I was flying home to Michigan after all the thunderstorm and tornado excitement of Sun ’n Fun. If there has ever been a major air show with worse and more severe weather than Sun ’n Fun this year, I can’t remember it. It’s just amazing that the loss of airplanes wasn’t greater, and that nobody was seriously injured.
But for my trip home the severe weather, thunderstorms, and springtime snow blast had moved up the East Coast, leaving a windy high pressure system in its wake. High pressure usually means good flying weather in terms of good visibility and absence of low cloud cover, but it can also be among the most miserable flying conditions ever. This was one of those miserable days because the air was so unstable. You can find unstable air anytime of the year, but in the spring, it is more common, and the degree of instability can be extreme.
The mark of unstable air is cooling with altitude. The warmer air at the surface rushes up into the clear colder air aloft. When the rising air cools to the dew point a layer of clouds – usually scattered or broken – will form. Depending on how much moisture is in the air, these clouds are often small cumulonimbus with the tops often stretching up to touch the freezing level.
When a strong instability exists you can bet your house on one thing – turbulence. The rising air is enough to disturb an airplane in flight. The parcels of air do not rise at a uniform rate so there are bumps when you transition from one lifting air current into another. But the real culprit is the wind.
As the wind blows through the rising currents of air it jumbles the air mass into a big turbulent mess. Visualize how a fast moving stream of water reacts when it encounters a rock in the stream and you can imagine how the air molecules are behaving when a strong wind blows through a rising mass of air.
The bad news is that the spring is also a season of sharp contrast between areas of high and low pressure, and of temperature extremes. That means strong winds are common. Blow a strong wind through an unstable air mass and you have hit your head on the ceiling turbulence. And that’s what I had to fly through most of the way from Florida to Michigan.
Clouds are often more turbulent than the clear air because it is the rapidly moving and cooling of the air that helps to form a cloud. But in a springtime unstable air mass the bumps can be almost as bad in the clear air.
However, there is usually smoother air above the tops of the clouds, if you can get there. The cloud tops typically mark a change in air temperature and indicate where the air has slowed its rise. With a more stable layer above the clouds, wind is the main turbulence producer, so if you can find a spot between layers of wind shear, there is a smooth ride.
But altitude alone does not guarantee a smooth ride. On this day there were plenty of complaints from jet crews flying above 30,000 feet. Sometimes the air gets stirred up by the contrasts of temperature and pressure all the way up.
There is, however, one advantage of flying through a springtime unstable air mass: free lift. The rising air pushes the airplane up and to hold altitude you have to put the nose down, so airspeed increases. On this day my cruise speed was averaging 15 knots faster than normal.
The only problem was that a few times I had to pull the power back because the indicated airspeed was close to the yellow arc and I didn’t want to fly into that yellow airspeed zone that is reserved for smooth air only. In the spring these areas of rising air can go on for many, even hundreds of miles. I guess at some point the rising air comes back down, but not along my route that day.
On a brilliant spring day with a bright blue sky filled with scattered puffy white clouds after the bad weather has moved on, a non-pilot looks up and says, “What a great day to go flying.” Little do they know what the air is really doing on a day like that. But we do.