants that bloom at different points in the year has shown that the seasons as defined by these plants have changed in Germany (Guido Cioni). According to their research, spring, summer, and autumn (as defined by phenology) are all starting earlier in the calendar year than they did in the past. This might sound odd, but if you think about how plants grow, it makes more sense. Once plants begin their life cycle of producing leaves, blossoms, and fruit, they progress through that life cycle on a regular schedule that is controlled by the climate that plant is growing in as well as by their genes. If spring starts earlier due to the warming climate, then the plant will grow and propagate on its own regular schedule and will finish its life cycle earlier than when the climate was cooler. This has some unfortunate consequences for insects, birds, and other animals that depend on the fruit of particular plants, since they may occur earlier in the year than in the past and that may not match the needs of those animals for migration or preparing for hibernation.

The seasons of your garden

If you don’t already keep a journal of what you are observing in your garden, I encourage you to start. That is a great place to keep track of how your plants (and insects and birds and wildlife) are changing over the year. If you live in the same place for long enough, you will have a historical record of how your local climate is changing and a source of enjoyment at how weather and climate may be impacting your garden’s growth from one year to the next. You may even wish to provide your observations to the NPN using their Nature’s Notebook program. That will allow you to compare your own observations to other gardeners around the country and the world. You can be a scientist as well as a gardener!

How did your garden fare this year? No matter how it grew, at least some of the impact was due to this year’s weather, although careful soil preparation and a good watering plan can fend off some of the worst effects. As usual, my last blog post of the year will be a brief review of this year’s climate and a look ahead to next year. Gardeners must prepare as well as dream! My review will focus heavily on the United States, but I will try to put in a little for our non-US readers too.

How warm was the earth in 2025?

After 2024’s record-setting warmest year on record for the globe, temperatures worldwide in 2025 were not quite as warm as in 2024, although they were certainly warmer than average. While the final average is not yet available, the first eleven months of the year were the second warmest on record, right behind 2024, so 2025 is likely to be in the top three warmest overall, but is not likely to be the warmest ever due to the extra warming from El Niño in the early part of 2024. If you are interested in seeing a time series of temperature for the globe, you can use the “Climate at a Glance” tool from the National Centers for Environmental Information to make your own plots. You can also see more about how El Niño and La Niña affect global climate here and here.

This year’s overall temperature was slightly reduced due to the presence of the La Niña that is currently occurring in the Eastern Pacific Ocean, causing a decrease in the sea surface temperature there (see graph above). As expected, the parts of the globe that are warming the most rapidly are the areas near the Arctic, since the loss of snow cover and sea ice make they warm up much more quickly than other areas with less frequent snowy conditions.

What was the US precipitation pattern like this year?

Again, the year is not quite complete yet, so we will have to wait for final numbers to be tabulated in January. However, using the “Climate at a Glance” tool, we can see there was a lot of variation for the first eleven months of the year in precipitation across the lower 48 states, with some areas like Kentucky and the Ohio River Valley and the Northern Plains receiving much above average rainfall while other areas like parts of the Corn Belt in Indiana and Illinois, Florida and adjacent areas of Alabama and Georgia, and most of the western US receiving much lower than normal rainfall.

Of course, an 11-month total does not describe how rainfall varied across the year, and some places like the Southeast where I live actually started out so wet it was hard for farmers to get out into their fields and ended the year so dry that exceptional drought formed in parts of that region. Since we did not experience any landfalling hurricanes this year and only one tropical storm, there was little impact on the rainfall pattern in the US in 2025, and the lack of rain may have contributed to the drought the developed later in the year in the Southeast due to the lack of tropical rainfall, which often provides a significant fraction of the precipitation in the summer months in areas affected by tropical weather.

What was the US temperature pattern like?

The temperature pattern of anomalies across the United States was less variable than the rainfall pattern and showed that most areas of the country were warmer than normal, especially in the western half of the Lower 48 states. You can find more detail, including maps for Alaska and Hawaii, in NOAA’s National Climate Report. A few areas in the eastern US were cooler than the 1991-2020 average but still above the long-term average. One of the things that struck me this year is how variable the temperature was, with some prolonged heat spells followed by cold outbreaks. This was especially seen in mid-November this year, when a warm start to the fall season was broken by an intense cold front that drove freezing temperatures nearly to the Gulf, ending the growing season in the Southeast earlier than expected in many areas. 

If you like to see satellite imagery of unusual weather or other activity like volcanic eruptions, be sure to check out NOAA’s annual gallery of interesting photos or videos of interesting events seen from the NOAA satellites, including wildfires, dust storms, atmospheric rivers, hurricanes, and even volcanic eruptions, many of which I have discussed in previous blog posts.

What should gardeners expect in 2026?

As of late December, we are currently in a weak La Niña, which means the ocean temperature in the Eastern Pacific Ocean is cooler than normal. That usually affects weather across the US by shifting the most active weather to the northern states, bringing a lot of rain, snow, and very cold weather to those areas while leaving Southern states warmer and drier than usual. But that is only expected to last into the early months of 2026 and after that we will return to neutral conditions. In fact, it is not unlike the conditions we expected last spring, when we were also leaving a weak La Niña and headed back to neutral conditions. Generally, when there is no El Niño or La Niña, the weather can be more variable and we can get more frequent swings between warm and cold conditions. There is likely to be some lingering effects from the La Niña that is currently occurring, so that means we will likely continue to see cold and wet conditions in northern parts of the country while southern areas are likely to be drier, sunnier, and warmer than usual for the next few months. One difference is that this year the predictions are that after several years of La Niña or neutral conditions, we are expected to swing to an El Niño, which will likely cause much different climate impacts next fall and winter than what we observed this year.

