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In the atmosphere table, I have added one more quantity that I find useful, although I have never seen it in another atmosphere table. This is the ratio of speed of sound to the kinematic viscosity. This quantity has units of 1/length. Multiplying this number by a reference length and then by the Mach number yields the Reynolds number.

For example, if I am cruising at M=0.8 at 34000 ft. in a jet transport with a chord of 16 feet, then from the table at 34000 ft, this ratio is 2.48 ft-1, and so the Reynolds number is 2.48*16*0.8=31.7 million (the million is built in). I realize that this only saves one division, but I seem to do it so much faster, probably because I don't have to deal with powers of 10. In the SI table this ratio is given in units of 1/m while it is 1/ft in the US table.

If you work in US customary units and you can remember that this ratio is about 7 at sea level and about 2-3 in the lower stratosphere and about 0.5 for SST cruise, then you can estimate Reynolds number in your head. Astound your colleagues!

PDAS Home > Contents > Std Atmosphere > Tables > Ratio
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