When the public has the opportunity to ask meteorologists those questions that have troubled their minds, several are most frequently asked: Why is the sky blue? Does lightning ever strike the same place twice? and Is it ever too cold to snow? Another frequent question is: "If water drops are heavier than air, why do clouds float?" With lovely forms of cumulus clouds popping up all around me on this summer's day, that is the question I will tackle this month.
The steady visage of clouds gives us the appearance that the cloud is floating gently on air.
Usually the beginning to that question states: "If the water droplets that make up a cloud are heavier than air...." Yes, liquid water is denser than an equivalent volume of air by about a factor of 1000. But density is not the only factor pertinent to this discussion. But first, let's look at the weight of a cloud. (I consider only purely water clouds here. While some clouds are made of ice and many are a mix of ice and water droplets, the same principles apply to them as pure water clouds.)
On a warm summer's afternoon, I see a typically small cumulus growing over the western hills. I guess its volume to be about a cubic kilometre — many that size are in this range — and its base lies at an altitude of about 3500 metres above the ground. It is not difficult to calculate the total mass of that volume and it weighs in at about one billion kilograms. Within that mass are billions of cloud droplets comprised of liquid water and a dash of solid materials that were used as condensation nuclei. They account for about one million of those kilograms, thus giving the liquid cloud itself a weight approximately that of 700 standard-sized automobiles, though the droplets are spread across the whole volume. The cloud may look rather dense because each droplet scatters, reflects, refracts and diffracts the light rays passing through the cloud mass. The sum total of all this cloud–light interaction delineates the visible mass.
We know liquid water is heavier than air, otherwise spilling a glass of it would not wet the floor but would wet the walls. From the above estimates, we have seen there is a lot of mass in the liquid cloud, so what keeps the cloud afloat? Why doesn't gravity pull the cloud down to earth?