Gardeners in southern and eastern areas should be on the lookout for potentially early blooms on fruit trees with the potential for a late frost that could kill off the blossoms. I would say to be cautious in planting your gardens too early because of the potential for cold weather returning after an early warm-up. In northern areas, spring could be late in coming this year. If it stays warm and dry in the Southeast this winter, a spring or early summer drought could occur, so make sure you have a good watering plan in place (of course, that holds true for most gardeners, since a dry spell can happen even in a season that is fairly wet).

How did the weather affect your own garden this year?

I am always interested to hear stories from gardeners about how the weather and climate affected their own gardens. Please feel free to share anything that affected your local conditions, from hail that damaged vegetables or fruit to unusual heat that caused problems with pollination. If your garden had an excellent year due to perfect growing conditions, I would love to hear that too! Be sure to say what state, country, or city you live in so we can compare the climate maps to what you observed.

Thanks for subscribing to “The Garden Professors” and let’s look forward to another interesting and productive garden year in 2026!

If you spend much time on social media, you have probably seen screaming headlines on Facebook, Twitter, or elsewhere about impending extreme weather. In the last few weeks I have seen wild stories about a late-season tropical storm forming in the Caribbean and an Arctic outbreak and snowstorm heading through the eastern US all the way to the Gulf. Neither of these had any real chance of occurrence but the people who post them are looking for clicks and attention. Garden Professors’ blog readers already know about misleading information in social media (Epsom salts and cardboard mulch, for example) but may not know much about weather forecasting and how it is misused in these click-bait posts to gain attention for weather that probably will never happen. This blog post will describe how weather forecasts are made so that you can understand which forecasts are the most reliable and useful for gardeners and others who work and play outdoors.

Types of weather forecasts

In general, forecasts can be categorized in several ways. They can be categorized by time period (nowcasting, short-range, medium-range, long-range) or by method (based on evolving weather conditions, statistics of past weather, or numerical weather prediction by computers). Forecasts related to time just indicate how close to the actual weather occurrence you are making the prediction. A “nowcast” is a forecast for the immediate future up to about six hours ahead. Short-range forecasts usually cover 1-3 days, medium-range 3-7 days, and long-range forecasts are made more than a week ahead. Long-range forecasts tend to provide more general descriptions of climate patterns and departures from climate conditions rather than specific weather conditions because accuracy decreases as you go farther ahead in time. This is one clue that a specific social media forecast map is likely to be unreliable because we cannot make specific predictions of atmospheric conditions more than about a week ahead due to lack of sufficient data, simplifications in the computer models used to make predictions, and chaos in atmospheric conditions that cannot be easily described by the input data to forecast model programs.

Methods for making weather forecasts can also vary. The simplest one says that today’s weather will be the same as what happened yesterday. It works well in regions and seasons when the weather does not change much from one day to the next but is poor where more dynamic weather occurs. One step up is what we call an advective forecast, which predicts changes in temperature, moisture, and other properties due to the horizontal movement of air by wind. Forecasters determine this by looking at wind direction and the temperature and other conditions of the air that will be arriving from upstream. If air is blowing in from the Gulf, for example, the weather is likely to turn warmer and more humid, while air arriving from the Arctic is likely to be colder and drier than what is already present. Similarly, you can sometimes use observations of clouds to make simple weather forecasts based on how they change over time. Climatological forecasts look at past weather occurrences on the dates for which you are forecasting and describe the statistical likelihood of specific temperatures and rainfall conditions. Some also look at analogs from previous weather situations that look like the current conditions and predict that the future weather conditions will occur again based on past performance.

Using computers to predict weather

Most forecasts used by modern meteorologists use numerical weather prediction to forecast future weather patterns by entering observational atmospheric data into complicated computer models that use the physical equations of motion to determine how the atmosphere will change in response to moving air, development of clouds and precipitation, and interactions with land and ocean surfaces at the ground and put the results into map formats that provide a graphical depiction of what the weather will be like in the future that can be interpreted by meteorologists to provide the expected local conditions (these are called deterministic forecasts). There are multiple weather models that are run by different groups, including US-based models as well as those from Europe, Canada, Japan, and other countries with large computing facilities.

Most models are run for a variety of different starting conditions to give a range of possible outcomes, and the combination of different model runs and different models lead to what is commonly called “spaghetti models” that show each individual model solution on the same map. The closer together all the runs are in the future, the more confidence we have that the models are in agreement, but if they are spread out, forecasters have low confidence in what will occur. Expected impacts at any one location change depending on which computer run is used. Many of the extreme weather events shown in maps show by social “media-rologists” are from a single computer run far in the future showing the worst possible case, even though there may be 99 other computer solutions that show something much less severe. The social media folk are showing whatever extreme case will get them the most attention (and clicks), which allows them to monetize their accounts by promoting the worst-case event, no matter how unlikely, not the most likely one. Long-range forecasts which look at all the solutions may use statistics to determine where the storm is most likely to go and base the probability on the range of individual model runs we see (these are called probabilistic forecasts).

How accurate are weather forecasts?

Despite the common belief that weather forecasts are seldom right, the statistics show that they are very accurate. People generally only remember the few occasions when they are wrong, not the many when they are correct. Today’s forecasts are better than in the past because computers are getting larger and more complex and more data are being collected to feed into the models, including satellite data that fill in holes in surface data over the oceans. A daily forecast now is likely to be good at least 7 days in the future, compared to 5 days twenty years ago. In the future, we may see even better results using artificial intelligence (AI) to fine-tune forecasts, as we did this year with hurricane track forecasts. But we will never be able to provide an accurate daily weather map 90 days in the future, so don’t count on the long-range forecasts to tell you what to expect on the day of your summer garden party when you are planning in January or even in April). If you need that forecast to plan for the likely weather, using climatological information is your best bet.

