The amplitude of the traveling thermal wave decreases strongly ( exponentially ) along its propagation direction, but if its propagation distance in the condensed phase is not too long, its amplitude near the gaseous phase is sufficient to create detectable pressure changes.
12.
Earlier, the pair spent more than five hours climbing to 9, 150 meters ( 30, 000 feet ) as they searched over a 135-kilometer ( 85-mile ) stretch of sky for the lift needed to reach mountain-soaring thermal waves some 1, 800 meters ( 6, 000 feet ) above them.
13.
In this way pollution of the measurement from adjacent regions would be avoided, i . e . the measurement window could be confined to a suitable fraction of the time of flight of the thermal wave ( using a Fourier analysis of the response could provide a similar outcome by using the high frequency components ).
