The Whys and Whats of Weather

By Steven P. Roberts

Weather science writer and analyst Steven Roberts invites you to come along as he goes well beyond discussing blizzards, climate change, El Niño, flash flooding and hurricanes. Here, you’ll learn about the Alberta Clipper, cell splitters, diurnal swing, omega blocks, snow bombs, train echo wave patterns, wintercanes and much more. Includes numerous links to online photos of weather phenomena.

• Language: English
• Publisher: Steven P. Roberts
• Release date: Mar 3, 2014
• ISBN: 9781311346810

Book preview

Dedication

This book is dedicated to Dorothy R. Donovan, whose undying belief in my ability to write it drove me through the tough days that the production entailed. This book would probably not have happened if it weren’t for Dorothy.

Dorothy, the book is as much your achievement as it is mine. The next time you feel like you have accomplished nothing, just know that you have indeed accomplished something great by helping me publish this book.

Acknowledgements

To my nephew Adam, who so ardently supported me in my quest to write this book.

To Pat, Pam, and Julie, whose unwavering support of my dream kept me going through the tough times.

To my Aunt Deb, who picked me up when I was falling down.

You have all been godsends to me.

To my niece Danielle, whom I affectionately refer to as Daniella or Yella, for expressing great interest in my literary efforts. Your interest was my encouragement. Thank you very much.

To my niece Kimberly Bower, who expressed constant curiosity in the progress of the project. Your curiosity kept me going and lifted my spirits.

And to my brother-in-law Sam Bower, who expressed great faith in my ability to do this.

About the Author

Steven Roberts has been interested in the weather since he could walk and talk and look out the window. His teachers fostered his interest in weather by encouraging him to read books and articles on the subject.

He lectured on weather-related topics at Middlesex Community College in Lowell, Massachusetts, from where he graduated in 1996 with a degree in business. He has also taught weather at the junior high school level.

Steven has been a regular contributor to the Talking Information Center (TIC) Radio Information Network in Marshfield, Massachusetts, where he has produced weather forecasts on a daily basis since 1997.

He worked as a weather analyst at WCAP Radio in Lowell for10 years, from 1996 to 2006.

For the last 17 years, he has had his own radio program on LAB/TIC Radio called Weather Wisdom Weekly, a weekly radio program on which he discusses the latest topics in weather, such as Superstorm Sandy, Hurricane Katrina, and the severe weather outbreaks of 2003 and 2011.

He is writing several additional books on various aspects of weather.

Contact information for Steven P. Roberts:

Email: [e-mail]

Phone: [phone]

His book-related website is: http:/www.dvorkin.com/stevenproberts

The website will be regularly updated to reflect new publications.

March 2014

CONTENTS

Dedication

Acknowledgements

About the Author

CHAPTER ONE

WHAT CAUSES WEATHER?

CHAPTER TWO

AIR MASSES AND FRONTS

CHAPTER THREE

LOOKING AT THE THREEDIMENSIONAL ATMOSPHERE

CHAPTER FOUR

SKY COLOR AND CLOUD COVER

CHAPTER FIVE

WINTER STORMS

CHAPTER SIX

UNDERSTANDING SEVERE WEATHER

CHAPTER SEVEN

HURRICANES AND TROPICAL STORMS

CHAPTER EIGHT

RIVER FLOODING SITUATIONS

CHAPTER NINE

WEATHER FORECASTING AND YOU

CHAPTER TEN

THE WHYS AND WHATS OF WEATHER WARNINGS

CHAPTER ELEVEN

UNDERSTANDING THE CLIMATE

CHAPTER TWELVE

EL NIÑO AND LA NIÑA: THE KIDS OF WEATHER

CHAPTER THIRTEEN

THE GLOBAL WARMING PROCESS

CHAPTER FOURTEEN

THE DYNAMIC, VOLATILE ERA:

CRAZY WEATHER AND CLIMATE CHANGE

CHAPTER FIFTEEN

OUR WEATHER AND OURSELVES

BIBLIOGRAPHY

Editing and Publishing Assistance

REFERENCES

CHAPTER ONE

WHAT CAUSES WEATHER?

The Three Elements of Weather

Weather requires three basic elements. They are the sun, the air, and the water. Remove any one of the three elements, and weather as we know it would cease to exist.