The best weather forecasts for gardeners

Your best source of accurate weather forecasts in the United States is the National Weather Service. Most other countries provide forecasts from their own government weather services. If you need to plan for specific times for gardening projects such as spraying products that require dry conditions or mowing, hourly forecasts for up to 6 days ahead can be found using the information at https://site.extension.uga.edu/climate/2018/03/where-to-get-hourly-weather-forecast-information/. Keep in mind that the farther ahead in time you get, the less accurate they will be. Weather information from private forecasters, including broadcast meteorologists and commercial companies that provide handy apps for cell phones, mainly base their forecasts on NWS predictions but often add value by providing descriptions of conditions, specific impacts on sectors like agriculture and transportation, or pretty graphics to make the forecasts look more appealing. Gardeners should keep in mind that most weather apps on phones are fine for daily planning but are not suitable for rapidly changing weather conditions like severe weather, since they are not updated often enough to factor in these short-term events.

Gardeners who want to maximize their outdoor activities need access to accurate and believable forecasts so they can plan the use of their time effectively. By understanding the nature of weather forecasts and the dangers of exaggerated predictions of likely future weather conditions, they can make the best use of their time and enjoy their garden work without worrying about overhyped extreme events that likely won’t happen.

This time of year, I frequently notice the change of the sun’s daily position over time, since my family room faces east. This is especially true as most of my trees on that side of the house are deciduous—as the leaves fall, I get a better look at where sunrise is actually occurring and how it is changing day by day. Sun angle and the amount of sunlight that reaches different parts of your garden can have a big impact on what kinds of plants you can grow and how your garden appears. This impact changes daily as well as seasonally and is based on your garden’s orientation. In this post we will discuss how the sun angle affects gardening and how you can use sun angle to help plan your garden.

How sun angle and day length change with the season

The tilt of the earth’s axis of rotation causes the path of the sun across the sky to change throughout the year. When the North Pole is pointed towards the sun (Northern Hemisphere summer), the sun is high in the sky, days are long, and the light is the most intense. This can lead to high temperatures and the risk of sun scorch or heat stress on sensitive plants where direct sunlight is strongest. When the North Pole is pointed away from the sun (NH winter), days are short, the sun is low in the sky, and the light is generally weaker. Shadows are longer and stretch further, which means that an area that might get full sun in summer may be entirely shaded in winter. This can limit gardening options in colder months even in areas where there is no frost.

In spring and fall, the sun’s angle is intermediate and changes more quickly from one day to the next. Of course, if your garden has deciduous trees, shade will also be affected by the leaf-out of those trees and will increase as the leaves grow and expand, so the type of surrounding tree cover will also be a factor.

You can find a useful tool to help you determine the direction of the sun at any time and place at https://sun-direction.com/.

Cloud cover and sunlight

In the real world, the sun doesn’t always provide much light if you are in an area with a lot of cloud cover. Some areas have a lot more cloud coverage than others due to the effects of mountains or water bodies that help form clouds. Cloud cover can also vary depending on the season and what types of weather are affecting a particular region. Where I grew up in western Michigan, the effect of lake effect cloud cover made winters and springs quite gloomy and any sunlight was welcome. However, in summer the prevailing wind shifted from the northwestern flow that occurred in winter to winds that were primarily from the south. As a result, our summer weather was much sunnier and warmer because of the increased sun due to fewer clouds. You can see a video of the seasonal cycle of cloud cover across North America here or see monthly maps over the United States in Brian Brettschneider’s Climate Blog. If you are in an area with a lot of cloud cover, especially during the growing season, you will need to factor that into your planning, since clouds reduce the amount of incoming sunlight and influence photosynthesis.

How sun angle and light exposure affect plant growth

The amount of light that hits a plant will directly control photosynthesis and plant development. Since plants use light energy for photosynthesis, the more light-hungry a plant is (like fruiting vegetables), the more hours of direct, intense sunlight it needs (typically 6-8 hours or “full sun”). Shade and insufficient light will result in weak, leggy plants with poor fruit or flower production. If the sunlight is unevenly distributed, the plant will grow and bend towards the light source, resulting in weaker stems or an uneven shape.

The amount of sunlight that an area gets will also affect the types of plants that grow in a given location. Areas with high-angle, intense sun develop thicker, shorter leaves to minimize water loss. These areas are also often areas of higher temperatures and drier conditions, which contribute to the types of plants that grow there naturally. Plants that grow in low-angle, dappled, or indirect light often have thinner, larger leaves to capture as much of the light as possible.

Using sun angle to help with garden design

Understanding the sun angle and light distribution in your garden is essential to good garden design. By observing the movement of the light and shaded patches in your garden over the course of the day and across the seasons, you will be better able to choose the best plants for the exposure you find in each part of the garden. You might even want to create a diagram of your garden to identify different areas of light exposure. You should match the plants’ light requirements (full sun, partial shade, full shade) to the appropriate areas you have.

In addition to determining where shaded and sunny areas are, you also need to consider the effects of trees, especially if they are deciduous and change over the course of the year. If you want to provide an area with more sunlight in your garden to produce crops like tomatoes, you may wish to consider some pruning of tree branches to provide more sunlight to those areas. You should also think about the impacts of taller garden plants on surrounding vegetation, so you should plant taller plants on the north side (in the Northern Hemisphere) so they do not shade shorter plants.

Protecting plants from too much sunlight

If your sunlight is too intense for some sun-sensitive crops like lettuce, you may be able to erect temporary shade structures to help protect them from the strongest sun. Farmers also use kaolin clay and similar products to help prevent sunburn in commercial plants by covering them with a white layer that reflects sunlight away and keeps the fruit cooler and less affected by strong sunlight. It can also help repel pests by creating a protective barrier on plant surfaces. You can tell if plants are getting too much light by observing leaf scorching (brown, crispy spots, especially on the edges), bleaching (leaves turning pale yellow or white), wilting, and stunted growth.

