KnowHOW team explains: The density of an ideal gas at a particular pressure and temperature is proportional to its molecular mass. Water vapour has a molecular mass of 18, and is, therefore, lighter than other major constituents of the atmosphere ? oxygen (32), nitrogen (28), carbon dioxide (44) and ozone (48). Clouds are formed, however, not simply from water vapour, but from vapour that has condensed to droplets of water and ice crystals under the influence of cold air at high altitudes.

They are denser than air but they do not actually float. Because the droplets and crystals are so small, and therefore fall very slowly, their lateral movement due to the effect of the wind is greater than their downward movement due to gravity. They are effectively carried along by the wind and thus, to an observer on the ground, they appear to be floating.

Also, though clouds are subject to the influence of gravity, the viscous drag of the air prevents them reaching any great speed. The downward velocity is usually much smaller than the upward velocity of the rising warm, wet air which created the cloud in the first place; so the cloud does not fall.

If a droplet grows big enough, or enough of them coalesce together to make a larger droplet, gravity will get the better of viscosity and the cloud will fall as rain.

The question was sent by Arun Kumar Mukherjee from Shillong