The main reason for this is that on the average, for most places in the middle latitudes, the air tends to be more stable in the fall than in the spring.
Air stability is one of those topics that can quickly mire you in some pretty heavy-duty science, but it is also easy to understand on a more intuitive level.
First, think of what you mean when you say that a person is "unstable." You usually mean that it doesn't seem to take much to cause this person to begin shouting, maybe threatening to hit someone or even start throwing punches - think of road rage.
An unstable atmosphere is more or less the same thing. When the air is unstable the weather goes wild with thunderstorms, or at the very least it produces up-and-down air currents that can make a ride in a small airplane bumpy.
For someone who's never been up in a small airplane, even light turbulence can be frightening. Also, a new student pilot might not be sure whether the left wing dipped because he moved the yoke or if the turbulence did it.
What makes the atmosphere unstable?
Without getting bogged down in details, the atmosphere tends to be unstable when the temperature drops rapidly with altitude. The greater the temperature contrast between cold upper levels and warm lower levels, the more unstable the air will be.
Think of it this way: As air cools, it becomes denser, or heavier. If you pile objects up with the heavier stuff on top, the pile is more likely to topple if you give it a push than if you had put the heavy stuff on bottom. The pile is unstable. While this isn't exactly how the atmosphere works, it's a good way to remember that very cold air atop warm air tends to be unstable.
A little more basic atmospheric science will help you see why fall days tend to be more stable than spring days.
Solar energy hardly warms the atmosphere as it passes through it on the way to the Earth's surface. Energy that's not reflected away warms the land or water that it reaches. As the land and water warm up, they heat the lower levels of the atmosphere, which is why the lower atmosphere is warmer than the air aloft even though the air aloft is closer to the sun. (Since the sun is around 93 million miles from the Earth, the six- or seven-mile depth of the lower atmosphere isn't enough to make much difference.)
As winter ends, both the ground and the air - near the surface and aloft - are cold. With spring days growing longer and longer and the sun higher and higher in the sky each day, the ground rapidly begins warming. The air aloft, however, warms much more slowly, which adds to the warm-cold contrast between the lower and upper levels of the atmosphere. The air becomes more unstable.
In fact, the up-and-down air movements caused by spring and summer instability help to warm the upper atmosphere by carrying warm air aloft and bringing cool air down.
By the end of summer, both the ground and the upper levels of the atmosphere have warmed. Now, as the days grow shorter and the sun is lower in the sky each day, the ground begins to cool off as heat radiates away into space during the longer and longer nights. This reduces the ground-upper atmosphere temperature contrast, making the air more stable.
The result on many fall days is much smoother air and more comfortable flights for a student who hasn't yet become used to small airplanes.
Across the northern parts of the country, pilots and passengers in small aircraft can enjoy fall foliage views that those viewing the seasonal colors from their cars can only dream about. Sometimes the air is so smooth that the carpet of color below seems to be scrolling by as your airplane hovers in the sky.
Fall does have some weather drawbacks, with fog probably the biggest one over the widest part of the country. Those longer and longer nights that cool the ground also are the perfect setup for fog to form.
As the air near the ground cools, the water vapor in it condenses into tiny water droplets that become a cloud that's sitting on the ground.
An even bigger concern is that the days are getting shorter and shorter. At 40 degrees north latitude - roughly from Philadelphia to Indianapolis to Denver to Reno, Nevada - there are about 13 hours and 5 minutes between sunrise and sunset on September 1. Add about 30 minutes of civil twilight before sunrise and after sunset and you have about 14 hours available for daytime flying. (During civil twilight it's light enough outside for outdoor activities without artificial light. The federal aviation regulations define night as "the time between the end of evening civil twilight and the beginning of morning civil twilight.")
By October 1, daylight flying time - including civil twilight - is down to about 13 hours at these latitudes, and by November 1 it's only about 11 and a half hours. These shorter days obviously squeeze flight scheduling.
Even more important, the nights are getting shorter even faster in northern Canada where the cold air that brings most of the United States its winter cold and storms is manufactured. At 60 degrees north, which is the southern border of Canada's Northwest Territories, the day is fewer than nine hours long by November 1 and rapidly growing shorter.
These short days allow the air to cool down to zero and below. You can think of the cold air "piling up" over northern Canada until the pile becomes too large and the cold air begins spilling south.
As November moves into December, blasts of cold air - often with snow and an increasing chance of icing aloft - are moving south across the United States more often. Unless you are in the South or on the West Coast, you are getting into the new game of winter flying.
In other words, anyone who begins learning to fly in the fall should try to get in as much training as possible before winter blasts in with its collection of challenges. A student who's mastered the basics on fall's stable, clear days will be more prepared to begin learning about winter flying.
Fall doesn't necessarily offer the best flying weather in all parts of the United States.
Florida and places along the Gulf of Mexico, where the seasons aren't as pronounced as they are farther north, are characterized more by wet and dry seasons, with summer being the wet season. A good share of the wet season rain comes from thunderstorms.
In Mobile, Alabama, for example, July averages 7.7 inches of rain with thunder heard on 18 of the month's 31 days. October, the driest month, averages only 2.8 inches of rain with thunder on only two days.
Go south to Orlando, Florida, and July is still the month when thunder rumbles the most - the same as Mobile, on 18 days, bringing a monthly average of 7.2 inches of rain. But in Orlando November, December, and January are the dry months, each with only one day of thunder. December is the driest with only 2 inches of rain. November averages 3.5 inches and February 2.8 inches.
While winter is the dry season in Florida and along the Gulf Coast, it's the wet season on the West Coast. San Francisco, for instance, averages zero rain in July but 4.5 inches in January.
Each region of the country offers its unique climate. If you've lived in an area for at least a couple of years you probably have a pretty good idea of which season should be best for beginning to learn to fly. Still, it doesn't hurt to check.
Of course, if you have decided to learn to fly you probably don't feel like waiting until your region's "best" flying season. No matter where you are, all times of the year will produce both good and bad days, and you should learn how to handle the weather year-round wherever you are flying.
No matter what season you're flying in, some parts of the day are likely to be better than others. In the fall, for instance, the chance of fog means that you are probably better off not scheduling dawn flights. At times of the year when the air is most unstable and thunderstorms are most likely, dawn flights are your best bet because the up-and-down air movements that make flights bumpy grow as the ground warms.
Most of us in our daily lives rarely give a thought to nature when we're planning our hourly, daily, or even yearly schedules. Flying, on the other hand, demands respect for the natural world of the weather. Even if you are by nature a "throttle to the firewall, charge ahead" type, your life as a student or low-time pilot will be a lot more comfortable when you listen to what Mother Nature has to say about when to fly.
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.
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