Author: Pam Knox

I saw an article describing an atmospheric phenomenon called the “pneumonia front” this week and it made me start thinking about local kinds of wind and their names. No matter where you live, in the United States or elsewhere in the world, you have wind patterns that are related to your local geography. These winds can affect gardens, especially if they are persistent over time, but I enjoy hearing about the different names for wind too.

What causes the wind to blow?

Wind is the movement of air from one place to another. The air movement is driven by differences in air pressure from one place to another—the atmosphere tries to even out the pressure so air molecules are always moving from areas with higher density and pressure to areas with lower density and pressure. Since density and pressure are related to temperature (remember your ideal gas law from high school chemistry?) and temperature frequently changes as the sun moves across the sky or lakes and oceans warm and cool, the air is nearly always moving except where there is locally no variation in pressure such as the center of a high-pressure area.

Two common types of local winds

Winds are often linked to specific geographic features. For example, sea or lake breezes are located along the shores of large water bodies and are driven by pressure differences related to the relative temperatures of the land and water. When the water is colder than the land (for example, on a hot summer day), air pressure over the hot land is lower than over the cold water due to rising air over land (you can often see clouds where this is occurring). Air from over the water blows onshore in response to the lower pressure on land, leading to a cool breeze flowing over the hot land, cooling things off. At night when the land cools off more quickly than the water, the flow reverses and becomes a land breeze. Monsoons like the ones in India, the Southwest US, and other places are the largest-scale version of a sea breeze over thousands of miles and develop over weeks instead of hours.

Another geography-linked local wind is the katabatic wind. Katabatic winds are related to differences in elevation that cause temperature variations that result in density differences in the air. In a katabatic wind, air at upper elevations cools off at night, creating a pool of very dense air that rushes down the sides of the mountains to pool in the valleys, creating pockets of very cold air. Vineyard owners know this and plant vines on the sides of hills so that the vines are not exposed to the coldest air (and to take advantage of sunlight, too). The recent frost in New England caused severe losses of apple blossoms in the bottom of valleys while orchards in higher elevations were less affected. In your gardens, this occurs on a small scale with frost pockets that can form in the lowest-lying areas of your yards and garden plots. Antarctica has some of the strongest katabatic winds, with shallow winds that can reach up to 200 mph due to extreme temperature and elevation differences in that continent.

Other local wind names

There are many other location-specific winds and weather patterns linked to wind that occur in other parts of the world. Some are driven by elevation differences, with wind blowing through gaps in mountain ranges (the mistral in France and the tehuantepecer in Mexico, for example). Others blow in specific directions where mountains prevent air movement in some directions, funneling the air into channels that bring characteristic weather to the local area. In northeast Georgia, for example, we have frequent incursions of cold air from the northeast, with air pushed south due to high pressure in northern latitudes that is prevented from spreading to the west by the Southern Appalachian Mountains. We call that phenomenon “the Wedge” due to the shallow and dense wedge of surface air that is pushed by the wind flow into our region numerous times a year. Areas with very persistent topography-driven winds often have trees with most of their limbs on the downwind side of the trunk.

How does wind affect gardens?

Wind causes many effects on gardens. It can blow frigid air into a region from the poles towards the equator, leading to advective frost which causes damage to fruit blossoms in spring. If the humidity of the wind is low, it can quickly remove soil moisture and desiccate plants where irrigation is limited or unavailable. When strong, it can rustle leaves, break limbs, and even topple entire trees, especially where wet ground weakens the anchoring of tree roots. In fact, one measure of wind speed, the Beaufort Scale, uses an empirical scale related to the appearance of waves (on the sea) and tree movement (on land) to categorize wind strength. Some wind is a good thing for many plants because it provides stresses that help strengthen the stems and trunks, but too much can cause a lot of damage from wind-blown debris or direct force on the plants.

What local winds do you see and what impacts do they have on your gardens?

This blog has reached 194 different countries with many thousands of unique visitors a year, so the variety of local winds you experience must be amazing. Some of them are variations on the winds described above, either topography-driven winds like katabatic or anabatic (the opposite of katabatic, with up-valley winds during the day) or foehn winds. Others may develop due to unique geographic features of your area such as the Columbia River Gorge with winds so strong it is a haven for windsurfers. We’d love to see a comment on your local winds and how they affect your gardens!

I close by quoting the famous poem from Christina Rossetti that provided our title for this blog post, one of my favorites:

Who Has Seen the Wind?

Who has seen the wind?
Neither I nor you:
But when the leaves hang trembling,
The wind is passing through.

Who has seen the wind?
Neither you nor I:
But when the trees bow down their heads,
The wind is passing by.

This time of year, I often get asked for a forecast for the coming growing season. Will we have a drought? Will it be warmer or colder than normal? Will we have any tropical storms in our area? All of these things affect how farm crops (and gardens) will perform over the next few months and how big the yield might be when it comes time to harvest. In this week’s blog, I will look at some of the factors that go into seasonal forecasting and how it all comes down to numbers.

How are seasonal forecasts made?

Seasonal forecasts use computer models to look for large-scale patterns in weather that affect what the climate is likely to be on monthly to seasonal time scales, but also use statistical methods based on observations from previous years to determine what the most likely climate conditions are. The predictions are usually based on several factors, including current conditions, long-term trends, and other regional variations like El Niño Southern Oscillation (ENSO), which we’ve discussed before.

Current conditions are important because they define the starting point of the prediction. If you are starting from a drought, for example, you would not expect that next month would be much wetter than normal because the dry conditions would make it hard for rain clouds to form unless you have an unusual event like a tropical storm or atmospheric river that comes along and changes conditions on the ground quickly. Long-term trends are important because they define the base state of how the climate is behaving. If you are on an upward trend towards warmer temperatures, as we are now with global warming, it is more likely that you will observe a month or season that is above normal than below normal. If there were no trend in temperature, then seasons that were colder or warmer than average would be equally likely. You can find climate trends for your local area using the Climate at a Glance tool and picking your own country, state or county.

