The Weather Never Sleeps

By: Jack Williams

Long winter nights

Use the season's short days wisely

Figure 1--The Earth's yearlong orbit around the sun defines a plane, or a flat, oval surface.

Figure 2--A cold front displaces warm air (red arrow)) with cold air (blue).

Figure 3--The atmosphere above Miami, Florida, is higher than that above Inernational Falls, Montana.

Figure 4--A winter storm jet stream on March 13, 1993. The swirling air helps to energize the storm at the surface.

As fall gives way to winter, shorter days and foul weather in many parts of the country conspire against those trying to learn how to fly.

Imagine you began flight training over the Labor Day weekend near Des Moines, Iowa. You've been going to the airport to fly once or twice a week--as your time and checking account allow--and by late November, you've logged more than 30 hours of flight time. You believe you are doing well but would be doing better if you had more time in the air.

In early September, the sun was going down around 7:45 p.m. each day, allowing you to fly after work. In late November, however, the sun sets before 5 p.m.; your flying is limited to weekends when the weather cooperates.

But the weather is less and less cooperative. Poor visibility, gusty winds, and even snow are cutting into flying time. In September you did preflight inspections in 70- or 80-degree temperatures. Now, you're fighting your inclination to rush the inspections to escape cold winds.

You learned in middle school that short days and colder weather are related--that makes sense--and changes in day length and temperatures have something to do with the way the Earth is tilted. This was kind of interesting at the time. Now, however, it's personal. You want to learn more.

You begin your investigation of winter weather by learning from a NASA Web site that the Earth is actually closest to the sun each year in early January, the depth of Northern Hemisphere winter. It's then about 91.7 million miles away. It's farthest away in early July, about 94.8 million miles.

Tilt tells the tale of the seasons. The Earth's yearlong orbit around the sun defines a plane--an oval, flat surface. A line between the North and South poles, the Earth's axis, is tilted by 23.5 degrees in relation to this plane. The Earth also spins around its axis each 24 hours, creating day and night. The North Pole always points toward the star Polaris, the North Star.

Figure 1 shows how this works. Let's begin on the left side with June 21, the summer solstice. The North Pole is angled 23.5 degrees toward the sun. Every place north of the Arctic Circle, which is 66.5 degrees north latitude, sees the sun 24 hours a day.

As you head south from the Arctic Circle, the nights grow longer until you reach the equator, where night is jut a few minutes less than 12 hours long. Continuing south, the nights grow longer and longer until you reach the Antarctic Circle, 66.5 degrees south latitude. From here on to the South Pole, the sun doesn't rise. By September 23, the Earth's 23.5-degree tilt isn't toward or away from the sun but parallel with it, if you imagine the sun as a flat disc. From pole to pole, sunlight bathes all of the Earth that's facing the sun. Day is now just a few minutes longer than 12 hours all over the Earth.

As the Earth heads for the winter solstice on December 21, Northern Hemisphere nights grow longer and longer, and each day the sun is a little lower in the sky at noon. And its rays are hitting the Northern Hemisphere from a lower and lower angle, which means the sun supplies less and less energy. Temperatures fall; in general the farther north you are, the quicker and lower they fall.

For Des Moines, this translates into the daily average high and low temperatures dropping from 81 and 60 degrees Fahrenheit on September 1 to 39 and 23 on November 30.

Obviously, the seasonal reduction in solar heating explains why places like Des Moines are colder in November than in September. Colder air means any precipitation is more likely to fall as snow than as rain. The drop in northern temperatures also accounts for why late fall, winter, and early spring are stormier than the summer and early fall.

January and June monthly average temperatures for Churchill, Manitoba, on the western side of Hudson Bay in northern Canada, and at Brownsville in southern Texas, will help us see why winter is stormier. Churchill's July average temperature is 53 degrees F, while Brownsville's is 83 degrees. The difference between the two is 30 degrees. The January average temperature is Churchill is minus 18 degrees, while in Brownsville, it's 59 degrees--a difference of 77 degrees.

Since north-south temperature differences supply most of the energy that powers storms which form outside the tropics, greater differences mean stronger storms. Figure 2 (p. 57), which represents a cold front where cold air is replacing warm air, shows how temperature differences can energize storms. The blue area is cold air, while the green arrow is warmer air that advancing cold air is pushing up.

The cold air pushes under the warmer air because the cold air is denser; that is, each cubic foot of it weighs more than a cubic foot of warmer air. The greater the temperature difference, the greater the density difference, which translates into stronger storms in winter.

Temperature contrasts also create upper air winds, including jet streams, which help to create and steer storms. Figure 3 is a simplified look at the atmosphere above Miami and International Falls, Minnesota, on the same day. The air above Miami is warm--not only at the surface, but also high into the atmosphere in comparison with air above places farther north.

Air, like most things, expands when it is warmed. The vertical red line shows that the top of the atmosphere is higher than at International Falls, as shown by the blue line, where the cold air has contracted. Air pressure at the surface is the same in both places, 1,034 millibars, which shows the columns of air in both places have the same amount of air. But, the surface temperature at International Falls is 31 degrees colder than Miami.

A weather balloon sent aloft over Miami found a temperature of minus 22 degrees F and a pressure of 324 millibars at 30,000 feet. The International Falls weather balloon found minus 58 degrees and 295 millibars pressure. The 30,000-foot pressure at International Falls is lower than Miami's because there's less air above this altitude over Minnesota. The pressure difference creates a pressure gradient force at 30,000 feet. This force pushes air from high pressure toward low pressure; it causes wind to blow.

As air moves at 30,000 feet over Miami toward the same altitude above International Falls, it turns toward the right as seen from the ground. If you could see the moving air but not the Earth's surface from space, the wind would be travelling in a straight line. But, the Earth is rotating under the wind. The result is that the wind--a jet stream--ends up moving roughly from west to east above the United States.

Jet streams tend to flow along the tops of boundaries between warm and cold air, and jet streams can help intensify storms. During summer and early fall, the polar jet stream usually moves generally west-to-east across Canada or the northernmost United States. As cold air advances south in late fall and winter, the jet stream follows it.

Often, a large mass of cold air will move south across the eastern United States while warm air dominates the West. In such a case, the polar jet stream will flow over the Pacific Northwest from over the Pacific Ocean, and then turn to head toward the south in the middle of the country. It will then swing back toward the northeast over the East or the eastern Atlantic, much like the pattern shown in Figure 4. Here, we see the jet stream making a sharp left turn over the Gulf of Mexico south of Alabama and Mississippi. As shown, such a turn causes the air to swirl counter-clockwise. This swirl in the upper atmosphere helps to stir up a storm at the surface.

Normal winters bring plenty of bad weather to a large part of the United States; even the Southwest may have more days with low ceilings or poor visibility than during the rest of the year.

Student pilots and their instructors have few choices but to use the longer nights to get in the three hours of night flying required for the private pilot certificate and to spend time in ground school learning more about aeronautics, the federal aviation regulations, navigation, and weather.

Eventually winter's darkness, cold, and storminess will give way to spring's longer days and friendlier flying weather.

Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.