thermal draft

Thermal updrafts are a natural phenomenon produced by localized heating of air. This causes air to rise and fall in relatively small localized areas.

A simple experiment to demonstrate this effect. The diagram on the left shows how the top half of a bottle can be used to trap heat from the sunlight. As the air warms up inside it rises and exits the top of the bottle. If a simple propeller is places above, it will turn due to the air passing through it. 

jpr: My back 10 acres is half meadow. The thermals cool my deck and may on occasional be strong under the shade trees.