How sun affects gardeners

We all love the sun and our gardens need it to grow, but too much of a good thing can be hazardous to the health of gardeners as well as plants. Make sure that you wear sunscreen and use hats and clothing to help protect yourself from the harmful aspects of sunlight and you will be free to enjoy your garden without fear of skin cancer and health issues. Let the sun shine and let our gardens (and gardeners) grow strong!

In my household, the weather is a common subject of conversation. That is only partly because I am married to a meteorologist. In fact, I have noticed that I can talk to almost anyone about the weather, and I suspect you can too. Weather is most captivating when something interesting is occurring, like liquid falling from the sky. When I give talks to master gardener groups, they are almost always consumed with how the weather is affecting their gardens. I get more questions about drought from these groups than almost any other topic, but rainfall, past or future, is also a frequent subject.

Today we will look at different types of precipitation, how they are formed, and how they affect garden plants. This is especially important as we move from summer, when rain is the most plentiful hydrometeor, into fall, when freezing rain, sleet, and snow become more frequent. Of course, this depends on where you are, and some Northern Hemisphere southern areas will experience almost no snow while for northern regions, it is the majority of what is observed.

What types of precipitation are there?

When I started writing my post, I looked online to see how many different types of precipitation were listed. I found that there is quite a variety in the number of types listed, ranging from two to seven varieties. I am going to lump them into two basic categories: liquid and frozen. But there are subcategories within these two basic buckets, especially in frozen precipitation, which includes snow, sleet, ice pellets or graupel, freezing rain, and hail. We will see how they are related to each other and yet distinct.

What is rain?

Rain is essentially liquid water that is falling from the sky and hitting the surface. If the air is dry enough, rain that develops in clouds evaporates before it gets to the ground, and that is called virga. Raindrops can form in tropical clouds through a liquid process that involves the collision and coalescence of small water droplets into larger drops that get heavy enough that they cannot remain suspended in the air and fall to the ground. But you might be surprised to know that most liquid rain starts as snow high up in the clouds, where the temperature higher up in the atmosphere is below freezing. We’ve addressed some characteristics of rain and how they affect gardens in previous posts here and here.

Clouds (except those in the deep tropics) are made up of a mix of supercooled water droplets and ice crystals that float around together, but it is easier for the ice crystals to grow by sucking up water vapor than for the water droplets to grow, so the ice crystals usually dominate the process of producing precipitation and snowflakes are the result. As these snowflakes fall towards the ground, they fall into warmer air near the surface and melt into the liquid water drops that make us wet. A light rain with small water droplets may be just a drizzle, but a thunderstorm with a cloud that is 10 miles deep could have water droplets as large as 0.34 inches, although I have heard unconfirmed stories about raindrops more than half an inch across in the heaviest rain events.

What types of frozen precipitation are there?

When precipitation freezes, it can take different forms depending on the temperature of the air that it falls through. If the air that the original snowflake falls through is below freezing through the entire depth of the atmosphere, then it remains as snow all the way to the ground. The shape of the snowflake depends on the combination of temperature and humidity that is present where the snowflake forms. They can take on an amazing variety of shapes beyond the typical dendrite that we usually associate with snow. The dendrites we often see falling in winter are ideal for blanketing the ground with a carpet of white, and their shapes make them able to trap a lot of air in the snow cover, providing insulation for the soil that keeps the temperature there relatively warm by protecting it from the cold and dry air the snow is falling through. In other words, it acts as a mulch to protect the ground and the plants there.

If the snowflakes from aloft fall through air that is above freezing, they transform from snowflakes to other kinds of precipitation, as shown in the diagram below. If the warm layer is deep, all of the snow changes back to liquid raindrops. If the air is above freezing but the surface is below freezing, then freezing rain occurs. The water sticks to trees, wires, and buildings, adding to their weight and collapsing them if the accumulations are thick enough. If the warm layer is relatively deep, but the air above the surface is below freezing, the water droplets may refreeze, leading to sleet or clear ice pellets. If the layer is thinner, the snowflakes may develop a coating of liquid water that surrounds the snow crystals from supercooled water in the clouds, encasing them in ice, leading to snow pellets or graupel, which are opaque instead of clear like sleet.

Hail is another variety of frozen precipitation that forms in summer thunderstorms, which can be as much as 10 miles high. In those storms, snowflakes can circulate vertically inside the storms due to strong updrafts, gathering a new coating of ice each time they move upward through the clouds, growing larger each time they cycle up and down. The largest hailstone ever measured was 8.0 inches in diameter and weighed 1.9375 pounds. It was discovered in Vivian, South Dakota on July 23, 2010. You will often find layers of clear and cloudy ice in hailstones as they travel up and down through the thunderstorms. Hail that is much smaller, as little as 0.25 inches, can cause damage to plants and crops as the hailstones shred leaves and cause damage to fruits and vegetables, leaving them unsightly and vulnerable to mold and pests.

How do different types of precipitation affect garden plants?

The most damaging types of precipitation for gardens are those that add weight to plants or cause impact damage when they hit something. Heavy snow and freezing rain can cause tree limbs to break due to increased weight, which trees cannot withstand, especially when they are still leaf-covered in fall or when they are not well maintained and have weak points in their structure. Garden plants can also be flattened or otherwise damaged by the heavy ice or snow cover. Previous advice in the Garden Professors by Linda indicates if the snow is really heavy, it should be removed from the plants before it can do damage, although lighter amounts can remain. On the other hand, snow cover on the ground can be a benefit to your garden if it incorporates enough air to serve as an insulating layer between the plants near the ground and the colder and drier air above it.

