|This article relies largely or entirely upon a single source. (July 2009)|
A thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection, specifically atmospheric convection. The Sun warms the ground, which in turn warms the air directly above it. Dark earth, urban areas and roadways are good sources of thermals.
The warmer air near the surface expands, becoming less dense than the surrounding air mass. The mass of lighter air rises, and as it does, it cools due to its expansion in the lower pressure of the higher altitude. It stops rising when it has cooled to the same temperature as the surrounding air. 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 said to be 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 cumulus 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 soaring and 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.
Thermals on the sun typically form hexagonal prisms (Bénard cells).
The most likely principle right now is detecting the motion of things carried along in the air. Doppler lidar (like radar, but with light) bounces light off dust particles in the air and measures their speed. This technology is already used in meteorological research to see thermals. Airborne versions are under development to see wind shear. Regular weather radar can see birds and insects, and measure their speed. Nexrad images are being used by researchers who study insect and bird migrations. Other basic principles might work too. Thermal gradients lead to differential diffraction of electromagnetic radiation, like the shimmers you see on highways in the desert. These can be seen on radar and lidar images. Thermals are more humid than the surrounding air. The same infra-red imagery that shows water vapor from satellites might show this signature of thermals. The FLIR infrared receivers used for night vision in some military applications see thermals. Radar could see circling birds much further than your eye, to say nothing of circling gliders. Computer processing of regular video might alert you to birds and gliders you wouldn’t pick up visually. A commercial anti-collision system is already on the market based on this principle. Some of these systems are currently too large and power hungry to fit in gliders. But electronics shrink before our eyes. Small, lightweight and low power versions of anything are only a matter of a short time. Also, passive radar and maybe lider might be used, avoiding the power and legal issues of sending a signal. Passive radar compares the primary and return signals generated elsewhere (weather radar, FAA) to produce an image.
- Bradbury, Tom (2000). Meteorology and Flight: Pilot's Guide to Weather (Flying & Gliding). A & C Black. ISBN 0-7136-4226-2.
- What do thermals look like? - Thermal Structure and Behavior by Wayne M. Angevine
- Time-lapse video of clouds caused by thermals forming and decaying