Knowledge about regional variations like ENSO also contributes to seasonal forecasts because the atmospheric and oceanic climate conditions which help define those oscillations can last for months or sometimes even years, making long-term prediction easier. The pattern of the atmospheric waves associated with those oscillations defines where warm and cold air are located as well as the position of the jet streams that blow storm systems around. In some respects it is like putting a rock into a river—when you have warm water in the Eastern Pacific Ocean as we do now in advance of the coming El Niño, thunderstorms develop vertically over that warm water, and those towers of rising air divert the flow of air currents that push weather systems around just as a rock diverts the flow of water in a stream. If the warm water were not there, the atmosphere would likely have a very different pattern.

Oirase Mountain Stream – Towada, Aomori, Japan, Daderot, Commons Wikimedia

Forecasts based on ENSO work best when it is at one extreme or the other and in areas where the differences between El Niño and La Niña conditions are most pronounced. That makes ENSO more useful for making seasonal forecasts in the Southeast United States and in northern states where statistical relationships are well-defined than in the central U.S., where there is a weaker statistical relationship between the ENSO phase and the local climate. Climatologists look at similarities between different El Niño years using statistics to identify recurring patterns that can help to predict the climate the next time an El Niño occurs, although other factors can come into play that throw off the forecasts.

How do seasonal forecast maps depict the future climate?

For the United States, seasonal forecasts are presented as maps with probabilities of above, near, or below normal temperature or precipitation. If there was no hint in any of the predictors of what would happen, each of the three categories would have equal weight, or a 33% chance of that climate category occurring. Those forecasts are based on current conditions, what they expect to happen in the months after the prediction made, the long-term trends that are pushing the temperatures or precipitation upward or downward, and the expected state of oscillations like ENSO. You can see the whole suite of 3-month forecasts for the next year for the United States at https://www.cpc.ncep.noaa.gov/products/predictions/90day/.

In the maps above, the outlook for December 2023 through February 2024 is shown. In the temperature map, the climate has a higher probability of being warmer than normal in the northern part of the country, especially the Northeast. This is consistent with the expected pattern of temperature in an El Niño winter. But the prediction of warmer than normal weather in the Southeast is not what we expect if we look only at El Niño. It also includes the effects of greenhouse warming, which is likely to overcome the cooler conditions that would be expected in the Southeast from El Niño alone. On the other hand, since there is not much long-term trend in precipitation, the precipitation outlook on the right shows a clear El Niño signal with wet conditions along the Gulf and South Atlantic coasts and dry conditions in the Northwest and to a lesser extent, in the Upper Great Lakes. In the Southeast, El Niño winters are expected to be cooler than normal because all the clouds associated with the rainy conditions keep daytime temperatures down.

What do we expect for this year?

Because the El Niño is coming on strong, I expect it to drive a lot of this year’s growing season climate. While it is still weak, it won’t have much impact on our regional climate variations, which makes summer difficult to predict. In that case, we look more towards the current conditions and trends to conclude that areas that are already experiencing drought are likely to get worse, and areas that are starting out wet may get a reprieve from drought conditions for a few months.

The biggest impact of an El Niño in the growing season is in how it affects developing Atlantic and Eastern Pacific hurricanes. In El Niño years, tropical systems in the Atlantic tend to be fewer in number and weaker because the strong jet stream aloft keeps tropical cyclone circulation from strengthening into tropical storms or hurricanes. In the Eastern Pacific, it is just the opposite, with more storms forming than usual. That can help feed moisture into the Southwest. Of course, it only takes one storm coming over your home and garden to cause tremendous damage, so you need to prepare for storms if you live in a hurricane-prone part of the United States or other Northern Hemisphere location.

By fall, the El Niño is expected to be well developed and areas where El Niño usually brings rain could see the wet conditions start earlier than usual. For farmers, that means harvest could be difficult if conditions are wetter than usual, and it might be hard to get that last cutting of hay. Winter could be drier than normal in the northern states since many of the rain-bearing storms will be farther south than usual, although you will certainly see some rain.

Last April 30, 2022, I wrote a post about what a third year of La Niña meant for gardens. La Niña, you might remember, is an atmosphere/ocean phenomenon driven by unusually cold water in the Eastern Pacific Ocean (EPO) that shifts the jet streams that steer storm systems around the world. In La Niña winters in North America, it is often linked statistically to colder and wetter than normal conditions in northern parts of the United States and north into Canada and warmer and drier conditions in the southern tier of states in the US. Just a couple of weeks ago, NOAA announced that the long La Niña has finally ended and that we are now in a neutral period. In last year’s post, I described the difference between La Niña and the opposite climate phase, El Niño, and how they affect climate conditions around the world. So now you might be wondering how this switch to neutral conditions and then potentially to an El Niño later in the year will affect your gardens in the coming growing season.

Elfen-Krokus (Crocus tommasinianus), AnRo0002, Commons Wikimedia

How good was last year’s forecast?

A discussion of how accurate the forecast for last winter was can be found in NOAA’s ENSO blog. As you can read, the temperature forecast was much better than the precipitation forecast (as it usually is). That is not surprising because many causes of rain and snow occur on very small spatial scales from a variety of physical processes that are not always well-captured in our current climate models. You can hear a more detailed discussion of historical ENSO patterns by David Zierden, the Florida State Climatologist, in this 15-minute video for the March 2023 Southeast Climate Monthly Webinar starting at minute 28:35).

As a result of the cold and snowy conditions in northern parts of the country, spring there has been delayed, and my friends in the Upper Midwest have only started to see daffodils and other early spring flowers, while in the Southeast, our azaleas and dogwoods are already in decline about a month earlier than usual (you can track this at the National Phenology Network website we’ve discussed before).

What happens next?

Now that La Niña has ended, we are in neutral conditions. That means it is difficult to predict the climate during the next few months because without La Niña (or El Niño) to give us statistical guidance on what climate to expect, almost anything can happen. The variations in climate in neutral seasons are caused by local variations, other interactions in climate on regional or global scales, and other factors that are not always well understood. In addition, we are in spring, which historically gives us the least trustworthy predictions for the coming seasons due to what is called the “spring predictability barrier.” That means that while we think we are likely to swing into an El Niño in the next few months, the atmosphere might have other ideas and could keep us in neutral conditions for quite a while before we switch to an El Niño.

