A wind that changes direction and velocity frequently indicates that thermals are drifting through and past your site. Dust devils are organized cores of plume thermals. Clouds associated with thermals include low to mid level cumulus clouds, and cumulonimbus clouds. Thunderstorms, tornadoes and hurricanes are also associated with strong lift but are best studied from a great distance. Structure of Thermals Thermals are convection systems that range in diameter from a meter or less near the ground to a thousand meters or more at several thousand meters above the ground.
Thunderstorms are giant thermal systems and often have more than one convection cell.
The first diagram on the right illustrates a vortex ring thermal, the diagram on the left shows a plume thermal. In each case, straight black arrows indicate wind direction, curved black arrows shows the circulation of air, and red arrows indicate the flight path of a butterfly. A series of crossbars on the red line indicate flapping flight, absence of crossbars indicate gliding, or soaring flight. The diagram on the right also shows that butterflies can fly under a vortex ring without encountering lift or sink. The diagram on the left shows that plume thermals lean downwind because wind velocity increases with altitude.