Author: Pam Knox

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.

In the Southeast, almost nothing gets more press than the Atlantic Tropical Season, including the outlooks, real-time events, and post-storm analyses. If you live in another part of the United States or the world, you may not be as directly affected as the Southeast is, but you might be surprised at how far and wide tropical moisture is spread, either directly by tropical storms and hurricanes or by the remnant moisture which can be carried with the winds for long distances away from their original sources. This week I will describe what tropical systems are and what they mean for gardeners. In my next post, I will review the current season, including any impacts that have occurred. Don’t let the early quiet conditions fool you—about 90% of Atlantic tropical activity occurs from mid-August through mid-October.

What is a tropical system?

I am using “tropical system” as a term that encompasses the life cycle of a tropical disturbance from birth to death. That may include formation by a tropical wave coming off Africa, developing in the Gulf of Mexico, or growing in the Eastern Pacific Ocean, then a tropical storm, a hurricane (maybe even a major one), and eventually a slow death as a tropical depression or extra-tropical low that has lost its tropical characteristics.

The map below shows historical tracks for all known storms across the globe. Note that in some regions they are called hurricanes and others, typhoons, but both are tropical cyclones, the generic name for rotating, organized systems of thunderstorms that originate over tropical waters and have closed, low-level circulations. The pattern of tracks shows some interesting information about tropical systems. For example, why are there almost no tracks in the eastern part of the South Pacific Ocean or in the southern Atlantic? Why do many of the tracks show a curve as the storm moves from east to west? Why are there more storms in the Northern Hemisphere (NH) than in the Southern (SH)? Why are there almost no tracks along the equator?

What ingredients are needed for tropical systems to develop?

Tropical systems form from areas of low pressure that develop rotation (counterclockwise in the NH and clockwise in the SH) as the storms’ pressure decreases. For the storm to strengthen, it needs favorable conditions to cause rising air in the center, which drops the pressure at the surface. One ingredient is ocean surface temperature of 80 F or higher (27 C)—that explains why almost no storms develop in the southeastern Pacific or southern Atlantic, since both are far too cold. Tropical storms and hurricanes are considered to be “warm core” systems with the air in the center of the storm staying warm all the way to the top of the circulation. The peak season of storms in the Atlantic is related to the cycle of ocean temperatures, with the highest likelihood of storms in the period from mid-August to mid-October. As ocean temperatures warm, this could mean a shift in hurricane season in the future.

For spin to develop, you need a force called the Coriolis force that affects the atmosphere due to the earth’s rotation around its axis. The Coriolis force is zero at the equator so any areas of low pressure that form in that area can’t develop the necessary spin to form storms. Another ingredient is light winds higher up in the atmosphere. This allows the vertical structure of the storm to develop a strong circulation that ultimately becomes a hurricane. This becomes important when we talk about the impacts of El Nino and La Nina on predictions of tropical seasons, since El Nino years have much stronger jet streams in tropical regions than La Nina or neutral conditions do.

The curvature of the tracks is due to the mid-level winds in the atmosphere that steer the storms as they go through their life cycles. The counterclockwise flow of air around ridges of high pressure systems push the storms along their edges. This often results in a C-shaped pattern as the storm travels around the western edge of the oceanic high pressure, although the position and strength of the high pressure will help determine the path each storm takes and when or if it recurves to the northeast.

Impacts of tropical systems

Some impacts of tropical systems are only found near the center of the circulation, but others can be found hundreds of miles away, so even if you are not in the main area that tropical storms affect, you are not without risk. These storms are not small whirls like tornadoes, but are much bigger and can take hours to cross a location near the center. If you are close to the storm’s center, especially if you are on the right side of the storm’s path, you are likely to experience strong winds, heavy rain, and, if you are near the coast, the chance of a storm surge coming inland from the ocean. Farther away, you can experience strong squalls that include small tornadoes, heavy rains, and gusty winds. These effects are made worse if you live in mountainous areas where lifted air can cause rapid flooding conditions. Some of the worst floods in U. S. history are from former hurricanes that traveled over mountains and dropped incredible amounts of rain, such as Agnes, which caused the death of 122 people mostly in Pennsylvania, almost exactly 50 years ago.

Even after a storm weakens to a depression or transforms into an extra-tropical storm, the blob of moisture within the remains of the storm can be transported by the atmospheric circulation a long ways. There have been records of typhoons in the Western Pacific Ocean whose watery remains crossed the ocean and brought heavy rain to the West Coast. Occasionally some will enter the central United States and drop flooding rain there, too, such as the remains of Tropical Storm Erin in 2007.