This year, we are already getting signs in the Eastern Pacific Ocean that a switch to El Niño will come quickly. The water near the coast is already quite warm and the warm pool is starting to stretch out to the west. In other words, El Niño-like conditions are already present in the EPO, but have not lasted long enough yet for an official El Niño to be declared. That usually takes several months of monitoring to make sure this is not just a short-term change.

What do neutral and El Niño conditions mean for the Northern Hemisphere growing season?

Usually, ENSO conditions do not strongly affect the NH summer climate. That is because both La Niña and El Niño are strongest in the winter months and tend to weaken or disappear in the summer. Local conditions including soil moisture variations such as drought, ocean temperature variations, and local weather systems have a much bigger impact on growing season weather than ENSO does in most areas.

However, the ENSO state does have one strong influence. That is in the tropical activity in the Atlantic and Eastern Pacific Oceans. When neutral conditions or La Niña conditions occur, the jet stream aloft is weak and it is easier for tropical waves to develop into tropical storms and hurricanes, so neutral and La Niña seasons tend to be more active and have more named storms. When an El Niño occurs, the jet stream is unusually strong and that keeps tropical waves growing vertically into strong storms, so the number of tropical storms and hurricanes in the Atlantic is usually lower in years when an El Niño is occurring. Of course, it only takes one storm (Hurricane Andrew in 1982 was in an El Niño year) to cause tremendous damage if it hits somewhere vulnerable. In contrast, the storm activity in the EPO increases in El Niño years due to the pool of warm water there.

This year, the likely storm activity may be more tied than usual to the ENSO state. If we see a quick switch to El Niño after just a few months of neutral conditions, the Atlantic may be most active early in  the season, while storms later in the season will be suppressed. By comparison, in the western US where some moisture enters the country through EPO storm activity, you may see an increase in thunderstorms that are fed by the water vapor entering the country from the storms in the EPO.

What does this mean for your gardens?

If you live in an area that normally gets rain from Atlantic tropical activity, even if it is not from actual tropical cyclones or hurricanes, you can probably expect drier conditions this year, or at least more variable rainfall from less organized systems as more precipitation will be produced from local influences. El Niño does tend to delay the onset of the Southwest Monsoon, so that could be something to watch next year, although it may not have much impact this year. In Texas, the switch to neutral conditions means rain in April through June is more likely, which would be great for avoiding drought. If you live in an area that does not receive much moisture from the tropics, it will be hard to make a good forecast because the statistics just don’t give much guidance. We do know that globally, El Niño years tend to be very warm, so it is likely that 2023 may be one of the warmest, if not THE warmest, since global records began in 1880 due to the long-term rise in temperature from greenhouse warming. Warmer weather will mean a longer growing season, more hot days and nights, more humid conditions (unless a drought occurs), and more diseases and pests that thrive on the warmer conditions.

What do we expect next winter?

If you like to plan far ahead, you can see seasonal forecasts from NOAA’s Climate Prediction Center at Climate Prediction Center – Seasonal Outlook (noaa.gov). If you are not from the United States, your own country’s weather service may provide similar outlooks for your region. Here in the US, we are likely to see wetter and cooler conditions in the southern US as the jet stream steers storm systems over us (days will be cooler because of the clouds, but nights won’t necessarily be colder than usual since clouds trap nighttime heat). In northern states, warmer and drier conditions will be more likely, and that could mean an earlier start to the growing season next year. Now that something to look forward to if you are a gardener!

Tulipa “El Niño” 2015, Retired electrician, Commons Wikimedia.

If you’ve been following the national weather this week, you might be wondering if the groundhog has developed a split personality this year. Is winter over or are we in for six more weeks of cold? While the eastern half of the United States is feeling the effects of record-setting high temperatures and one of the earliest springs on record, the western U. S. is observing cold and snowy conditions all the way down into Southern California. I’ve heard reports of snow reaching all the way down to Tucson in southern Arizona.

What is an atmospheric wave?

Some of you may be wondering how we can have some tremendous differences between the eastern and western U. S. at the same time. The reason we see such differences is linked to the wavy pattern of the upper atmosphere, which helps direct warm or cold air into different regions. Today I will discuss atmospheric waves and how they affect the surface weather.

Since air is a fluid, it should not be surprising that it has waves in it. In fact, there are many different types of waves that can occur in the atmosphere, ranging from small-scale pressure waves downwind of mountain ranges to the largest planetary-scale oscillations in pressure. Sometimes you can see some of those smaller-scale waves in clouds occurring above you.

Where do atmospheric waves come from?

What ultimately drives the atmospheric circulation is the temperature difference between the equator which is warm due to direct sunlight and the poles which are cold because whatever little sun they get, especially in winter, is of low angle and there is not much solar energy reaching the surface to heat things up. This temperature difference causes a difference in atmospheric pressure that makes winds blow to try and equalize those differences.

Variations in land versus sea, the rotation of the planet, and differential heating from other causes like drought all contribute to the atmospheric developing waves that look like large swings in the pressure patterns at mid-levels in the atmosphere. Areas where the pressure is low are called troughs and areas where it is high are called ridges, like the patterns we see in topographic maps. Surface fronts are located near the boundary between troughs and ridges where the contrast in temperature and humidity is often the greatest.

How is the current wave pattern driving these big temperature swings?

This week there is a tremendous ridge of high pressure located in the eastern US while a very deep trough is located over the western part of the country. Warm air blowing up from the south under the ridge has brought record-setting high temperatures to many parts of the East, while cold air blowing in from the north behind a strong surface low pressure center has caused winter storm and blizzard warnings all the way south to the mountains near San Diego. Record-setting low temperature and wintry conditions are occurring in those areas.

The warm temperatures in the East are of special concern for gardeners since they have caused the first leaves and blossoms to occur as much as a month early in some locations, according to the National Phenology Network. Since the average frost date for those areas is a month or two later, the likelihood of damage to fruit crops is high since freezing temperatures could significantly reduce the production of fruit in gardens and orchards if they occur in the next month or two. It could also cause damage to many other flowering plants that are fooled into blooming early due to the unusual warmth.

Will atmospheric wave patterns change in a warmer world?

