thermal

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thermal

1. of, relating to, caused by, or generating heat or increased temperature
2. (of garments or fabrics) specially designed so as to have exceptional heat-retaining properties
3. Meteorol a column of rising air caused by local unequal heating of the land surface, and used by gliders and birds to gain height
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

thermal

[′thər·məl]
(meteorology)
A relatively small-scale, rising current of air produced when the atmosphere is heated enough locally by the earth's surface to produce absolute instability in its lower layers.
(thermodynamics)
Of or concerning heat.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

thermal

A column of rising air heated by the sun or other phenomena. Sailplane and glider pilots use it for soaring. The pilots recognize the presence of a thermal with the help of a variometer—an instrument having two tubes with red and green indicators. A red means a downward air current, and a green means an upward air current, or a thermal. See variometer.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved