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Title: Thermals  
Author: World Heritage Encyclopedia
Language: English
Subject: Lapse rate
Publisher: World Heritage Encyclopedia




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.





Thermal detectors

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.

See also


External links

  • What do thermals look like? - Thermal Structure and Behavior by Wayne M. Angevine
  • Time-lapse video of clouds caused by thermals forming and decaying
