Weather

For my first few afternoon lessons flying from the Ledgedale Airpark just outside Brockport, New York, in 1976, the wind was calm and the air smooth. I was comfortable and believed I was getting the hang of things as the Cessna 150 responded to the controls as my instructor said it would.

For my third or fourth lesson, a few puffy clouds were overhead and a little wind was blowing. Once we were aloft, the airplane hit unexpected bumps now and then. Flying the pattern and lining up with the runway to land wasn’t as easy as it had seemed before.

I understood the need to compensate for the wind, which was trying to push the Cessna to one side of the runway. But the bumps were disconcerting.

Fortunately, I had read enough about flying to know the turbulence I was feeling wasn’t going to make us crash. Nevertheless, it made concentrating on learning to fly a little harder.

Anyone who wants to fly—either as a pilot or even as a relaxed, comfortable passenger—has to get used to turbulence. Most of the time the air is moving at least a little, horizontally and often vertically. Often it’s moving smoothly, but not always.

Turbulence that hits airliners in cruise flight can be so strong that it can injure passengers who don’t follow instructions to keep their seat belts fastened when they’re seated, and flight attendants whose jobs require them to move about the cabin.

Forget about ‘air pockets.’ To begin understanding turbulence, destroy any ideas about “air pockets.”

The idea of “air pockets” could have originated with early pilots. It’s easy to imagine Bob Barnstormer thinking the bumps in the air he felt as he flew his Curtiss Jenny were like the holes in the road that rattled his 1924 Stutz Bearcat. Bob was a great stick-and-rudder pilot, but not well versed in atmospheric science.

The sudden bump you feel when your airplane hits turbulence can make you feel as though the wings ran out of lift because no air is flowing around them. This isn’t what’s happening.

Turbulence means air is moving roughly. Like flowing water, moving air is sometimes smooth, sometimes turbulent. Imagine the water in a whitewater river. When the water flows over a rock it first goes up, and then comes down, creating a wave downstream from the rock.

Nature—sometimes with human help—has several ways of breaking up the smooth flow of air to create turbulence.

A good, simple example is what’s called “mechanical turbulence.” Wind blowing around hangars, trees, and other obstructions near a runway becomes turbulent as it whirls and moves up and down.

Climb to escape fair-weather cloud turbulence. A common kind of turbulence is caused by air that is rising because it has been heated by the warm ground or water. Such columns of rising air are called thermals, and glider pilots love them because they are a way to climb without an engine.

As the air rises, it cools, and the water vapor in it condenses into clouds. These are called fair-weather cumulus clouds because they form on mostly sunny days, which should remain calm for at least a few hours.

Under the right conditions, these clouds don’t grow higher and will fade away late in the afternoon as the sun goes down and the ground begins to cool. The flat bottoms on some of the clouds show where air that is rising into them becomes cool enough for water vapor to condense. Clouds with ragged bottoms no longer have air rising into them.

On a day like the one pictured above, you’re likely to have a bumpy ride until you climb higher than the tops of the clouds. On the other hand, if the clouds are growing into towering cumulus clouds, the air could be turbulent up to at least the tops of the clouds, and you need to be alert to thunderstorms forming.

Wind shear causes turbulence. Wind shear is defined as “a change in wind speed and/or direction over a short distance. It can occur either horizontally or vertically and is most often associated with strong temperature inversions or density gradients. Wind shear can occur at high or low altitude.”

The illustration above shows how winds in close proximity flowing in different directions stir up the air to create turbulent eddies, which are one kind of wind shear.

If the winds were blowing in the same direction but with faster winds at one level, they also would create wind shear. Finally, winds blowing roughly the same speed don’t have to be blowing in opposite directions to create shear, but can be blowing at an angle to each other.

The mathematics of wind shear can become complicated because differences in both speed and direction have to be taken into account.

A good rule of thumb you can use to anticipate wind shear when planning a cross-country flight is to check the differences in speeds and directions of the winds aloft, which are given for every 3,000 feet. If the difference in wind speed between two reported levels, such as for 6,000 feet and 9,000 feet, is more than 18 knots, wind shear could make a flight between those altitudes bumpy.

Wind shear is common near the frontal boundaries separating air masses with different characteristics, such as a cold front with advancing cold air replacing warm air. This is one reason you should be ready for some turbulence if you fly across a front.

A general rule is that the greater the difference in temperatures on opposite sides of the front, the more wind shear turbulence you should expect. Since the air is usually cloudy near fronts, this is something you shouldn’t have to worry about until you have an instrument rating.