Thermals

On a summer's day, you can see birds circle upwards without flapping their wings. They are "thermalling". A thermal is a volume of air that has been heated by the sun more than the surrounding air - imagine if you were standing on some sunlit concrete, you would feel warm! As you know, hot air rises, and it is circling within this air that allows birds - and gliders - to go upwards. Next you may ask "so how do you know where the thermals are?". Well, sometimes this is educated guess work, based on how you imagine ground features below you are warming up. However, often cumulus (cotton-wool type) clouds form at the top of the thermal, marking where the thermals are. Another good indicator of a thermal is other pilots! If you are out in the country on a sunny summer's day, particularly if you are near a gliding club, you may sometimes see a group of gliders sharing a thermal.

Occasionally, you may be joined in the thermal by a bird - from a swift to an eagle. You can never do it as well as they can, but it's great fun trying! Thermals are used in cross-country flying - you climb in a thermal to gain the height to move forwards to the next thermal on track (or thereabouts).