A thermal is a bubble or column of air which is warmer than the surrounding air, and thus of lower density, so it rises in search of equilibrium. Thermals typically form on sunny days with surface winds below 20 MPH. The sunlight heats a patch of ground, and the ground heats the overlying mass of air. The heated air has a lower density than nearby cooler air and very warm thermals can break free of the surface because of small disturbances such as a gust of wind, a passing tractor-trailer, etc. Thermals of lesser temperature differential to the ambient air can be pushed along the surface by gentle breezes without breaking apart - air masses of different densities mix surprisingly little.  If the thermal is pushed up onto a slope or over cooler ground the resulting increased temperature or pressure difference can act as a trigger, releasing the thermal. Thermals can rise thousands of feet.

Thermals cool as they rise, since the pressure is decreasing, as does the surrounding air, but as long as the thermal is warmer than the air around it, it will rise. Sometimes the rate of cooling with altitude of the air is less than the rate of cooling of the thermal and the thermal comes to a stop. Other times a thermal will stop when it encounters a temperature inversion, which is when there is a warm layer of air above a cooler layer. Thermals can also be broken apart by winds at higher altitudes. In moist climates thermals rise until they cool to the dewpoint at which time they form clouds.

Hawks, vultures, and pilots of sailplanes, hang gliders, and paragliders all use thermals to soar - to gain or maintain altitude when flying. The birds are able to conserve energy by not flapping, and the gliders are able to stay aloft without any mechanical power source. As long as a thermal is rising faster than the glider's sink rate (gliders are always sinking through the air) the glider can climb - typical climb rates are 100 to 1000 FPM (feet per minute). Birds and gliders stay in thermals by circling, and those with smaller wingspans and slower airspeed can fly smaller circles at lower sink rates than their larger counterparts, so they can utilize smaller, weaker thermals.

Thermals were virtually unknown (by Man) until adventurous glider pilots in the early 20th Century, who had been confined to soaring in ridge lift along mountains, began venturing away from the slopes as their aircraft became more efficient. The discovery of thermals has allowed us to understand why and how birds circle lazily in the air and has led to glider flights covering very long distances, as well as enhanced comprehension of certain meteorological phenomena.

Ther"mal (?), a. [L. thermae hot springs, fr. Gr. , pl. of heat, fr. hot, warm, to warm, make hot; perhaps akin to L. formus warm, and E. forceps.]

Of or pertaining to heat; warm; hot; as, the thermal unit; thermal waters.

The thermal condition of the earth. J. D. Forbes.

Thermal conductivity, Thermal spectrum. See under Conductivity, and Spectrum. -- Thermal unit Physics, a unit chosen for the comparison or calculation of quantities of heat. The unit most commonly employed is the amount of heat necessary to raise the temperature of one gram or one pound of water from zero to one degree Centigrade. See Calorie, and under Unit.


© Webster 1913.

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