The sun warms the ground, and the ground warms the air. The sun also warms water, initiating the process of evaporation. Perhaps the biggest impact the sun has in creating the weather comes from the disparity in solar heating that takes place from pole to equator. The equator receives the sun’s most direct rays and therefore its greatest warming effect, while the poles receive the sun’s least direct rays and therefore its least warming effect.

The resulting disparity in air temperatures causes the cold air at each pole to move toward the equator, while the warm air at the equator is always moving toward each of the two poles. This process keeps the weather machine in high gear all the time. As one air mass overspreads an area, it is introduced by a frontal boundary. A front can rudely crash through an area with powerful thunderstorms. The clash of air masses can also cause storm systems to develop.

A storm requires water to develop. The sunwarmed water results in the process of evaporation. The reverse of evaporation is condensation. Condensation results in the production of clouds and precipitation. The production of precipitation requires the atmosphere to have enough lifting power to transport moisture to cloud level. Heat and moisture combine to provide the lift needed to create clouds and precipitation.

Water’s phase changes result in the removal and return of heat to the environment. Condensation adds heat, while evaporation subtracts heat.

Heat and Moisture Are Partners in the Production of Weather

Heat and moisture work hand in glove in the process of weather production. Regardless of whether you are looking at a thunderstorm or a hurricane, heat and moisture work together to make those storms form.

When heat is added to the atmosphere, the lifting capacity of the air is enhanced. This is because when heat is introduced to an air mass, the air in the air mass becomes lighter. The introduction of heat into the air also increases the ability of an air mass to store moisture. This is because the molecules of air in a warm air mass are more widely separated from one another. This allows water vapor to get into the air mass by going between the air molecules.

As moisture is introduced into the air, the capacity of the air to lift is further increased. Now we have heat and moisture working side by side to enhance the lifting capacity of the atmosphere. The combined impact of heat and moisture on the buoyancy of the atmosphere are crucial to the formation of clouds. Heat and moisture cause the air to rise from the surface. As the air rises, it expands and cools. The cooling of the air causes its moisture to condense into clouds. The clouds then go on to produce precipitation in one form or another.

The water cycle results in the removal and return of heat energy. In this way, the water cycle is also a thermal cycle. When water evaporates, it takes heat out of the air. Once that water condenses, it releases its stored or latent heat into the atmosphere. When moisture condenses, it releases as much heat to the atmosphere as was taken from the atmosphere during evaporation, hence the thermal cycle. Regardless of what is going on outside your windows, heat and moisture are working together to produce the weather.

A Never-Ending Battle of Feuding Factions

The atmosphere is in a constant state of war. This war takes the form of weather. Weather is our atmosphere’s attempt to even an inherently uneven score. The differences in temperature, pressure, and moisture all act in concert to drive the process we refer to as weather.

The sun warms the planet in an uneven way. The tropics receive the most direct rays of the sun and are therefore warmed the most. Conversely, the polar regions of the planet receive the least solar radiation and are therefore warmed the least. The differences in sun fall can cause striking differences in temperature from place to place. For example, the average daily temperature can vary significantly from Cape Hatteras, North Carolina to Cape Cod, Massachusetts, let alone from the Arctic to the equator.

The cold air is constantly trying to take over as much of the planet as it possibly can. The warm air would also like to take over as much of the world as it possibly can. With both warm air and cold air acting on the same agenda, the stage is set for an arduous and never-ending battle for global control. These battle lines are drawn on the weather maps as warm fronts (the leading edge of warm air) and cold fronts (the leading edge of cold air). See Chapter Two for more about frontal boundaries.

The next battle involves the disparity between high and low air pressure. The property of any gas, including the gases that make up our atmosphere, is to go from a state of high concentration to lower concentration. The air in our atmosphere goes from high to low pressure all the time. In an ideal setup, the air in an area of high pressure would go straight into an area of low pressure, but because of the earth’s rotation, that air is deflected to the right in the northern hemisphere and to the left in the southern hemisphere.

In the northern hemisphere, high pressure centers rotate in a clockwise direction and low pressure centers rotate in a counterclockwise direction. The close proximity of high and low pressure can cause very high winds to occur. If you were in the state of Michigan when a big storm was intensifying in New England, you would have a strong northerly wind. Now, if there were a strong area of high pressure over Wisconsin, the winds in Michigan would be even stronger out of the north because of the pressure difference between the areas of high and low pressure.