Is the unusual warmth due to global warming? This is a hotly debated topic (if you will excuse the pun) with both proponents and skeptics weighing in. Many scientists believe that as the arctic regions warm up faster than the equator (a process known as “Arctic amplification”), the atmospheric wave patterns will become more amplified, with both deeper troughs and stronger ridges occurring. That could mean more extremes in both hot and cold weather as these waves occur. Other scientists caution, however, that the computer models used to study the large-scale wave patterns don’t always agree with global warming as a root cause. If we do have generally warmer conditions that are punctuated by significant cold outflows our gardens will have to be able to survive the increasing variability of the weather, especially in winter and spring when there is still a significant pool of cold, dry air near the poles available to flow south into mid-latitudes.

Please feel free to share how this crazy weather has impacted your own garden. Are your trees blooming early, or has snow covered everything? If you are not in the United States, what do you see going on in your own neighborhood? We are interested to know!

If you have fruit trees in your garden, then you may already be aware of the importance of winter cold for the development of blooms and fruit. Some fruit varieties like apples, cherries, peaches, and blueberries all need a certain number of hours of temperatures below 45 F to prepare the trees to bloom in the spring. Surprisingly, other trees you might not think of as fruit trees also need a period of cold conditions to produce a good crop, including pecans and olives. Many other garden plants also require cold periods to provide the best blooms, such as lilac and hydrangea. I miss seeing lilacs here in the Southeast because we just don’t get cold enough to meet their chill requirements, although new varieties that can flourish in warmer conditions are being developed.

Why do some plants need winter cold?

The cold conditions over winter cause the plants to go into dormancy. That helps protect the plants from harsh conditions over the winter. Once the plants experience the number of chill hours required for that plant, they are ready to begin the blooming process once the temperature warms up. If the fruit trees do not get enough hours of cold over the winter, they do not bloom well in the spring and blossoms may be delayed or bloom at irregular times or not bloom at all. Leaf emergence may also be affected. The potential yield of fruit is reduced due to the lack of a strong uniform bloom.

Scientists count the number of hours a tree or orchard is below 45 F and measure the accumulation of those hours as “chill hours.” Some publications call them “chilling hours” instead. They are usually accumulated starting around October 1 and go through the winter into the spring bloom period when bud break occurs. An alternate method for calculation only counts the hours between 45 and 32 F. Chill hours are different from cold hardiness, which is the lowest temperature that a tree or plant can tolerate without dying. Recent research has allowed scientists to develop more sophisticated methods that dynamically calculate “chill portions”—these calculators take into account variations in temperature over time and can reset the calculation if cold conditions return after a warm spell.

Picking the right fruit variety for your garden

Most types of fruit other than citrus and each variety has a preferred minimum number of chill hours for that variety to set a good fruit crop. If you live in the Southeast, you may be able to plant varieties of blueberries that require only a few hundred chill hours, where in the north you may be able to plant a more cold-hardy variety that requires over 1,000 chill hours for the best bloom. Because of the cold outbreak we had this December, some of our blueberry varieties are already blooming this year in the warmer weather we have had in January.

If you are planning to plant a fruit tree in your garden, you will want to pick a variety that matches the expected number of chill hours for your area. If you plant a variety that expects a lot of chill hours but your location gets only half that amount, you will not get very good yields from your trees since the plants won’t break dormancy correctly. But if you pick a variety that requires a low number of chill hours for your area, then the tree will be ready to bloom as soon as the chill hour requirement is met, which could put them at risk for a frost if they bloom too soon. So picking the right variety for your area is crucial! Most nurseries can provide the recommended number of chill hours for the fruit variety you pick.

In the future, as temperatures get warmer under the influence of greenhouse warming, most areas will see a decrease in the average accumulation of chill hours over time. We are seeing this at my weather network stations in Georgia and in many other states as well. If you plant something that has a long lifetime, you may want to plant varieties that require fewer chill hours than your current climatological average to make sure they will thrive in a warmer climate in the future.

Where can you find information about chill hours for your location?

For average numbers of chill hours for the United States, check out the map below. Keep in mind that your local microclimate will affect the number of chill hours your garden will receive.

Here are some sources of current chill hours for this year:

The Georgia Weather Network has a tabulation of stations with current accumulations since October 1 at https://weather.uga.edu/aemn/cgi-bin/AEMN.pl?site=AAAA&report=ci. Many other state agricultural weather networks provide similar information. You can find a list of many of them listed under the Partners tab at the National Mesonet Program website.

The Midwestern Regional Climate Center has a current map for the contiguous U.S. similar to the at https://mrcc.purdue.edu/VIP/indexChillHours.html. An interactive version based on GIS maps is at https://mrcc.purdue.edu/gismaps/vipstndata.htm.

AgroClimate has a chill hour calculator for Florida and Georgia at https://agroclimate.org/tools/Chill-Hours-Calculator/ which allows you to choose the chill hour model and the time period of interest.

Don’t forget to check out our archives!

If you are interested in this topic or have other questions about gardens and gardening, we encourage you to explore our archive of blog posts to see if you can find the answer to your questions. Here are a few of the previous stories about winter chill you might find interesting:

Chill hours and bud break in Christmas trees:

https://gardenprofessors.com/and-now-for-something-completely-different/

https://gardenprofessors.com/the-walking-dead-christmas-tree-edition/

Chill hours and lack of flowering in landscape plants like lilac:

https://gardenprofessors.com/why-doesnt-my-plant-flower-part-1/

As we close out 2022, I thought I would spend a few minutes reviewing the weather and climate of the past year, both the average conditions and some of the extremes we saw. While this is skewed towards the United States, I did include some events happening in other parts of the world for our non-US readers.

What were the average climate conditions in 2022?

Since the year is not quite over, I can’t provide a final average for temperature or precipitation for the complete 365 days, but there are some websites that allow me to look at all but the last few days. The images below are from the High Plains Regional Climate Center for January 1 through December 28. They show the temperature departure from normal and the percent of normal precipitation for the continental United States. (You can see the global temperature statistics for January through November 2022 at the National Centers for Environmental Information.) In most parts of the U.S., the temperature was warmer than the 1991-2020 normal; the exception was the north central part of the country, where temperatures were colder than normal. This pattern is consistent with the La Niña that we have been experiencing for most of the year, although individual months did vary.

