The unequal heating is greater in winter than summer; it's like turbocharging the "engine" that drives the weather. Warm days can bring their own nasty weather, of course, but it's more likely to come and go relatively quickly, as with thunderstorms, rather than hanging around for days as during the winter.
Most of us enjoy summer for its long days and warmth. Pilots in northern areas - where summer days are longer than farther south - can often leave work at 5 p.m., drive to the airport, and get in a couple of hours of flying before dark. Those long summer days are paid for during the winter when the sun might be already going down when you leave work at 5 p.m.
The day's length and the sun's height in the sky determine how much solar energy reaches the Earth's surface to warm the land and oceans. During the Northern Hemisphere winter the farther north you go the shorter the time between sunrise and sunset. After the sun sets at the North Pole on September 24, it doesn't rise again until March 19. At Barrow, Alaska, the northernmost airport in the United States, the sun sets on November 18 - not to rise again until January 23.
As northern nights grow longer and the sun drops lower in the sky, the Northern Hemisphere loses more and more heat to space compared to the amount of incoming solar energy. This loss of heat means that by January locations along the U.S.-Canadian border commonly have temperatures that bounce around near the zero degree Fahrenheit line on thermometers.
Meanwhile, to the south, the sun is almost directly overhead at noon and is in the sky around 12 hours a day. All of this sunlight is why tropical and subtropical temperatures stay in the 70s and 80s around the clock, even in January. To illustrate the contrast: In July the difference in average daily high temperatures between International Falls, Minnesota, and Cancun, Mexico, is only 12 degrees - 78 degrees F in International Falls to 90 degrees F in Cancun. In January, however, International Falls' average daily high is 69 degrees colder than Cancun's 81 degrees.
Such huge temperature contrasts between north and south around the Northern Hemisphere supply the energy for winter's wild weather.
Figure 1 shows in a greatly simplified way how temperature contrasts supply energy. It represents a fish tank that's divided by a partition, which can be easily removed. When you pull out the partition, the cold, dense water on the left begins flowing under the warm, less-dense water on the right. If the tank was on a rotating table, the moving water would swirl, just as the Earth's rotation causes moving air to swirl.
In the language of physics, the differences in water density on the two sides of the partition represent potential energy. When you pull out the partition, this potential energy becomes the kinetic energy of moving water.
The atmosphere, of course, doesn't have removable partitions between warm and cold air, but temperature contrasts represent tremendous amounts of potential energy that becomes the kinetic energy of storms as cold air pushes under warm air, which rises and moves over cold air.
Energy from heat contrasts does most of the work of powering extratropical storms, the large storms that form outside the tropics such as winter's blizzards. Extratropical storms move across the Earth's middle latitudes all year, but those occurring in winter are generally stronger than summer storms.
Temperature contrasts are the direct cause of upper-air winds, including the jet streams, which help to create and steer storms. In addition to temperature contrasts, weather energy is also supplied by the latent heat released when water vapor condenses into liquid water or sublimates directly into ice. Release of latent heat supplies most of the power for thunderstorms and tropical cyclones, such as hurricanes, but only a small part of the energy of winter storms.
Students learning how to use altimeters find out that any particular barometric pressure setting will indicate a higher altitude above sea level in warm air than in cold air. Or, looking at the same thing from another viewpoint, at any particular altitude the air pressure will be higher in warm air than at the same altitude in cold air. This occurs because air expands as it warms. In the expanded column of warm air, there is more air above any particular altitude than the same altitude in cold air. Since air pressure depends on how much air is above the place where it's being measured, the pressure is going to be higher in the warm air.
Weather balloon measurements on a fall evening illustrate this. When a balloon was launched at International Falls, Minnesota, the temperature at the ground was 41 degrees F. At 30,000 feet, the temperature was minus 58 degrees F and the air pressure was 295 millibars. At the same time, the temperature on the ground in Miami was 72 degrees, while at 30,000 feet, the air temperature was minus 22 degrees F and the pressure was 324 millibars. This was part of a general pattern of warmer air and higher pressures at 30,000 feet over the Southeast than over the north-central states and central Canada.
Let's look at just the Miami and International Falls 30,000-foot pressures. The difference creates what meteorologists call a pressure-gradient force; that is, a force that pushes air from high toward lower pressure. The greater the pressure contrast, the greater the force and the faster the resulting wind.
As air begins moving from high above Miami toward International Falls at 30,000 feet, the Earth's rotation causes it to turn toward the right in relation to the Earth. All of the forces balance out to create a general southwest-to-northeast flow of air over the central United States, including 100-knot winds 30,000 feet over Iowa and Illinois.
During the summer, when the temperature contrast from north to south is smallest, the jet stream is usually far to the north, over Canada, as shown in Figure 2. All upper-air winds, including jet stream winds, tend to be slowest in the summer.
As fall turns into winter, upper-air winds begin growing stronger and the jet stream begins to migrate south, as in Figure 3, because the jet stream tends to be above the zone of greatest temperature contrast. Jet streams that flow generally from west to east with only small north-south excursions, as in Figure 3, are found when the weather is calm because the warm-cold contrasts are smaller.
When you see an upper-air forecast calling for a jet stream like the one in Figure 4, expect trouble. In a case like Figure 4, warm air has pushed northward across the western United States into Canada while cold air is moving from Canada across the north-central United States and Great Lakes into the Southeast.
In this case, the air could be coming all the way from the Arctic to smash into much warmer, humid air over the South and East. Winds will be strong at the upper altitudes and on the surface. Depending on the temperatures and where you are, rain, snow, and ice could be widespread.
An upper-air map like the one depicted in Figure 4 could bring you a couple of days when flying a small airplane might not be a good idea. Those days would be good for curling up with a good meteorology text to learn how nature turns the potential energy of temperature contrasts into the kinetic energy that's grounded you.
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