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In regard to viscosity, it may be noted that air is highly viscuous—relatively much higher than water. Density for density, the viscosity of air is about 14 times that of water, and consequently the effects of viscosity in air are of the utmost importance in the calculation of resistance of moving parts.

Atmospheric air at sea level is about 1/800 of the density of water. Its density varies with the altitude and with various atmospheric conditions, and for this reason the density is usually specified "at sea level" as this altitude gives a constant base of measurement for all parts of the world. As the density is also affected by changes in temperature, a standard temperature is also specified. Experimental results, whatever the pressure and temperature at which they were made, are reduced to the corresponding values at standard temperature and at the normal sea level pressure, in order that these results may be readily comparable with other data. The normal (average) pressure at sea level is 14.7 pounds per square inch, or 2,119 pounds per square foot at a temperature of 60° Fahrenheit. At this temperature 1 pound of air occupies a volume of 13.141 cubic feet, while at 0° F. the volume shrinks to 11.58 cubic feet, the corresponding densities being 0.07610 and 0.08633 pounds per cubic foot, respectively. This refers to dry air only as the presence of water vapor makes a change in the density. With a reduction in temperature the pressure increases with the density increase so that the effect of heat is twofold in its effect.