Precipitation was more variable, as the map of precipitation percent of normal below shows. The driest areas in California and the Central Plains are consistent with the extensive droughts that covered those parts of the country throughout the year. The eastern Coastal Plain also shows overall drier than normal conditions for the year as a whole but the timing of wet summer and dry fall caused a lot of problems for the farmers there. The wettest areas were New Mexico and Arizona (due to a vigorous monsoon), the Southern Appalachians and Mississippi, and the Florida Peninsula due to the heavy rain associated with Hurricane Ian.

What extremes did we see in 2022?

The averages show the overall conditions that occurred this year, but don’t begin to capture the extremes in temperature and precipitation that occurred. These extremes get washed out in the averaging process but are far more likely to cause serious impacts than deviations from normal conditions over the whole year. These extremes caused 15 billion-dollar disasters in the U. S. alone as of October 11, and I certainly expect that extreme events since then, including last week’s cold outbreak and snowstorm, will add to that number.

In 2022, we experienced a number of heat waves with record-setting temperatures, including unusual warmth in South America, Europe, Asia, and Australia, where their highest temperature ever recorded (50.7 C or 123.3 F) was tied in January, their peak summer month. The United Kingdom experienced their hottest year ever, including temperatures in excess of 40 C for the first time. Much of Asia was also very hot in 2022. In the United States, the Pacific Northwest saw heat waves in both August and October, with the Southwest experiencing blistering heat in September and the Northeast in August. By comparison, cold outbreaks occurred both in January and early February and in December, with an Arctic outbreak spreading southeast from Alaska down to southern Florida, bringing extensive freezing conditions that caused significant damage to citrus in Florida and Georgia, bursting water pipes, and a lake effect blizzard in Buffalo NY that eclipsed their previous record-setting snow event set just a month earlier.

Precipitation was just as variable, with floods and droughts occurring around the world this year. Some of the more notable flood events include the rainfall in Pakistan in August that put a third of their country underwater, the floods that destroyed the northern entry to Yellowstone Park in June, and the local flash floods that occurred in eastern Kentucky in summer and fall, and the southwest monsoon that began in June with precipitation 200-800% of normal, easing drought in that area. At the very end of the year, as I am writing this, an atmospheric river event on the West Coast is bringing heavy rain to areas of California that have been plagued by drought all year. That may provide some relief from the dry conditions going into 2023.

Many other areas of the world experienced significant droughts in 2022. They include an extreme drought that occurred over most of Europe, causing damage to many crops and limiting navigation on local rivers. This was also true in the United States, where the long-lasting drought in the central United States led to record low levels on the Mississippi River, stopping barge traffic that usually transports grain from the Midwest down to the Gulf of Mexico. Drought covered over half of the United States for many months in 2022, although it waxed and waned in some areas with the movement of rain-producing systems.

How about the tropics?

While La Niña usually means that the Atlantic tropical season is active, this year was oddly quiet between early July and the end of August, with no storms observed during this period for only the third time since 1950. But once the season resumed, we saw Hurricane Fiona (affecting Puerto Rico and Nova Scotia, although it stayed offshore for the continental eastern U. S.) and Hurricane Ian in September. Ian caused tremendous death and destruction to southwestern Florida as it crossed over the peninsula, dropping feet of rain before it moved into the Atlantic Ocean and then recurved west into South Carolina as a weakened storm. In November, Hurricane Nicole brought devastating coastal flooding to areas that were previously affected by Ian, although it caused less damage than Ian did. Damage from tropical systems was not confined to the tropics, however, as the remains of Typhoon Merbok hit the west coast of Alaska in September, causing significant coastal damage with its incredibly strong winds.

What does the past year teach gardeners?

Most of the United States as well as the rest of the world experienced a warmer climate in 2022, so gardeners will continue to need to choose plants that are appropriate for their warming climate zones. But they will also need to prepare for extreme conditions; devastation by individual storms as well as climate variability will continue to affect home gardens through water stress caused by drought and extreme heat as well as damage caused by floods, high winds, and freezing temperatures. Building a resilient garden that can withstand these extremes will allow your garden to thrive through whatever conditions the atmosphere throws at it.

Thank you for another great year!

Finally, I want to end this year by thanking you all again for your loyal readership and your thoughtful questions and comments on many topics. I encourage you to share your 2022 garden challenges (weather or otherwise) in the comments along with your plans for how you plan to address them in 2023 and beyond. I look forward to reading them! We will see you again in the New Year.

If you follow current weather news, you have likely read the astounding story of the recent lake effect snowfall in Buffalo, New York, and other areas downwind of the Great Lakes, where over 6 feet of snow fell in just a day or two in some locations. My mom, who still lives in Grand Rapids, Michigan where I grew up, reported that in her city some areas got up to 30 inches during the same time frame. So this month I want to discuss lake effect snows and how heavy snows can affect your trees and gardens.

What is lake effect snow?

Lake effect snow is snow that is caused or enhanced by differences in the temperature of warm water in the lakes and the cold Arctic air that blows over it. Calling it “lake effect” is a bit of a misnomer, since cold, dry air blowing over a warmer ocean can cause the same effect. In the United States, it most often occurs downwind of the Great Lakes, especially in fall when the lakes are still warm and the air blowing in from the north is much colder and drier than the lake surface. It can even sometimes occur downwind of smaller lakes or reservoirs if the conditions are just right.

As the cold dry air crosses the warm water, copious amounts of water vapor evaporate into the air mass and once that warmer, moister air blows onshore again clouds drop huge amounts of snow in the areas downwind of the lakes. The snow usually falls in heavy bands that drop snow in areas that are highly dependent on the direction of the wind. Often the bands are just a few miles wide but if you drive through one your visibility can drop to near zero in just a short distance. When I lived in Valparaiso, IN, near the south end of Lake Michigan, winds blew straight from the north for much of the month of December 2000 dropping 32.0 inches of snow when Decembers there usually get just a few inches, thanks to the lake effect snow that occurred. (I moved to Georgia the next month, although it was not because of the snow—mostly.) As winter progresses and the lakes get colder with more ice cover, lake effect snow is reduced because of the decrease in available water vapor so fall and early winter are the prime times of year for the heaviest lake effect snow.