Hail, especially heavier stones, can cause significant damage to leaves and can defoliate gardens or farm fields completely in just a few minutes if the hail is intense enough. Even small hail can destroy tender plants, fruit, and flowers or at least damage the skin or leaves enough to decrease their value as crops due to cosmetic blemishes and places where diseases or insects can enter the plants. Gardens should be assessed for hail damage soon after a storm occurs, since the damage can be harder to spot over time.

How else can precipitation help gardeners with their gardens?

A previous post on GP noted that observing your garden after a heavy rain can be helpful in determining what the drainage patterns are and what might need to be addressed. While you might not be able to do much work in your garden right after a heavy rain event, it provides an excellent time to make future plans to make your garden more weather-proof.

Rain and snow, along with the other varieties of precipitation we experience, provide valuable moisture to our gardens as well as protection to plants in winter, but can also produce damage that can harm our garden plants. Enjoy the rain (or snow) when it comes, but be aware of the negative impacts it can have on your garden, too.

The cooler weather that many parts of the eastern United States are experiencing this week is causing many gardeners to think about what this fall will be like. In fact, many farmers in Georgia are already planting fall crops, and I am sure that many gardeners are also busy with their own fall planting if they live in the Northern Hemisphere mid-latitudes. In this blog post we will discuss seasonal forecasts and how you can use them to plan and plant your garden.

What kinds of long-range forecasts are available?

In weather and climate, there are two basic types of forecasts. The first is a deterministic forecast, which gives an outlook that describes the specific weather that is expected to occur at a discrete time in the future. Deterministic forecasts are commonly used for weather forecasts for the next few days and are based on computer model predictions that are grounded in the physical equations of motion, thermodynamics and other properties of the atmosphere. Generally, deterministic forecasts are most useful within a few days after the forecast is made. As you go farther out in time, they become less accurate due to lack of updated weather observations and drifts in the models due to the chaotic nature of the atmosphere. In general, a deterministic forecast is not very accurate more than a week ahead of when it is made. They are better than they used to be, and the accuracy of a 7-day forecast now is as good as a 5-day forecast was a couple of decades ago, but we will likely never have a perfect deterministic forecast more than ten days out.

Longer-term forecasts are usually given as probabilistic forecasts. In other words, the forecast will give a likelihood of occurrence of general weather conditions such as wetter or drier and colder or warmer than normal. For meteorologists, “normal” is a 30-year average of temperature and precipitation at a location and is currently based on the 1991-2020 period (they are updated every ten years due to the amount of work it takes to produce a clean dataset). Most probabilistic forecasts are based on multiple model results that start from slightly different conditions and are built with different methods of creating rain and clouds, moving temperature and humidity around, and start with different surface conditions. The more the models agree, the higher the probability of occurrence of a particular type of weather.

How to interpret the NOAA monthly and season forecast maps

In the NOAA map for September 2025 above, for example, there is a dark brown area centered on the Great Salt Lake in the western United States. This is an area that has a strong probability of being drier than normal in the month of September based on the available model output. The fact that it is dark brown does not mean it will necessarily be much drier than normal, but that we have a strong likelihood that it will in the lowest third of years in terms of how many inches of precipitation it is likely to experience. These forecasts start from even odds of having near normal precipitation (34%), above normal precipitation (33%) and below normal precipitation (33%). If the climate forecast models indicate mostly dry conditions at a point, then the percentages shift to something more like 34% near normal, 50% below normal, and 16% above normal precipitation. Because there is so much uncertainty in the atmosphere, you never know with total accuracy which category the season will fall into, you just have some confidence of which way it is likely to fall. Since this is the average for an entire month or season, there can be periods within that time span that are significantly different weather that what the probabilities suggest are the most likely to occur. Here is a list of what the seasonal forecast maps do NOT tell us.

Deterministic versus probabilistic forecasts

If you are planning an outdoor event like a garden wedding months ahead, a deterministic forecast is what you would probably want to know. Do we need to rent a tent? Should we expect hot or cold weather when we purchase a dress? Unfortunately, this is not possible a long time ahead and so you need to use previous or average weather conditions on that date to decide what kind of weather is most likely to occur and be aware that you could be wrong. There are a few commercial forecasting sites that provide specific deterministic forecasts up to 90 days ahead. I asked a friend who works for one of these companies why they do it when research shows that there is no skill involved, and he told me they do it for “entertainment value.” In other words, it is not real information, it is just click-bait. The same thing happens with wild hurricane forecasts on social media that show a single deterministic forecast of a huge storm hitting somewhere like Tampa, conveniently not showing the 99 models with no such storm present. Don’t believe them, they are harvesting clicks, not providing useful information.

The Farmers’ Almanac and the Old Farmer’s Almanac

Every year, including this one, publishers release almanacs each fall which claim to show what the weather will be like for 1 to 3-day periods for the winter and next year. Their winter outlook maps get a lot of press about what to expect weatherwise for the next few months (I won’t post any links in this post but you can search online if you are really curious). If you read them carefully, you can see they are quasi-deterministic since they reference storms or heat waves occurring at specific areas over just a few days, although they are usually written in broad enough language that they can be interpreted in several ways. Many people use these for planning their gardens, and they do contain useful information about climatological conditions, average frost dates, and sunrise and sunset information. But scientists have shown that they are only about 50% accurate, which amounts to pure chance. They base their forecasts on secret formulas that cannot be scientifically studied or verified so we have no way to know what they are really using to determine what those forecasts will be. So if you buy one this year, I challenge you to keep track of what they forecast and what weather actually occurred at your house and see if they did any better than chance in 2026. Let the buyer beware and use them for “entertainment value” only.

What can we expect this fall?

