Air Viscosity by Sutherland's Law

Dynamic viscosity measures how strongly a fluid resists being sheared, and for air it is the one property a Reynolds number for the wind past an object needs that a weather report does not give. This page finds it from the absolute temperature alone. Air gets more viscous as it warms, the opposite of a liquid, because a gas carries momentum by molecules colliding. Pressure does not appear in the equation, and the standard's own table uses it from sea level to 86 km.

The equation is $\mu = \beta T^{3/2}/(T+S)$, with $\beta = 1.458\times10^{-6}$ kg/(m s K^(1/2)) and Sutherland's constant $S = 110.4$ K. Because $S$ is a temperature that is added to $T$, the temperature must be absolute, so the page works in kelvins and the unit pickers convert from degrees Celsius or Fahrenheit. The two constants are the values the standard prints. This is a working formula, not an exact law: the standard itself warns that it fails at very low and very high temperatures and at great heights, so treat answers far outside the weather range as extrapolation.

The source is the U.S. Standard Atmosphere, 1976 (NOAA, NASA and the U.S. Air Force), which prints this as equation 51 and builds its table of viscosity by height from it. The page agrees with that table. At 288.15 K (15 degrees Celsius) it gives $1.78938\times10^{-5}$ Pa s, where the table prints $1.7894\times10^{-5}$. At 216.65 K, the cold air at 11 to 20 km, it gives $1.42161\times10^{-5}$ against $1.4216\times10^{-5}$. Two more rows, at 223.254 K and 236.215 K (temperatures from the standard's lapse of 6.5 K per km in the lowest layer), give $1.45767\times10^{-5}$ and $1.52711\times10^{-5}$ against the printed $1.4577\times10^{-5}$ and $1.5271\times10^{-5}$. The table prints five significant figures, so the agreement is to that precision.

Once you have the viscosity, divide by the air density for the kinematic viscosity, or put it in a Reynolds number with a speed and a length. As a hand check by a different route, at 300 K the equation gives $1.846\times10^{-5}$ Pa s, a little above the 15 degree value because the air is warmer.