So what is a thermal anyway? Well it’s pretty simple, a thermal is created when the sun heats the earth which in turn heats the layer of air directly above it. As this layer of air warms up the molecules spread out causing it to become less dense and therefore lighter than the air surrounding it. When the temperature difference becomes great enough the air breaks away from the surface and begins to rise, much like a hot air balloon.
However as a thermal begins to rise the surrounding air pressure decreases which allows it to spread out and now it’s temperature begins to drop. This drop in temperature do to a decrease in air pressure is known as Adiabatic cooling.
As long as the air temperature inside the thermal remains higher than the temperature of the surrounding air the thermal will continue to rise.
To read more information about thermals and paragliding weather in general see Understanding the Sky by Dennis Pagen.
When flying in a thermal it is very helpful to have a reliable vario like the Flytec 6005.