The final part of the battle includes the differences between moist and dry air. Warm air can hold more moisture than cold air can, so the capacity of the atmosphere to accommodate water vapor is highly variable from one air mass to another. If the atmosphere were to transport moisture into an area of cold air, the result would be the production of precipitation: rain or snow. This is because a cold air mass will wring out the moisture it cannot accommodate in the form of precipitation of one sort or another.

The differences in atmospheric moisture concentration go far beyond the different air masses and their varying capacities to hold water vapor. A cold air mass can have a large volume of water vapor stored within it. Those of us who live in the northeastern United States or the Pacific Northwest can clearly attest to that fact. It is also possible to have a hot air mass with little or no moisture stored within it. If you live in one of the cloudless, arid deserts of the southwestern United States, you can clearly identify with this atmospheric reality.

When a cold air mass becomes saturated with moisture, one of three things can happen. First, clouds form in the lower levels of the atmosphere, creating light rain or drizzle. This often occurs when a northeasterly wind sets up in New England, or when a westerly wind sets up in the Pacific Northwest. The second outcome involves the development of extremely dense ground fog. This takes place when air that is slightly warmer than 32 degrees Fahrenheit advects (moves horizontally) over deep snow cover. The 40-degree air is warm enough to evaporate the moisture out of the snowpack, but it is of such limited evaporative capacity that it reaches saturation level relatively quickly. The fog bank is moisture that has condensed in the lower atmosphere as a result of evaporation from the snowpack. The final outcome is the production of extremely heavy precipitation. When a lot of moisture is rapidly introduced to an air mass with a limited capacity to store moisture, the excess is wrung out as precipitation. This occurs when a nor’easter throws Atlantic moisture into a bitterly cold air mass over New England, or when a Pacific storm comes crashing on shore in California, producing one to two feet of snow in the mountains.

An air mass can be hot and dry. For example, the air in the southwestern deserts can warm up to 100 to 120 degrees in the summer, yet have very little moisture concentration within it. As this air rises, what little moisture the air has condenses, creating high-based thunderstorms, also called dry thunderstorms. These showers produce little or no rain at the surface, because the precipitation evaporates on its way down to the surface. This is what we refer to as virga. However, these socalled dry thunderstorms can produce lots of lightning that can start forest fires. The limited rain produced by these storms enables the lightning to do its dirty work.

To see some dramatic images of virga, do an online search on that term or click here: https://www.google.com/search?q=virga&client=firefox-a&hs=EmT&rls=org.mozilla:en-US:official&channel=np&tbm=isch&tbo=u&source=univ&sa=X&ei=U36zUo_2GMbmyQHV04D4Aw&ved=0CDEQsAQ&biw=1661&bih=935

Hot, dry air does not lift as vigorously as warm, humid air. When humid air is impacted by a dry line, which is a boundary introducing warm, dry air into a region inhabited by a warm, humid air mass, the result can be severe thunderstorms. This is because the warm, humid air already has considerable buoyancy. Anything that would lift the atmosphere further has the potential to trigger some truly tremendous thunderstorms. Dry lines are lifters capable of destabilizing the atmosphere under the right conditions. During our severe weather events, hot, dry air and warm, humid air are often observed dueling it out, with spectacular results.

The Uneven Heating of the Globe

The uneven heating of planet Earth is driven by two separate but comparably important processes. First, the tropical regions of Earth receive much more solar radiation, or sunlight, than either polar region; therefore, the tropical regions of Earth are much warmer than the polar regions of the planet. Second, the land warms and cools much more rapidly than do the ocean basins. This allows for a constant disparity between sea-surface temperatures and continental temperatures. These temperature differences significantly impact overlying air temperatures.

As was stated above, the cold air of both polar regions moves in the direction of the tropics, and the warm air over the tropics tends to move in the direction of either polar region. Here is where weather starts to happen. The clashing of warm and cold air masses can cause thunderstorms and midwinter storms, as well as the formation of fronts that introduce both warm and cold air to various regions of the planet.