What gardeners (and everyone else) should know

Preparing a garden for a hurricane is no different than preparing for other types of extreme weather. Survey your property before the season to make sure that no objects that could blow around in high winds damaged are present. If a storm is coming, make sure that your yard is free of garden gnomes, rakes, damaged tree limbs, or other loose objects that could become airborne. Make sure your roofs and gutters can shed heavy rain and have a place on your property to contain rainwater safely. Have supplies of batteries, non-perishable food, and water for people and pets. Put together a plan to recover later by making inventories of your property, including outdoor equipment that you store online. And make a family plan for how to evacuate if you live in an area of flooding and how to contact each other later if you get separated. Cell phones often do not work after strong wind events, so you can’t count on them to bring your family back together quickly after a storm.

What’s next?

I expect to see the Atlantic tropical season start to pick up by mid-August, when the African dust that is currently inhibiting storm formation clears and the ocean temperatures get even warmer. In fact, today’s 5-day outlook map shows an area of possible development in the eastern Atlantic (as of 8-6-2022). This is expected to be another active season, and even though we’ve only had three named storms so far, we will likely see many more storms before the end of November, when the official season ends. In my next blog, we will look at the season so far in more detail.

“Some people feel the rain. Others just get wet.”—Bob Marley

If you follow the weather news carefully, you might have noticed a little factoid in this week’s headlines: Mawsynram, Meghalaya, in northeastern India reported the highest all-time single-day rainfall in the month of June on the 17th. At 1,003.6 mm (39.5 inches), it eclipsed the previous highest rainfall of 945.4 mm (37.2 inches) recorded on June 7, 1966, according to the Guwahati-based Regional Meteorological Centre (RMC). The extraordinary rain fell as part of the Indian monsoon while other parts of the country are in drought and have received less rain than usual so far, although monsoon rains have been picking up. Seventy percent of India’s rain comes from the monsoon so if the monsoon fails, agriculture and water supplies are severely impacted.

What is a monsoon?

If you ask a layperson what a monsoon is, they will probably tell you something about very heavy rain and might even mention that it is a seasonal rain event. The most famous monsoon occurs in India each year, but monsoons are found in other parts of the world as well including China, Africa, and the United States. In the U. S., the Southwest Monsoon season usually starts in mid-June. If you look at the weather forecasts this week, it is right on time. This period is characterized by heavy showers and strong moisture flowing into the region bringing wet conditions to areas of the country that seldom see rain in other seasons. But officially, a monsoon is not the rain itself but a change in the atmospheric flow driven by differences in land-sea heating in summer. Rain falls in one half of the cycle but the switch to the opposite flow pattern often means dry weather as high pressure forces moist air away from the land.

How a monsoon forms

Monsoons are driven by temperature differences between land and water caused by heating from the sun. The hot land causes air over it to rise, leading to a net low-pressure area. Air rushes in from other locations to even out the pressure. On a small scale this type of circulation occurs as a sea breeze that you might feel along the shores of a large lake or ocean. The land heats up much more quickly than the water during the day, causing rising air over land that creates a circulating cell of air that blows cooler air from water to land in afternoon when the temperature difference is strongest. You can often feel a sea breeze front as the cooler air moves inland, and sometimes thunderstorms form along that boundary between land and marine air masses.

In a monsoon this pattern happens on a much larger scale. In Southeast Asia, for example, the whole of India and surrounding countries heats up under the direct summer sun much more than the Indian Ocean to the south. That causes air to rise over the land and pulls in air from the ocean to the south. In India, meteorologists track the progression of the monsoon “front” from south to north across the region and celebrate when the monsoon finally arrives, bringing copious showers that bring much-needed rain to India. In the Southwest U. S., the coming of the monsoon is also watched carefully because it provides welcome moisture and cooler temperatures to the region due to increased cloud cover. More than 50% of the annual rainfall of Arizona and New Mexico falls during the Southwest monsoon, so it is an essential part of the seasonal cycle. But it can also bring lightning strikes that spark forest fires in areas that receive little rain, causing widespread destruction in those areas.

How monsoons are important to gardeners

If you live in a part of the world that is affected by monsoons, you need to be prepared for the variation of rain over the course of the year. During the wet part of the monsoon, you may experience very heavy rains that can erode your garden and wash out plants. If you have low-lying areas, roots can be affected by standing water to the detriment of your garden plants unless you put in good drainage. Conversely, you need to also be prepared for periods with little to no rain at all, sometimes for significant periods of time. That means you either need to use plants that are adapted to the wet-and-dry monsoon conditions or be prepared to irrigate them regularly to keep them in good shape.

Monsoons are one example of how earth’s geography, including sun angle, land-sea distribution, and differential heating all work together to create diverse climates across the globe. They have inspired local music and literature as essential elements of culture in India and other parts of the world. And they serve as a critical driver of plant life by providing needed moisture to growing plants and crops. So for those of you who live in monsoon climates, do a little dance when the rains begin. The monsoon is here!

