Thermals on Earth
The warmer air nearer to the surface expands, becoming less
dense than the surrounding air. The lighter air rises and cools due to its expansion in the lower pressure at higher altitudes. It stops rising when it has cooled to the same temperature as the surrounding air. Dark earth, urban areas, and roadways are good sources of thermals.
Associated with a thermal is a downward flow surrounding the thermal column. The downward-moving exterior is caused by colder air being displaced at the top of the thermal.
The size and
strength of thermals are influenced by the properties of the lower atmosphere (the
troposphere). Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and can rise like a hot air balloon. The air is then referred to as unstable. If there is a warm layer of air higher up, an
inversion can prevent thermals from rising high and the air is said to be stable.
Thermals are often indicated by the presence of visible
clouds at the apex of the thermal. When a steady wind is present, thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by
glider pilots. Cumulus clouds are formed by the rising air in a thermal as it ascends and cools, until the
water vapor in the air begins to
condense into visible droplets. The condensing water releases
latent heat energy allowing the air to rise higher. Very unstable air can reach the
level of free convection (LFC) and thus rise to great heights condensing large quantities of water and so forming showers or even thunderstorms. The latter are
dangerous to any aircraft.
Thermals are one of the many sources of
lift used by
soaring birds and