Based on NOAA’s probabilistic forecast, we can expect the temperature across most of the United States to be warmer than normal, with some areas more likely to be warm than others. This is likely due to the continuing greenhouse warming that is occurring, making every year (on average) warmer than the previous one, although there is a lot of variation within that trend from one year to the next. Precipitation in the Southeast for the September through November period is leaning towards wetter than normal conditions, and that is probably related mostly to the second half of the Atlantic hurricane season, which could bring heavy rain to parts of the Southeast during fall even though parts of it will certainly be missed. The southwestern United States centered on the Four Corners area is expected to be drier than normal in an area that is already quite dry climatologically.

For the December through February period, the forecast is showing a pattern of climate conditions that is consistent with the La Nina that is expected to occur as we head into winter, with warmer and drier conditions in the southern US and wetter and cooler conditions in the northern US. If you live in a different part of the world, you can find images of the expected La Nina conditions here. Choose plants according to the climate conditions you expect and be prepared to manage them for those conditions, so that if you are in the South, you may have to water more frequently because of the drier than average conditions.

Seasonal outlook maps can provide clues to the kind of temperature and rainfall conditions you are likely to experience in your neighborhood over 3-month periods, although they will not give you specific details about when the first frost might occur or how many heavy rain events you can expect to see. But knowledge of the likely pattern of conditions you can expect will allow you to plan what kind of plants to put in the ground and how you are likely to have to take care of them as they grow.

Just an additional note: If you like to track the fall colors, check out this interactive fall foliage map at https://www.explorefall.com/, now including Alaska. It’s a great resource for planning weekend trips to areas you think will be experiencing beautiful fall foliage.

Have you ever stopped while you were gardening to look at the clouds? Clouds, like flowers, come in a variety of shapes and sizes that can form beautiful patterns in the sky. But clouds are not just pretty, they can also be used to make predictions about the weather in the coming days. In this week’s post, we will look at the different types of clouds and how they relate to coming weather. You can use that to prepare for your garden work by knowing when it will be sunny and predicting when rain is coming.

How are clouds classified?

Clouds were first classified by Luke Howard in 1803 in his “Essay of the Modifications of Clouds”. Howard classified the clouds by their shape, using Latin names for wisps (cirrus), lumps (cumulus), or sheets (stratus) to describe how they looked in the sky. Clouds that are precipitating either rain or snow are called nimbus clouds. Some clouds are a combination of types, such as stratocumulus clouds, which look like a layer of lumpy clouds, or cumulonimbus clouds, which are the tall thunderstorms that form in summer. You can find a great gallery of cloud photos at the Cloud Appreciation Society. Wikipedia has an exhaustive list of cloud types online as well.

Clouds are also classified by heights. Most clouds form in a single layer of the atmosphere, but you can often see multiple layers of clouds when you look at the sky. These layers may be caused by different mechanisms. The shape and height of the clouds provide clues to what is going on in the atmosphere and are especially related to the presence (and sometimes absence) of warm and cold fronts that are harbingers of precipitation and a change in wind and temperature conditions.

To estimate how high the clouds are, you can use a literal “rule of thumb” that works well for cumulus-type clouds. Hold your hand out towards a cloud. If the individual cloud is the size of your fist, then it is probably a low cloud. If the lump of cloud is the size of your thumb, then it is probably a mid-level cloud. And if it is the size of your little pinky joint, then it is probably a high cloud. Note that the clouds are the same size, but the difference in height makes them look like they are different sizes.

For stratus clouds, you can estimate the height by how transparent it is. A cloud layer that allows you to clearly see where the sun or moon is located is probably a high cloud. A layer that lets you see where the sun is but shows it with blurry edges (we sometimes call this a “ground glass” appearance), then it is probably a mid-level cloud. If it is so thick that you cannot see where the sun or moon is, then it is probably a low-level cloud. But please be careful not to look directly at the sun, since it can damage your eyes!

How do clouds form?

Clouds form where moist air cools off to the point that the water vapor condenses into small drops that become visible to us. Since the atmosphere generally cools off as you go up, the clouds form where the air is rising. The rising motion can be provided by heating from the earth’s surface, lifting of the air by a mountain, or large-scale upward motion caused by warm and cold fronts, where masses of air at different temperatures interact to create areas of rising air. The highest clouds usually form in the coldest air and form as ice crystals, leading to their wispy appearance. Lower clouds appear in warmer air where the water condenses as liquid droplets, leading to their more robust appearance.

How are cloud shapes related to atmospheric winds and structure?

The most interesting weather (at least to me, as a meteorologist) is where there are differences between masses of air at the surface and above the surface that are interacting. Boundaries between these air masses are called “fronts” and are named because they act like battle fronts between enemy armies. In a cold front (left side of diagram above), cold dense air near the surface pushes beneath a layer of warm, humid air, causing it to rise and cool. Clouds ahead of cold fronts tend to be relatively tall and energetic and form cumulonimbus clouds that can drop a lot of rain in a short time but generally tend to move through an area quickly unless the front stalls due to other atmospheric dynamics.

Warm fronts are large masses of warm, humid air that are pushing over the top of a layer of colder, more dense air (right side of diagram above). As the warm air rises, it slowly forms clouds as the layer cools to the temperature of condensation. Since the entire layer is rising, the clouds that form are often sheets of clouds rather than individual clouds. The higher clouds indicate that the moisture from an approaching warm front is present high in the atmosphere and shows that a warm front is likely to be approaching, signaling a change in the weather from cool to warmer conditions that could also drop rain as the warm front gets closer to your location. As the warm front gets closer, you should see the high clouds replaced by mid-level clouds like altostratus and then lower clouds like stratus clouds and nimbus clouds if they start producing rain. This usually happens over a day or two depending on the speed and strength of the surface warm front.