Other resources:

British Meteorological Office video: What is a monsoon?

NOAA SciJinks: What is a monsoon

Monsoon video series of SW monsoon imagery set to music by Mike Olbinski—spectacular!

Chasing the Monsoon: A Modern Pilgrimage through India—my favorite book on tracking the monsoon through India

While drought is part of the natural cycle of the climate and many native plants depend on drought to propagate, it is the bane of gardeners everywhere because of the increased need for water. I have previously written about the four types of drought. Today I thought I would focus on drought monitoring and a way that you, as a citizen scientist, can help report local conditions that the official drought monitor authors can use to fine-tune their depictions of drought. This will be especially helpful this year with so much of the country in drought conditions.

What is the Drought Monitor?

The U. S. Drought Monitor (I’ll call it the DM here) is the source of the “official” drought status across the country. Of course, you don’t need an official status to be in drought but it is commonly used by media, government agencies, and scientists to categorize the strength of drought over time and space. The official DM map shows a single map for the United States updated each Thursday morning that is color-coded by five levels of dry conditions ranging from Abnormally Dry (D0) to Exceptional Drought (D4). The drought categories show experts’ assessments of conditions related to dryness and drought including observations of how much water is available in streams, lakes, and soils compared to usual for the same time of year. If there is no color shown then that region is not officially in drought. If it is in D0 (bright yellow on the map), then it is considered dry but not currently in drought. Often a D0 designation means either that drought is imminent or that there are lingering impacts from a drought that is ending. You can read more details about what kinds of impacts are seen in each level of drought and how the weekly map is produced on their “About” page. You can also find links to state-specific impacts there, since a drought in the Southeast does not look like a drought in the Southwest, for example.

Issues with the DM

One of the shortcomings of the DM is that there is just one weekly map that is supposed to be a complete depiction of drought across the United States. If you read my last post on drought, you know that drought comes in different varieties that occur over different time periods. Agricultural droughts are caused by dry spells and hot temperatures during the growing season and can come on very quickly (“flash droughts”), while the long-term precipitation averages might not reflect that lack of water and so would not be captured on the DM map. Hydrological drought is related to a long-term lack of rainfall that reduces the water levels of lakes, streams, and reservoirs over many months or even years. It might not affect gardens and farms significantly as long as enough rain falls at regular intervals to keep the plants alive.

While the DM was not created to be a legal means for defining drought, it has become one in legislation passed by Congress to provide drought relief to affected farmers. The program I deal with the most in my work is the Livestock Forage Disaster Program, which provides a payment to forage producers to offset economic losses if drought stops the growth of pastures, especially early in spring when hay supplies are depleted over the winter. The law is written in such a way that if the drought level is severe drought or worse (D2+) for eight consecutive weeks, then they can receive one month’s payment for the lost forage. If it is in extreme drought (D3) somewhere in their county for any time during the growing season, then they can get payment for three months.

As you can imagine, when a flash drought happens and the grass stops growing the forage producers need the relief to help purchase hay to keep their livestock alive. But the DM does not typically depict this quick-changing drought because it is based on longer-term indicators that do not change much over just a couple of weeks. So there is a disconnect between what the DM is showing and how it is being used for legal purposes, at least in the case of forage and pastures.

As you might guess, this results in some attempts to game the system to make it look drier than it is to get the DM authors to declare a D2 or higher drought. The authors tell stories about being harassed over the phone about where they draw the drought lines on the map, finding observers who report no rain when radar clearly showed it occurred, and other creative ways to make the drought appear worse than it is so they can get access to the payments they need.

How can you monitor and report drought?

Here is where you come in: Citizens like you have an important role to play in keeping an eye on local vegetation and monitoring it for signs of drought or impacts from saturated soil. Gardeners are already especially attuned to what conditions are normal and what conditions are not. Folks who monitor local climate conditions are much like storm spotters who monitor weather systems for strong winds and tornadoes—they watch how things evolve over time and provide that information to the authors of the DM and others who need hyper-local information, although on a much longer time-scale that storm spotters. If you are a CoCoRaHS rainfall observer, you probably already know about the Condition Monitoring report that you can submit through their website. Another great place anyone can submit official Condition Monitoring Reports (called CMOR which is pronounced “see more”) is the CMOR site on Drought.gov.

As you can see from the map above, there are many parts of the country with no CMOR reports to provide information about conditions at those spots. By providing these reports, you are helping the DM authors with unbiased, fact-based local observations that can support other drought indicators like streamflow, precipitation deficits, and groundwater losses. Thanks for providing this service to the DM authors and other scientists!

