The Weather Never Sleeps

Tropical Cyclones And Extratropical Cyclones

Pilots who spend more time than they'd like during the winter watching television weather, hoping for some good news, often see what looks like a parade of Ls marching across the country. An L on a weather map, of course, denotes an area of low atmospheric pressure. The ones we see crossing North America are what meteorologists call extratropical cyclones. In the United States they're generally called lows or storms. Other parts of the English-speaking world often call them depressions.

Sometimes a low will slip into Montana from Canada's Alberta Province, arc across the Midwest, and zip across the Northeast as the television people talk about an Alberta clipper. It comes from Alberta and "clips" across the country, usually spreading light snow. Another time, a Pacific Ocean storm will hit Oregon or California, weaken as it crosses the West's mountains, and pull itself together again on the Plains just east of the Rockies. From here it lumbers to the east with blinding snow across the Plains and Great Lakes into Canada.

Other lows are born over the Gulf of Mexico and move up the East Coast, gaining strength over the Atlantic Ocean off Cape Hatteras, North Carolina, before bringing the Northeast to a halt with piles of snow. At times, a storm will move north along the west side of the Appalachian Mountains and then "center jump" to the ocean off the North Carolina coast and strengthen. When this happens, the storm's upper-level lifting energy moves across the mountains, while the mountains break up its lower-level winds. Once over the ocean, the spinning upper air stretches down to kick up winds at the surface.

Fortunately, most lows cross the country without causing big problems. They bring clouds, rain, snow, and then colder air, but normal life goes on - as long as you stay on the ground. In the summer they are more likely to cross Canada than the United States. During the coldest part of winter they're more likely to roam the southern United States.

Whether weak or strong, these storms often set the agenda for the day-to-day local weather that pilots experience.

Meteorologists use the term cyclone for any large-scale weather system with winds blowing around and into a central area of low atmospheric pressure.

Tropical cyclones form over warm oceans. They include hurricanes in the Western Hemisphere and typhoons over the western Pacific. There's really no difference between a hurricane and a typhoon. It's just that tradition has left us with a habit of giving western Pacific cyclones the "typhoon" moniker. Hurricane or typhoon, these storms draw their energy from the warm, humid air over warm oceans and begin to rapidly weaken and die when they move over cool water or any large body of land.

Extratropical cyclones, as the name implies, form outside the tropics. They are born and thrive over any ocean or land. Heat released when water vapor condenses into clouds and rain supplies most of the power for tropical cyclones.

Contrasts between masses of warm and cold air supply most of the energy that powers extratropical cyclones. They form along the boundaries - called fronts - between large masses of warm and cool air.

A fairly simple demonstration can show how such warm-cold boundaries supply energy. Start with a fish tank that can be divided into two equal sections by an easily removable partition. Pour hot water mixed with red food dye into one side. Cold water, with blue dye and salt to make it denser, goes in the other.

Now, pull out the partition. The blue, dense, cold, salty water will dive under the less dense, warm, red water. After a little sloshing back and forth, the water will settle down with the blue on bottom, red on top. If you put the tank on a spinning turntable, the water would swirl as the cold water slid under the warm water, much like warm air and cold air do as a real storm is forming.

The real world obviously is more complicated, but this gives you an idea of where the energy of extratropical storms originates.

As a real storm is forming, low atmospheric pressure develops in the center of the swirl, and air begins flowing toward the low center from the surrounding higher pressure. If nothing else happened the air pressures would balance out when enough air flowed into the low center, and that storm would die.

Even if a strong warm-cold air boundary exists, an extratropical cyclone won't spin up unless upper-altitude winds are helping to pull air up from the surface to keep the air pressure in the storm's center low at the surface.

Air is also rising elsewhere in the storm, and this creates clouds and precipitation as the rising air cools. The swirl of warm and cold air around the storm's center creates the fronts that are always a part of an extratropical cyclone by pushing masses of cold and warm air along the surface.

Extratropical storms always have a warm front - where warm air is advancing - and a cold front, where cool air is advancing. Tropical cyclones, which are always embedded in a mass of warm air, have no fronts.

Much of an extratropical storm's action is along the fronts, with the cold front's weather usually being more dramatic than what's happening along the warm front. The cold front usually stretches to the southwest, maybe to the south or west, from the central low-pressure area. Here, thunderstorms are generally created where cold air is shoving under warm air. But the most violent thunderstorms are often ahead of the cold front. This happens when squall lines form a hundred or so miles ahead of the cold front.

A storm's warm front usually stretches to the east, maybe the southeast, from the low-pressure center. Warm front weather includes widespread areas of low clouds and more or less steady rain or snow to the north of the warm front's surface location. This area can also bring icing and thunderstorms that are hidden by other clouds - these are called embedded thunderstorms.

The fronts move in relation to the center, pivoting around it. At the same time, the entire system is traveling across the country with the low-pressure center heading to the east or northeast, sometimes even toward the southeast.

Since the more dramatic weather action tends to be concentrated along the fronts or ahead of them, you might think the weather to the north of the low-pressure center, where there are no fronts, would be easy to handle. You would be seriously mistaken. The generally counterclockwise flow or wind around the low-pressure center brings warm, humid air over the warm front and wraps it around to the north side of the storm's center. This means that a pilot who flies into the northern part of the system could run into thick clouds, turbulence, and dangerous icing.

Combining what you see on weather maps that show winter's parade of extra-tropical storms across the country with what you see out the window of an airplane - or your house when the weather is too foul for flying - is a good way to learn how the weather works.

The mental "model" of what's going on in an extratropical cyclone is a good way to increase your understanding and appreciation of weather in all of its complexity.

Jack Williams is the weather editor of USAToday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.