If you see lumpy cloud forms, then they are most likely related to the rising motion of air due to columns of warm air rising from the surface (“thermals”). Air between the columns is sinking, which leaves clear spots between the clouds. Fair-weather cumulus clouds are the tops of these thermals, when the rising air cools down enough for the water vapor in the column to condense, forming the cloud. These types of clouds usually form when the ground is heated, most often by sunlight during the day but sometimes by pavement or wildfire as well as mountainous areas. They usually form on days when you are in a mass of warm, humid air that is far from a frontal boundary, although if they grow taller, then a cold front is likely to be approaching.

If the air is very hot and humid and the surrounding air is cooler than the rising column, then the clouds can grow vertically to great heights before they hit a layer that is warmer than the rising air and stop growing. These tall clouds are called cumulonimbus clouds because they often drop heavy rain as they develop. These often form along and ahead of cold fronts and indicate that the wind is likely to shift from a south wind to a colder wind coming from the northwest (in the Northern Hemisphere). In the worst cases, the rain can cause erosion or damage to fragile plants when it is very intense. Plants that live in rainy areas evolve to have such things as pointed tips or shiny leaf surfaces that shed the water quickly. Cumulonimbus clouds are also sometimes associated with high winds, hail, and tornadoes, all of which can damage garden plants and trees as well as harm humans and animals.

How can you predict the weather by watching the clouds?

As you work in your garden, take a look at the clouds above you. If they are high and wispy, then moisture high in the atmosphere may indicate that a warm front and a chance of rain is likely in a couple of days, especially if they get lower and denser over time. The picture of daisies at the beginning of this post shows high, wispy clouds that could indicate a warm front is approaching. The glowing ring of light that appears in cirrostratus clouds in the picture just above this paragraph may also be a sign that there is moisture on the way, leading to the saying that “a ring around the moon means rain in 48 hours”.

The chance of rain may affect your plans to spray your gardens or lawns for pests or fungal diseases, since many garden treatments have weather-related requirements for when to use them. Some work better when applied to wet leaves after rain falls, but others need a period of dry weather for the chemicals to be most effective. Make sure you read the labels to know what kind of weather they need. It may also tell you that it would be a good idea to mow the grass before it rains in the next 24-48 hours.

If you are already in hot and humid conditions and you see cumulus clouds getting taller and more numerous over time, a cold front may be approaching. This could indicate a period of strong winds, heavy rain, some possible lightning, and sharply cooler temperatures which could be either a curse or a blessing depending on just how hot it has been. The very shallow clouds in the picture above (cumulus humilis) are most likely seen after the cold front has passed and there is minimal lifting to cause clouds to form.

Don’t forget to look up

A good gardener should always be keeping an eye on their garden but should also be watching the environment around it to see how the conditions might be changing in the future. Who knows what delights you will see if you just look around you? But don’t forget to look up, too, because the sky is full of wonders and can inform you about the future as well as strike you with awe.

If you’ve been paying attention to the weather across the United States this past week, you may have noticed that most of the eastern U. S. is experiencing extremely hot temperatures, especially when you factor in the effects of humidity. At the same time, in the western U. S., it has been snowing in the mountains, even though it is almost July! In this week’s blog post we will look at why this pattern of hot and cold conditions occurs so often and what is causing it.

Heat in the East

We have talked about persistent areas of very high temperatures several times in past blog posts (for example, here and here). Those areas most commonly form in summer under stagnant areas of high pressure that have sinking motion in the middle of the high. The sinking motion of the air keeps clouds and rain from developing, leading to very hot and dry air being trapped near the earth’s surface, raising temperatures and reducing wind speeds. Often those areas also include a lot of humidity, which makes the temperatures more oppressive because sweating does not cool you off efficiently if the humidity is high, especially if winds are also light.

When this happens, the National Weather Service will put out heat advisories or other statements reminding people to be careful to keep cool and stay out of the hot sun as a way to prevent heat-related illnesses like heat stroke. This is also important for gardeners, who like to spend time outside tending their plants and gardens. You can monitor the atmospheric conditions associated with heat to identify times when it would be better to stay inside where it is cool by using the National Weather Service’s HeatRisk map or the Southeast Regional Climate Center’s Wet Bulb Globe Temperature forecast tool (which is for the entire country, not just the Southeast).

Note that heat domes don’t always occur in eastern parts of the country. A deadly heat dome occurred in the Pacific Northwest (PNW) in late June 2021, breaking numerous high temperature records and killing 136 people in Washington alone in the period from June 26 through July 6, 2021. In that case it was the West that was under unusually high temperatures while the Eastern U. S. was cooler than usual. They also occur in other parts of the world, and in fact this week Europe is also experiencing very high temperatures due to another region of high pressure parked over them.

It is also interesting to note that in winter, areas of high pressure are often the coldest areas of the country due to the lack of cloud cover, which allows heat from the earth to escape to space, leaving colder conditions at the surface at night when no sun is there to heat the ground.

Cold and snow in the West

On the other end of the country, cold air was trapped on the other side of the frontal boundary between the eastern high-pressure center and the low-pressure trough that was present in the western U. S. The cold air was so intense, especially at higher elevations, that snow fell in some mountainous areas of western Montana and surrounding states. The Going-to-the-Sun Road in Glacier National Park had to be closed from June 20 to June 23 due to the heavy snow.

What connects these two extremes together?

The common connection between the heat in the East and the snow in the West is the large-scale atmospheric wave that is linked to both the low pressure in the west and the high pressure in the East. The atmosphere is a fluid and is constantly adjusting its pressure fields by forming waves with ridges of high pressure as well as troughs of low pressure. Sometimes these patterns get locked in place for a few days (or even longer in rare cases), which leads to more extreme effects, but they usually move on in a few days, causing the weather to go back to normal conditions or even flip-flopping to the opposite pattern. In fact, we discussed atmospheric waves back in July 2021 following the PNW extreme heat event in this blog.

