Articles on Thermal Science

    STRUCTURE AND BEHAVIOR

Getting to the Core

Wayne Angevine tells us, based on observations, that the normal thermal is from several hundred meters wide to two kilometers wide. We know for our flight experience that thermals are large. The clouds are often large and the bumping and gusting associated with them is spread over a pretty large area. But we also know that the core, let's define that as the strong lift that is usable for climb, is small. Maybe less than 500 feet in diameter in the average Eastern thermal. Because these cores are what we're really interested in we usually call them thermals.) We like to post an article that bridges the gap between Angevine's large thermals and the small cores we use. How and why does the core form?

Centering and Climbing

Tighten up

Doug Jacobs does a little exercise on thermal size in his advanced cross country classes. He asks: How big do you think is the diameter of the average thermal. 200 feet? 500? 1,000? 2,000? The guesses can be all over. But then he says: “A speed of 60 knots is about the same as 100 feet per second. So let’s see what a 1,000 foot diameter thermal would sound like on the audio. Somebody time me for 10 seconds.” He then does a pretty good impersonation of the  sound a variometer makes to signal climb.

Before the 5^th second has passed the exercise has already become laughable. The thermals we’re used to in the East are clearly smaller than 500 feet across.

Now that you know how small a thermal is, how big is the circle you fly in your glider?