Flying without an engine, gliders need to find a way to stay up in the air. One of the most common ways to do this is to seek out rising columns of air called ‘thermals’. When the sun shines onto the ground, it heats up the air just above. Occasionally, this warmer air breaks away from the ground and rises like an invisible bubble. Glider pilots learn how to find these rising columns of air. By circling tightly within such thermals, gliders can be lifted into the air, often gaining 300-400 feet a minute or more.
Clouds often mark the tops of thermal columns and pilots are taught to analyse the way clouds are forming and decaying around them to help find the best lift. Thermals are also formed on “blue” days, when there are no clouds, but they’re more difficult to find. Sometimes birds will help and glider pilots often find a good thermal by watching the behaviour of buzzards and seagulls flying nearby.
Thermals are normally used as a means of lift “up to cloudbase” and in this country will allow pilots to soar for many hours at heights up to about 5,000 feet, although, when flying in hot countries abroad, it is not unusual to thermal soar to above 12,000 feet and to find climbs of 1,000 feet a minute.