In this case, weather forecasters were well aware of the potential for record-breaking snow because of their knowledge of the lake temperatures plus the computer-generated forecasts of wind direction and persistence over time. Winter storm warnings and maps of predicted snowfall were produced well ahead of time. Even so, the amount of snow that was produced from this historic event is still amazing.

Source: Carolyn Thompson / AP Photo

Why did Buffalo experience such extreme snowfall amounts?

Buffalo is known for its incredible snowfalls due to its position downwind of Lake Erie, a long and shallow lake that is usually warm well into fall. The long distance of the wind blowing over the lake (called the “fetch”) allows the air to pick up tremendous amounts of water that becomes snow as it hits the land NE of the lake; the exact location of heaviest snow depends on the direction of the wind over the lake (see my poorly drawn map annotated on a screen capture of the Earth Nullschool streamline map for the day of the heaviest snowfall below). In this month’s case, the lake had temperatures well above the long-term average, and the wind across the lake was very consistent over a few days, allowing the snow to pile up dramatically. In some locations snow was falling at the rate of several inches an hour and the extended period of snowfall allowed it to build up to over six feet in some locations in just a day or two, while other areas not along the direct path of the wind received much less. The area of heaviest snowfall shifted as the winds changed direction over time.

The result of this weather event was the nearly complete shutdown of Buffalo and other areas affected by the heavy snow. Even a city that experiences as much annual snow as Buffalo does can be stopped in its tracks for a while by the sheer volume of snow that has to be removed. The weight of the snow also caused problems for a number of building roofs and caused some power outages as well. Even the professional football game between the Buffalo Bills and the Detroit Lions had to be moved from Buffalo to Detroit because of the impossibility of clearing out the open-air stadium and the roads around it for fans to get there safely (or at all).

Does climate change affect lake effect snow?

A warming climate does have some impact on the conditions that make lake effect snowfalls most likely. The lakes are generally staying warmer later into the fall, so when cold continental air does develop over Canada and move across the lakes there is more potential for large amounts of water vapor to be evaporated, increasing the chance of heavy snow. It is likely that there may be some reduction in the production of the coldest, driest air in polar regions, but it will still occur often enough for lake effect snow to continue to be a climate factor downwind of the lakes. It is more difficult to say how or if the weather patterns that determine the direction of wind flow will change as the climate gets warmer.

Source: Dan Taylor-Watt, Commons Wikimedia

How does heavy snow affect trees and gardens?

Lake effect snow is often very wet and heavy which makes damage to trees and power lines more likely. An average snowfall may have about one inch of water equivalent in ten inches of snow, but in a lake effect snow it is often more like six inches of snow to one inch of water equivalent which means it is very dense stuff to shovel. Wet snow may weigh up to four times as much as newly fallen regular snow per square foot. No wonder most heart attacks from clearing snow occur when this very wet and heavy snow has fallen.

The weight of this much snow can easily collapse the roofs of buildings. It can also do a lot of damage to tree limbs and shrubs, especially when the wet snow sticks and freezes to either needles or leaves adding to the weight on the limbs. Trees and bush varieties that are brittle or have poor branching structure are especially vulnerable to damage from heavy snow. Snow on the ground can help insulate the plants from very cold weather, but the moisture that is left after the snow melts can cause saturated soils that can negatively impact roots. Salt added to help melt the snow from paved surfaces can also harm plants and the deep snow cover in some lake effect storms can also provide cover for voles and other critters that like to nibble on bark.

For me, lake effect snow in my Michigan winter when I was growing up was the ultimate fluffy Christmas snow, with big fat flakes drifting down like a picture postcard. But when the flakes come down fast and heavy the holiday snow becomes a problem that can affect travelers, home owners, and gardeners too. I hope that as you travel over the holidays this winter, the snow that you see, whether you stay or go, is a delight and not an obstacle to spending time with your friends and family.

Here in the Southeast we were surprised last week by a much earlier than usual freeze, putting an end to many gardens full of tender plants, although the cold was not deep enough to kill more cold-hardy species. In many parts of the region the frost came earlier than the 10% probability of frost indicating that early freezing conditions like this will come in fewer than one in ten years. Of course many of you in more northern interior parts of the United States have already seen your first frost this year, but here we never seem to be ready for it. In fact in parts of southwest Georgia last year’s first fall frost did not turn out to be until well into January, which caused a lot of problems for gardeners and farmers who had to deal with pests and diseases that easily overwintered the mild conditions.

Frost versus freeze

One of the questions I often get this time of year is what is the difference between frost and freeze? The National Weather Service (NWS) puts out both frost and freeze warnings but has different criteria for each. For a frost warning, the predicted temperature may not even get down to 32 F (0 C), but may hover in the mid 30’s. For a freeze warning the predicted temperature is expected to get down to 32 F or below and for a hard or killing freeze it usually gets down to 28 F or lower. Once the area has gotten down to 28 F or lower, the NWS usually stops issuing additional freeze warnings since at that point all but the most cold-hardy plants have completed their growing season and are either dead or dormant.

How does frost form if the temperature does not get down to freezing?

To understand frost formation and when warnings are issued it helps to know both how frost forms and how temperatures vary near the ground. Frost crystals form on surfaces that get down to freezing and have something on the surface that is conducive to seeding crystal formation. This can happen even when the air temperature is above freezing in conditions of light wind and clear skies that allow surfaces to cool to freezing temperatures by emitting heat radiation out to space at night when there is no incoming solar radiation. Conditions for this can occur with temperatures anywhere in the 30s with a reasonable amount of water vapor in the air and as long as the surface (a metal car body, an asphalt roof, or a blade of grass) can cool to the freezing point. At that point, anywhere on that surface that has an appropriate scratch, particle, or other imperfection can serve as a place for ice crystals to form and start to grow. These are called nucleation sites and allow the initial formation of an ice crystal upon which more ice can grow into delicate but visible frost.