The surface weather associated with these areas of high and low pressure are what cause the big changes in observed conditions that gardeners and farmers have to deal with since they can have big impacts on flowers and crops. The atmospheric wave pattern can lock in place for a number of reasons including conditions in other parts of the earth such as unusually cold or warm water in the ocean or droughts or floods in other areas. Fortunately, these stationary patterns usually shift or break down after a few days, leading to big changes in local weather conditions that might be more welcome.

How do cold and hot outbreaks affect gardens?

Most garden plants are fairly resilient to the changes in temperature, humidity, winds, and cloudiness that come with the shifts in the atmospheric waves and their associated surface weather. Plants respond differently to heat than humans and other animals do because they don’t sweat. High heat can cause the plants to close the stoma in their leaves to retain moisture, but a long enough period of high temperatures and dry conditions with little soil moisture leads to wilting and eventually, death of the plants. Of course, most gardeners are watering their gardens to avoid this worst case!

Cold spells in summer do not cause as much damage as frost and snow in spring and fall because they just don’t get cold enough to damage the plants (unless you are in the mountains and the temperatures drop below freezing), but they can slow the plants’ growth and reduce their flower or fruit production. Food crops that depend on a significant number of growing degree days (a measure of the accumulation of heat over time based on daily temperatures) will grow more slowly, resulting in late harvest of whatever crop is being grown. In the worst cases, it could delay harvest so late in fall that fall frosts become a consideration, but most home gardeners do not need to worry about this as much as commercial farmers because they are not farming hundreds of acres of crops, just their own patch of land.

I encourage you to visit some of the links in the article above to learn more about heat domes and atmospheric waves as well as their impact on your garden plants. You can also use the search field to find additional sources of information about any topic of gardening that you might want to learn more about, especially one that relates to the science of gardening. It’s a great resource!

I am traveling in Colorado this week, so my thoughts naturally turned towards the mountains. Mountains affect gardening in a number of ways, many of which include a weather or climate component. They also provide some special challenges for gardeners because of the harsh conditions and short growing seasons that are often found in and near mountainous terrain.

How the mountains affect weather and climate

It is said that mountains create their own weather, and there is a lot of truth to that. Mountains interact with the atmosphere in several ways, and that interaction can change both the atmosphere itself and the conditions on the surface of the mountain that is being affected by the air.

One way that mountains affect the atmosphere is by providing a physical barrier to the flow of air. Obviously, air can’t flow into a mountain slope, so it must either go up the slope, around the mountain crest, or through the valley between peaks. Air that is pushed up the mountain usually results in the formation of clouds on the upwind side, since rising air cools as it goes up and eventually the moisture in the air condenses to form cloud droplets, and eventually rain or snow, called “orographic precipitation”.

On the downwind side of the mountain, the air typically sinks and dries out, since the moisture has been wrung from the air by the rain or snow left on the upwind side. That can lead to a “rain shadow” effect with low precipitation amounts downwind of a single mountain or a range of mountains such as the Rockies. Flow over the mountains can also result in the development of lenticular clouds downwind as the air rises and sinks in waves produced as the wind moves over the peaks. You can see how rainfall affects climate across the United States at NOAA’s “The highs and lows of climate“.

If the air is diverted sideways around the mountain it can block the wind from hitting some locations downwind of the peak. Our fearless leader Linda told me she experienced this just a few weeks ago when a strong low-pressure center moved into the Northwest, bringing strong winds to the region. However, Linda noted that in her location, those winds were blocked by Mount Rainier, resulting in much lower local wind speeds due to the shelter from the massive mountain.

Wind flow up and down the mountains due to temperature variations

Mountains can heat or cool the air around them, depending on the time of day and the season. In summer, the peaks warm up and provide a heat source that helps lead to the formation of thunderstorms. You can see this almost every summer day in the western US with storms that develop over the mountains as the sun warms them. Those storms then move out over the prairies, leading to scattered rain or even virga, rain that evaporates before it falls to the ground. At night or in winter, the air near the peaks cools quickly and the denser air flows down the mountains into the valleys, resulting in katabatic winds that can cause freezes in low-lying areas when the coldest air reaches the lowest elevations. In this situation, the valley floors may experience frosts while areas on the slopes remain above freezing because the dense air drains through them relatively quickly. The winds can also increase evaporation rates, limiting the amount of available moisture and causing water stress on garden plants.

Temperature variation with elevation and orientation

The surrounding atmosphere also affects conditions on the mountain terrain. Atmospheric temperatures decrease with height, so as you go up in elevation on a mountain, the temperature will drop. This can lead to cooler climates and shorter growing seasons due to the increased likelihood of frost with the colder temperatures. This limits the types of plants that gardeners can grow because the climate of that location has limited suitability for plants that grow well on the flatlands.

Another aspect of mountainous terrain is the number of microclimates that are present in the rocky, uneven landscape. Mountaintops and sides offer a range of microclimates, from sunny, well-drained slopes to shady, cooler areas, influencing plant growth in different locations. North-facing slopes get little direct sunlight and can pool pockets of cold air that result in frosts every month of the year, while sunny south-facing slopes could be much warmer and more suitable for a variety of plants. Any mountain gardener has to be especially aware of the local microclimates in their area and account for them when choosing what and when to plant.

Of course, there are other characteristics of mountains that can also affect garden success. Soils are often shallow, rocky, and low in organic matter. Lower pressure and humidity may cause problems with plants’ ability to thrive in the harsher conditions. Sunlight can be very intense and shade from taller plants may be limited. Some areas may experience extensive periods of snow, which can provide insulation but can also damage plants when it slides downslope.

Gardening in the mountains can provide challenges for many gardeners due to the difficult environment that the mountain air provides, but it can also allow gardeners to create unique collections of alpine vegetation that can provide enjoyment for years to come, all set in a diverse and scenic landscape. Be sure to look for additional information on alpine or mountain gardening on the web to make sure your garden is well-suited to your local conditions.