found: Work cat.: Cone, C.D. The design of sailplanes for optimum thermal soaring performance, 1964:p. 1, summary ("The meteorological aspects of isolated buoyant convections (thermals) are discussed in relation to efficient soaring design ...")
found: McGraw-Hill dictionary of scientific and technical terms, 2003("thermal ... 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")
found: Chambers dictionary of science and technology, 1999("thermal --- (aero[nautics]), (meteor[ology]) --- an ascending current due to local heating of air ...")
found: Academic Press dictionary of science and technology, 1992("thermal ... a localized air current that rises aloft when the lower atmosphere is heated by earth's surface enough to produce an absolute instability over a certain area; a common source of low-level clear-air turbulence")