Frost will not form if the humidity is too low because there is not enough moisture to produce visible crystals. Often frost does not damage the plants a lot because most of the frozen water is confined to the surface of the plant and does not affect the interior cell walls, although there may certainly be some damage where the ice forms. Large formations of ice crystals can sometimes form on trees or fences if the conditions are right; this is called hoar frost.

Frost forecasts are also provided with the understanding that the NWS is forecasting temperature values for their thermometer heights of about 2 meters or 6 feet high, since that is how they verify the accuracy of their forecasts. In light winds and clear skies the temperature at the ground level is often colder than the temperature at the thermometer height due to cold air sinking so the ground in your garden may be colder than the forecast would predict. Frost is also more likely to form on elevated surfaces that don’t have contact with the ground, since soil temperature keeps the ground surface warmer in Fall than later in the year due to residual heat from the summer warmth. Blueberry farmers that I work with tell me that you can sometimes see quite a difference in frost damage to their bushes from top to bottom due to the different temperatures that the plant may experience at different heights above the ground. Bridges often have signs that they freeze first for the same reason.

Freeze damage to garden plants

The NWS issues freeze forecasts when the temperature is expected to get down to or below 32 F. The damage that the freeze does to plants depends on how long the temperature drops below freezing and how susceptible the plant is to cold temperatures. If the temperature barely gets down to 32 F for a short period damage is likely to be minimal since the water inside the plant cells did not have sufficient time to freeze. But if it lasts longer the water in the cells freezes and, as you undoubtedly know, ice expands and breaks the cell walls causing irreversible damage to plant leaves and stems that leads to their death. John Porter provided a useful table of how different garden vegetables respond to cold temperatures in his 2020 blog on spring frosts, which underlines why some vegetables like spinach and cauliflower do better as late-season vegetables than tomatoes and melons.

The discerning reader who looks at John’s article will also note some differences between the first frost map he published in his blog and the map above, because they cover different time periods. John used the map for 1980-81 to 2009-2010, since that was the current one at the time of his post. The map here uses the 1990-91 to 2019-2020 period since the normal temperatures have been updated since John’s blog was published. Average frost dates change over time as you can see especially in some areas like eastern Oregon and northern New York State and generally, as the earth gets warmer, the first frost of fall is occurring later in the year than it did in the past (although there are a few exceptions such as parts of northern Georgia).

With winter on the way, we are sure to see many more examples of frosts and freezes in the coming weeks for almost everyone other than those who live in tropical areas. For those of us who enjoy chilly weather, the magic of frosts and freezes is something we look forward to as it paints our dying gardens in icy white.

In my last blog post in late August, the Atlantic tropical season was just beginning to wake from a long nap, with several areas of interest appearing on the National Hurricane Center’s (NHC) map. Since that time, the season has become incredibly active, with Hurricanes Fiona and Ian causing tremendous damage in North America. Other parts of the world have also seen damaging storms, including Hurricane Kay in the Eastern Pacific, post-typhoon Merbok in Alaska, and Typhoons Hinnamnor in South Korea, Nanmadol in Japan, and Noru in the Philippines. So, with apologies to those who live far from hurricane parts of the world, I want to talk one more time about tropical systems.

Where do we stand with the tropical season now?

As I am writing this on Thursday morning, September 29, I hear the sound of Ian’s wind in the tulip poplar trees outside my house in Athens, GA. Ian is still in central Florida, just about to come off the coast into the Atlantic Ocean, so that gives you an idea of how far the influence of a tropical storm can spread, especially with a strong high-pressure center to our north that is increasing the pressure gradient driving the winds. Ian made landfall yesterday afternoon near Fort Myers, Florida, as an almost-category 5 storm, with winds of 150 mph (some reports say 155 mph). The videos I saw yesterday showed the power of the storm, with tremendous wind gusts and a storm surge that surpassed 10 feet in some places. I know the damage is horrific, and some of those areas will never recover completely from the storm, as buildings have been washed away and even shorelines may have changed due to the force of the wind and water. Since Ian is expected to curve back toward the East Coast on Friday and may strengthen again, its effects are not over yet. Fortunately, a weaker storm has lower impacts, but folks along the Georgia and South Carolina coasts will be feeling those impacts in the next few days before Ian moves out of the area and dissipates.

Hurricane Fiona lashed Puerto Rico and the Dominican Republic with rain of up to two feet in some places around September 18-19 before moving rapidly to the north and slamming into Nova Scotia as a post-tropical cyclone on September 24. It caused tremendous damage in both places from storm surge, wind, and rains. The floods in Puerto Rico destroyed a lot of local farms and gardens in the southern half of the islands where the rain was heaviest and in doing so, eliminated an important source of locally produced food as well as disabling a fragile power grid that had not yet recovered from Hurricane Maria in 2017. The storms in other parts of the world have had similarly bad effects on the lands over which they moved, with loss of trees and buildings and high storm surges wiping out coastal infrastructure. Not all hurricane impacts are bad, however, since the rain from Hurricane Kay in southern California helped reduce drought conditions there in a time when not much rain usually falls in that part of the world.

Predicting the tracks of hurricanes

One of the questions that have arisen with Hurricane Ian has been the prediction of where the hurricane would go. Predicting the track of a hurricane is an art that includes the use of multiple computer models that simulate conditions over the life of the storm. That includes sea surface temperature, vertical atmospheric structure, and the surrounding wind field which will push the storm around. On the news you will often see maps that show all the individual model results on one map, which ends up looking like a mass of spaghetti noodles, hence the term “spaghetti models”.

Forecasters look at all the individual model tracks together to see how consistent they are with each other and where the differences lie. Then the human forecasters use their knowledge of how well those models behave under different weather conditions to create a “forecast cone” that shows the region where the center of the storm is likely to go.

No one model is right all the time because they weigh different weather factors differently. In the case of Ian, the models run by European weather services did better, but that is not always the case. Generally, they say that 2/3^rd of the time, the central low pressure will stay within the predicted cone, although the storm itself is usually much larger than the cone and hazards like high wind, heavy rain, tornadoes, and storm surge can and do occur far outside the cone. If there is a lot of spread in the models, then the forecast cone is wider, indicating that they are less certain about where the storm will go.