The next thermal disparity is caused by the varying rates at which continental and oceanographic areas respond to seasonal forcing. During the fall, when the northern hemisphere is cooling off in its transition into winter, the continents cool off much faster than do the oceans. This is because the oceans hold onto the heat they acquired during the summer months much more effectively than do land masses. During the spring, when the northern hemisphere is in the process of warming up, the land areas warm much faster than do the oceans. This is because the oceans warm at a much lower rate than do the continents, regardless of how the atmosphere is forced. Land masses respond more rapidly to seasonal forcing than do the oceans. This particular disparity causes a whole host of weather effects. During the summer, the coastal areas of the U.S.—and of the world, for that matter—have sea breezes. As the land warms very quickly relative to the ocean, the warm air rises up off the land, creating a void that gets filled by the advecting cool marine air. A sea breeze can create frontal boundaries called sea breeze fronts. These fronts can cause strong to occasionally severe thunderstorms.

During the winter, storms can hit the coastal regions with a variety of precipitation. This is because the warming influence of the ocean causes snow to change to sleet or rain. On the other hand, communities set back from the coasts will often receive very heavy snowfall, because the warming influence of the ocean does not extend to the areas being buried by snow. This is a classic example of how land/sea temperature differences can influence the weather from one location to another.

The Differences in Air Pressure

Within our atmosphere, there are areas of high and low pressure. There are two things that have to be understood in order to appreciate the interactions between these areas. First, gases tend to go from areas of high concentration to those of low concentration. Second, nature abhors a vacuum. That is, the atmosphere tends to fill in areas of low pressure with air.

In the atmosphere, there are areas of high atmospheric pressure, air that exerts a lot of pressure on the surface of the earth. The air in a high pressure area rotates from the center to the outside of this fair weather feature. In the northern hemisphere, high pressure rotates in a clockwise direction. The tendency of the gases in our atmosphere is to go from high concentration to low concentration. Air tends to sink in areas of high pressure.

Low pressure areas are regions of low atmospheric pressure, air that exerts a low level of pressure on the surface of the earth. In the northern hemisphere, low pressure areas rotate in a counterclockwise direction. The air in a low pressure area tends to move toward the center of low pressure. Air also tends to rise in areas of low pressure, hence the reduction of pressure at the earth’s surface.

The interactions between areas of high and low atmospheric pressure can cause some interesting weather effects. For example, an area of high pressure and one of low pressure in close proximity to one another can cause extremely high winds. Let’s say that there was a powerful ocean storm to the east of New Jersey and a strong area of high pressure over the state of Ohio. The winds would blow out of the north in Pennsylvania and New York State. In this case, the high winds would be caused by the contrast in pressure between the high and the low pressure areas.

On the other hand, if there were a high off the northeastern coast and a low over the Great Lakes, strong winds would blow out of the south because of the pressure contrast between both weather systems and the rotational characteristics possessed by each of the two weather systems.

The weather systems in the United States move from west to east. The high over the North Atlantic may cause the storm over the Great Lakes to stall out. This is because the high will block the forward motion of the area of low pressure.

The Process of Seasonal Change

During the summer, the temperatures across the United States do not vary all that much. For example, the temperatures along the Gulf Coast may be in the 90s, while the temperatures along the Canadian border states are in the 80s. This temperature setup results in a thermal contrast of 10 to 20 degrees. The small temperature gradient (difference) results in fewer and weaker storms. This is because storms are driven by the differences in temperature that set up in the fall and winter months.

During the early fall, the nights start to grow longer around the Arctic Circle. The longer nights allow for very substantial cooling to occur. The increasingly cold air in the north advects to the south on northerly winds. As this air plunges south out of the North Pole, the temperatures across Canada and the northern U.S. start to fall. Meanwhile, the temperatures in the southern tier of the United States are still very warm; air of 80 and 90 degrees is still in place along the Gulf Coast. The cooler air in the northern United States and the warm air still present in the South causes a growing temperature contrast in the middle latitudes (the United States and Europe). The increasing temperature contrast helps in the formation of storms.

The process of seasonal change has many impacts on our weather. During the early fall, the jet stream starts to move to the south. This causes storm systems and cold fronts to move further south than they did in June and July. Another impact of seasonal change is greater daytoday weather variability. During the summer, the variation in weather from one day to the next is rather small. One day may feature 90-degree heat, while the next day sees 75 degrees with low humidity. During the early fall, one day may be sunny with temperatures in the 90s, while the next day may be rainy and cool with temperatures in the 60s. During the fall, we have greater day/night temperature variability. During the early fall, the sun is still strong enough to warm the atmosphere significantly during the day. The nights are long enough to allow the heat of the day to escape back into outer space and bring about substantial cooling.

