Empirical Earth · Gravity, Buoyancy & the Air

Why clouds stay up

The claim

“A cloud weighs millions of pounds, so it should fall — it floats, so gravity is wrong.”

What is measured

Clouds float because moist air is buoyant in a gravity-stratified atmosphere; their flat bases mark the condensation height, typically ~1–2 km up.

What would show this is wrong

clouds that ignored air density and temperature, or stayed aloft with no buoyant support.

Sources

  1. Cloud microphysics & droplet terminal velocity. Stokes' law for small spheres; cloud liquid-water content and droplet fall speeds. Rogers & Yau, A Short Course in Cloud Physics ; NOAA/UCAR a link
  2. The buoyancy of moist air. Because water vapour (18 g/mol) is lighter than dry air (~29 g/mol), humid air is less dense than dry air at the same temperature and pressure — so moist air rises link
  3. Cloud droplets settle slowly; rain is droplets that grew. Below ~40 µm a droplet’s Stokes fall speed scales as radius squared (~1 cm/s at 10 µm), so gentle updrafts hold a low-density water link
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