Weather

The dark, cold, and stormy days of winter can be a terrible time for flight training across the northern United States. Large civilian flight schools and military flight training are concentrated in the southern states because calmer weather gives students more time in the air.

On the other hand, learning to fly during a northern winter is bound to make a new pilot much more weather wise than a pilot who concludes, based on experience in Florida or Arizona, that afternoon thunderstorms are a pilot’s main weather concern. Learning to fly during a northern winter gives a student pilot plenty of practical experience in obtaining weather briefings during “interesting” weather.

Earth’s changing seasons occur because the planet’s axis is tilted at a 23.4-degree angle to its orbit around the sun. In late December the Northern Hemisphere is tilted away from the sun, and in late June it’s tilted toward the sun. From late June until late December the time between sunrise and sunset decreases, and each day the sun is slightly lower in the sky at noon than the day before. This process reverses on the winter solstice, with the time between sunrise and sunset slowly increasing and the sun climbing higher in the sky each day.

The table shows the difference in the time between sunrise and sunset in Marquette, Michigan, on Lake Superior, and Mobile, Alabama, on the Gulf of Mexico, in the middle of July—the warmest month of the year, on average—and the middle of January the year’s coldest month on average.

The sun’s heat keeps the Earth in a livable temperature range. The combination of shorter days and the sun being lower in the sky lowers temperatures where those conditions are experienced. Since the Earth takes a while to cool down or warm up, the year’s coldest temperatures in the Northern Hemisphere, on average, occur in January, not in late December when the sun is lowest in the sky. The warmest average temperatures are generally in July, a month after the summer solstice.

The northern United States is cold in winter not only because it receives less sun than places further south, but also because it’s closer to the Arctic, which receives the least winter sun of anywhere in the world. During the Northern Hemisphere summer, it’s winter south of the equator and the least-sunlight title goes to Antarctica.

On the winter solstice the sun does not rise anyplace north of the Arctic Circle, which is latitude 66 degrees 33 minutes north. As you travel north from the Arctic Circle the number of days each year with no sunrise increases until you reach the North Pole, where the sun rises once—on the March equinox—and sets once, on the October equinox, each year.

As cold air travels south from the Arctic the air is warmed. Blasts of cold air reach as far south as Mobile, Alabama, from time to time, but have lost much of the punch they had when they crossed the border from Canada.

The temperature differences between the northern and southern parts of the contiguous 48 states are much greater during the winter than in the summer. Such temperature differences supply the energy for the kinds of large storms that dominate the cooler parts of the year.

Meteorologists call such storms extratropical cyclones, which means they form outside the tropics or the subtropics. Extratropical cyclones include masses of both cold and warm air with fronts separating them. The greater the temperature contrast between the cold and warm sectors, the stronger the storm can grow.

Such storms begin along a generally stationary boundary between warm and cold air known as a stationary front. The heavier cold air begins flowing under the lighter warm air, pushing the warm air up. The greater the temperature difference, and thus the density difference, the stronger the push of cold air under the warm air. As the air moves, the Earth’s rotation gives it the counterclockwise (in the Northern Hemisphere) circular motion that characterizes storms.

An extratropical cyclone’s mix of warm and cold air helps it create the variety of precipitation that grounds VFR training and can make instrument training dangerous. At any time such a storm can be producing rain, freezing rain, snow, dangerous winds, and low ceilings and visibility. Such storms can also send lines of thunderstorms across the southern states, grounding training flights in places such as Florida for at least part of a day.