The models are run every six hours or so, and each time the cone is updated to include model results that include new weather data observed since the last forecast was issued. As this happens, people that are in or near the cone must respond to the forecast by deciding whether to evacuate or stay in place and where to go if they do leave, since they don’t want to evacuate to a location that could be hit by the storm if the cone shifts. When the forecast is especially tricky, as it was with Ian, the movement of the cone over time can become overwhelming to people who just want to find a place they will be safe. The forecasts of where the storm is likely to travel are improving over time, but the tracks will never be 100% accurate because the atmosphere is a complicated place that we can’t simulate perfectly using even the best computers, so confusion is likely to continue to occur in future storms.

Dealing with flooded gardens

Since this is a blog about gardening, I want to end up mentioning what impacts these storms have on gardens. Coastal areas where there is a storm surge will see inches to feet of seawater flow over their land. The water contains salt but can also contain toxic chemicals from boats and tanks that are damaged by floating debris or strong waves. The salt and chemicals can kill garden plants but also may get deposited in the soil as the water sinks in, leaving toxic residue behind. The physical motion of the water on and off the land can also scour the topsoil and change the soil structure or deposit sand on top. Saturated soils can drown the plants by keeping oxygen from reaching the roots of plants. And of course, the howling winds can snap the plants, bushes, and trees above the ground, leading to damage that can be taken advantage of by pests and diseases. In areas where there is heavy rain and freshwater flooding, salt is not usually a problem, but all the other problems with too much water can occur there, too. For those who live where storm damage is heaviest, helping their gardens to recover will be a long process even if their houses survive the storm.

The Atlantic tropical season is not over yet for us in the Southeast, but I know that in other parts of the United States and the world, the seasons march on, so in the next months I will move on to talk about fall frosts and the upcoming winter. Thanks for bearing with me as I explore tropical storm systems. Please keep all of those affected by our storms this year in your thoughts and prayers as they work to recover from damage and disaster.

In my last blog post in early August, I noted how quiet the Atlantic tropical season has been so far this year. In fact, the period from early July through this week has been one of the quietest on record, with no named storms since the short-lived Tropical Storm Colin formed along the South Carolina coast and dissipated less than 24 hours later on July 3 in eastern North Carolina. The last time we had so few named storms was 40 years ago, so while it is not unprecedented, it is certainly unusual. And we are definitely later than the average date for the first hurricane of the year. By comparison, in 1992, a strong El Niño year, Hurricane Andrew (an “A” storm, so the first of the year) had formed and taken its devastating track through southern Florida and Louisiana by this date.

All of that is about to change, and hurricane forecasters are relieved after predicting a season of above-normal activity based on warm ocean temperatures and the current strong La Niña. They could still be correct. The National Hurricane Center’s 5-day map (as of 8-28-2022) is now showing four areas of potential development, with one area that has a 50% chance of development into a tropical depression within the next 5 days. Just in time for the peak of the season, according to the timeline we discussed earlier this month.

Why have the tropics been so quiet?

What caused the very quiet period in July and August? Hurricane climatologists point to several factors: the continuing clouds of dust that have blown off Africa and across the Atlantic Ocean towards the west, dry air moving in from Europe, which is experiencing its worst drought in 500 years, and the lack of strong waves moving off of Africa to act as seeds for tropical storm development. But the presence of warm sea surface temperatures and the lack of a strong jet stream (which is consistent with the presence of the La Niña) were expected to contribute to a stronger season than we have seen so far. If we can’t understand why this season has been so quiet so far, it means we still have a lot to learn about hurricane climatology and behavior.

The second half of the 2022 season is likely to be a lot more active than the first half, although forecasters have dropped the predicted number of storms from the early forecast due to the past two quiet months. If you live in an area affected by Atlantic hurricanes, you should be prepared for a more active pattern—don’t let the last two months fool you! If you live in another part of the world that is affected by tropical storms, you should also understand their climatology and likely impacts on where you are as well.

Some resources for following hurricane weather

For those of you who are fascinated by tropical storms and hurricanes, even if you don’t live in an area that is prone to them, there are a few resources that you can use to track potential storms and follow them as they develop and move through areas that could be severely impacted by them. The first site I use is the National Hurricane Center, the source of official forecasts and outlooks for the season as well as specific storms as they form. Their website has a lot of information about past storms as well as educational resources on tropical systems. You can also find a lot of maps and climatological information at Mike’s Weather Page if you just need a quick look at maps and other images related to tropical weather in the Atlantic and Pacific Basins.

On social media, I follow Bryan Norcross and Brian McNoldy on Facebook and Twitter; they may be on other social media as well. Bryan Norcross is the television meteorologist who was working in Miami at the time of Hurricane Andrew; it has been fascinating this week to follow his timeline of Andrew from tiny disturbance to monster storm as it hit Miami and then Louisiana over the past week back in 1992, thirty years ago. Brian McNoldy is a senior hurricane researcher who works at the University of Miami and has done some interesting climatological work on past hurricanes as well as provides insight on the current season. There are also plenty of great local resources for local impacts if you live in a hurricane-prone area.

How to visualize the wind

If you are interested in looking at the wind patterns associated with storms, both tropical and extra-tropical, then there are three sources of fascinating maps that allow you to visualize the flow of air across the United States or the world:

United States current surface winds Hint.fm/wind. This site has a current map of the surface winds across the continental United States showing the wind speed and direction in motion. It is based on a near-real-time computer simulation to provide seamless coverage across the country.

Global earth interactive wind map https://earth.nullschool.net/. This interactive map allows you to look at current winds anywhere on the earth by dragging and zooming on the globe. You can use the menu on the lower left to pick higher levels in the atmosphere; this will allow you to look at jet streams aloft as well as surface winds.

Windy global current and forecast winds https://www.windy.com/. This site provides global current and forecast winds as well as other weather information that will allow you to view the weather and plan for future weather conditions at home or away.

These sites provide you with information about both wind speed and direction. That can be very useful for gardeners who are spraying or need wind information to track where the air hitting their gardens has come from. Wind drift of agricultural chemicals also causes damage to crops and outdoor workers. Exposure to chemicals such as weed killers can affect gardens adversely, and it can be important to know where those chemicals are coming from. If you don’t have access to local wind observations, these maps can provide you with useful information.