Dynamic Viscosity of Air at 20 °C

Air's dynamic viscosity is tiny, but the number that governs flow regimes is the kinematic viscosity ν = µ/ρ, and because air is a thousand times less dense than water its kinematic viscosity is 1.51 × 10⁻⁵ m²/s — fifteen times water's. That is why, at the same velocity and duct size, airflow has a Reynolds number fifteen times lower than water, and why boundary layers in air are comparatively thick.

Gases invert the liquid temperature trend: air gets more viscous as it heats, since viscosity in a gas comes from momentum transfer by molecular collisions rather than from intermolecular attraction. Sutherland's law gives about 21.8 µPa·s at 150 °C. Pressure has almost no effect until you approach vacuum. Use this value for duct friction, fan-law corrections, particle settling in air-cleaning equipment and drag calculations — and remember that the density term in Reynolds number changes with altitude and temperature far more than the viscosity does.