NOAA recently announced that La Niña is favored to continue through summer and fall this year and could last through next spring. This forecast is bound to strike fear in gardeners in the western United States, since La Niña is associated with drought in the western parts of the country which sorely needs more rain. Los Angeles has announced some stringent watering restrictions due to impending water shortages, and that means gardeners will have to be especially careful there to use the water they have wisely.

What is La Niña?

Many people have heard the terms La Niña and El Niño but for those who don’t, let me take a few minutes to describe them. You can also read more in my blog post from last fall when this winter’s La Niña was just getting going. El Niño and La Niña are two opposite phases of an oscillation in the atmosphere and ocean in the Eastern Pacific, with neutral conditions in between the two phases as the oscillation swings back and forth like a seesaw. When that region’s sea surface temperature is warmer than usual near the equator, rising air above the warm water creates thunderstorms which act like a rock in a river diverting the flow of air along the southern US, especially in winter when El Niño and La Niña are usually strongest.

In El Niño winters, the Southeast is usually wetter and cooler than usual due to the presence of the subtropical jet stream overhead. It pushes storms with their associated rain and cloudy conditions through the region, recharging soil moisture for the next growing season. In La Niña winters, the jet stream is shifted to the north over the Ohio River Valley, leaving the Southeast warm, dry, and sunny. That means conditions for severe weather are more favorable in the Southeast than in other phases; we have certainly seen plenty of that this year so far. The lack of a strong jet stream also means that tropical activity in the Atlantic Ocean is more frequent and stronger than in El Niño years. In northern parts of the country La Niña winters are usually cold and snowy with a late start to spring, as we have seen this year. The Pacific Northwest is often wet, which also matches what has occurred in their coastal areas this year.

How often does a third year of La Niña occur?

The atmosphere usually swings back and forth between El Niño and La Niña roughly every 3-5 years. Right now we are ending a second consecutive winter of La Niña; with its predicted continuation, that would make it three years in a row. This is not unprecedented, but it is certainly unusual, since 1950 we have only had two “triple-dip” La Niñas. Since there are so few direct comparisons it can be hard to determine exactly what to expect this growing season and on into fall and winter. Our best bet is to assume that typical La Niña conditions will occur. The 3-month composites of the expected anomalies (differences from average; MAM means March-April-May, etc.) show the seasonal variability of El Niño and La Niña for temperature and precipitation across the US. La Niña and El Niño’s effects stretch far beyond the US and affect global weather patterns.

What does this mean for the growing season across the United States?

Typically effects from a La Niña are weakest in the summer because sea surface temperature anomalies are not strong and is often switching from La Niña through neutral conditions to an El Niño the next year. However this year the La Niña is still going strong, so this seems less likely. That means the pattern of warm and dry southern states are likely to continue, which is trouble for the already drought-ridden Southwest including California (where severe water restrictions are now in place). With the high temperatures, low rainfall, and low humidity, that means water stress on gardens will be higher than normal, and drought and wildfires could dominate that part of the country for the next few months.

In the Southeast, the active spring severe weather season will likely give way to an active tropical season in summer and fall. Rainfall in the Southeast in summer is dominated by tropical systems and small-scale convective rain events that provide only hit-or-miss rain. If you are in the path of a tropical storm, you can experience several inches of rain while areas a few counties away can see none, resulting in a feast or famine of rain. In the Pacific Northwest wet conditions in coastal areas will give way to drier conditions in the summer but may return again in the fall, while inland areas may continue to see very dry conditions that will lead to increasing drought and water shortages. The Northeast could see wetter than normal conditions so a drought there this year seems unlikely. The central part of the United States could be the hardest hit by drought conditions and the drought that is already present across a large part of the central and western US is likely to get worse over the next few months with little rain expected. That will affect not only gardeners but the farmers of the main grain-growing area of the US, at a time when Ukraine, normally a big grain producer, is not likely to be able to produce a regular crop this year because of the ongoing war.

Managing your garden in La Niña

Gardeners in the Southwestern US will have the most difficult conditions to manage this year due to the water restrictions and ongoing drought there. Proper use of irrigation and conserving soil moisture through mulch and appropriate choice of plants are good ways to keep water use lower. This may also be true of gardeners in the central US, where the drought could also be severe this summer. In the Southeast, the summer rain you get will depend on tropical activity and where the storms go so you could see either wet or dry conditions. Managing your garden for both dry periods and potentially heavy rains is a challenge that you may need to deal with this year. In the Northeast, the climate may be easier to contend with this year but even short-term dryness can be a problem for plants that need regular infusions of water. In the Pacific Northwest, predictions for a warmer and drier than usual summer mean you should pay careful attention to water-conserving measures, especially in inland areas where drought is already a problem. If you are outside the US, then make sure you understand how La Niña is likely to affect your region and manage your garden accordingly.