By the time that we get into mid-autumn, the process of seasonal change is well underway. The storms that impact the United States are rather vigorous and sizeable. There is now a great deal of variability in the weather on a national basis. During any given day, the weather maps may feature a snowstorm in one part of the nation and summerlike weather in another part. By the latter part of October, the weather nationally is more likely to be cold than warm. November is the coldest of the three fall months. This is when things can get very interesting. The storms that traverse the United States can produce rain or snow, and some snowfalls can be immense. In the northern United States, temperatures can be in the 20s, while in the southern U.S., temperatures of 70 and 80 degrees can still be found.

As the seasons change, the atmospheric setup changes. This alters the way weather behaves from day to day. During the early fall, temperature contrasts from north to south are rather small. As the autumn progresses, the temperature contrasts from north to south increase. This results in an increase in the frequency and magnitude of storms that form from those contrasts. As the fall progresses, the weather from day to day in a given area can become more and more variable. For example, during the month of September, it can be 90 one day and 65 the next, with clouds and rain. During the month of November, it can be 65 one day and 30 the next. Seasonal change alters the nature of weather by altering the nature of the war that causes weather in the first place.

The Rotation of Planet Earth

The rotation of the globe has many impacts on our weather. Let’s start with day and night, which can have quite an impact on the weather. As the earth rotates on its axis, the sun shines on various parts of the blue marble. The parts of the planet exposed to the rays of the sun are experiencing day; conversely, the parts of the planet facing away from the sun are experiencing night.

After sunrise, the air temperature can rise 20 to 30 degrees, and in some cases, even more. As the sun sets, the amount of heat received by the atmosphere declines. Once the sun has set and night has commenced, thermal input to the atmosphere stops. The cessation of warming leads to the onset of radiational cooling. At night, the heat that the atmosphere acquired during the day rises back up into outer space.

Another impact of earth’s rotation is the Coriolis Effect, also called the Coriolis Force. The Coriolis Effect causes air that is moving in a straight line to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere. Once again: In the northern hemisphere, areas of high pressure rotate in a clockwise direction, while low pressure areas rotate in a counterclockwise direction.

In order to understand and appreciate the influence the Coriolis Effect has on the weather, you must first understand that the atmosphere exhibits two similar but separate tendencies. The first is that gases in our atmosphere tend to go from high concentration to low concentration. The second law of the atmosphere is that nature abhors a vacuum. The tendency of the atmosphere is to flatten the hills and fill in the valleys created by areas of high and low pressure. For the purposes of this and future analysis, think of high pressure as being hills and of low pressure as being valleys.

If the earth did not rotate, the air in high pressure areas would move straight toward the nearest area of low pressure, like a ball rolling from the top of a hill to the valley below. But that is not how the atmosphere works. The rotation of the globe causes the moving air to be deflected to the right of a straightforward path in the northern hemisphere. Because of the deflection caused by the Coriolis Effect, the atmosphere has to spend more energy to transport air from high to low pressure.

Think of it this way: The shortest distance between two points is a straight line. Taking the shortest way from Point A to Point B will require the least amount of energy possible. Because the atmosphere has to use so much of its energy to transport air from high to low pressure, high winds are produced. The wind is the atmosphere’s method of transporting air from place to place. The rotation of earth and the resulting Coriolis Effect drive the atmosphere to its wild and windy ways.

The high winds typically produced by the atmosphere result from high and low pressure areas which are close to each other. The pressure contrast between these two weather systems can cause very high winds to occur, sometimes as high as hurricane force. In this case, the high winds are caused by the air in the high pressure area moving into the nearby area of low pressure. Because of how air rotates around high and low pressure areas, the air going from high to low pressure is deflected in the process of transport. The deflection results in more energy consumption by the atmosphere. Without the rotation of planet Earth, air would go directly from high to low pressure with little fanfare.

The Impact of Land and Sea

The land and the oceans work together to create weather. First, the continents respond more rapidly to the process of seasonal change than do the oceans. Second, the land and oceans are frictionally different from one another, which causes changes in the way air advects around the planetary surface.

The continents warm and cool more rapidly than do the ocean basins. This causes temperature contrasts to set up in the winter and summer months. It is possible to see the north- central United States freezing in 20-degree