Thermals and Thermalling

It is raining today, Monday, but the rest of the week looks like good soaring weather with a possible ridge day on Friday.

Iíll have my hernia surgery tomorrow, so I may not be able to fly for a few days.

A new printing of our popular Glider Pilot Logbook is finished, so we should have a new supply in the next day or two.

The 2006 edition of the popular Segelflug Gliding Calendar is on its way to us. You can order from our web site now.

Thermals & Thermalling

Iíll summarize the recent thermalling ideas today and proceed to glider safety while I am recovering.

There are many myths in soaring. Some are dangerous and contribute to accidents while others simply do not permit pilots to fly as well as possible. Thermals and thermalling include a couple of less serious myths.

Thermals are air made less dense, forced to rise by gravity pulling down more dense (dry/cold) air from aloft. The sun heats the ground causing heating, but more important, evaporation of moisture in/on the ground. Moist air is less dense than dry air, thus more buoyant.

Rising air has considerable buoyancy, and mass, thus once it begins to move upward, it has lots of inertia. A cloud 3,000 feet in diameter weighs roughly 9 tons! (See text below.)

Anyway, the surface of the earth is heated, which warms the thin layer of air above it, but as soon as the air begins to rise, it quickly mixes with the ambient air, so the temperature of a thermal a few hundred feet above the earth quickly becomes the same as the surrounding air. For the soaring pilot, thermals are probably best thought of as a column of bubbles of air breaking loose from the earth, each bubble rising at different rates, mixing with, and passing by bubbles that preceded them.

This explains why it is seldom a pilot can make more than a couple of turns before needing to adjust the center of the circle to be in the better lift.

Air on the outside of the thermal is descending, so a glider passing nearby would have the wing close to a thermal forced down. It is a myth to believe a rising wing indicates a thermal on that side. Once inside a thermal, I suppose it is possible for a wing to be lifted by better lift, but with the turbulence associated with thermal lift, I doubt if it is possible to accurately interpret such wing wags as signs of better lift.

Understanding thermal lift production helps a pilot understand the blue areas of the sky without clouds are signs of sinking air, and are just as important to the glider pilot as watching the clouds as signs of lifting air.

* * * *

According to a meteorology text I have, the water content (density) of clouds varies quite a bit, from about 1/10 gram per cubic meter to over 5 grams per cubic meter. It is even harder to come up with an "average" cloud volume, as this varies even more widely than water content. Nonetheless, let us use Dr. Topper's formula to get at least a rough order-of-magnitude estimate. We can model the cloud as a sphere of, say, 1 kilometer radius. This gives a volume of about 4 billion cubic meters. Then if we use 1 gram per cubic meter as a "representative" water content, we get an estimate for the mass of the cloud of 4 billion grams, or 4 million kilograms.