Why don’t clouds fall to the ground?

Posted on Jan 12, 2021      300

Everyone knows that clouds comprise tiny water droplets or ice crystals. The water droplets in a cloud vary in diameter, from fractions of a micrometer to several millimeters. No matter how small the ice droplet is, it is still heavier than air. Therefore, a natural question arises: How are the water droplets (and with it the cloud) held in the air and do not fall to the ground? Another question arises: under what conditions do water droplets stop being held in the air and fall to the ground as rain?

Let’s start with the smallest droplets with a radius of fractions of a micrometer. Such droplets are prevented from falling down by random blows from air molecules in a chaotic thermal motion. Such motion is called Brownian motion - after English botanist R. Brown, who discovered it in 1828. Air molecule strikes force the droplet to bounce in many directions; as a result, it moves along a freakishly jagged trajectory.

The heavier the droplet, the harder it is for the air molecules to move it, and hence the smaller the role of Brownian motion. But the influence of the Earth’s gravity increases. When the drop’s radius becomes more than a micrometer, its motion ceases to be Brownian. The drop falls under the action of gravity, gradually speeding up. Then an additional factor that prevents the drop to fall downwards - resistance of the air environment - starts playing an important role. Simultaneously with acceleration of the drop, the force of air resistance acting on the drop appears and grows. It is opposite to gravity and proportional to the velocity of the drop.

As the drag force increases, the velocity of the droplet increases more slowly. When the force of air resistance is equalized in modulo with the force of gravity, the further increase of the velocity of the drop stops, and further the drop falls evenly. Such a uniformly moving droplet can be slowed down and even thrown upward by an ascending stream of warm air. And the earth, which is heated by the sun, is a constant source of such upward air currents.

In addition, in falling, the droplet can simply evaporate. Or break up into smaller droplets.

But the droplet may, on the contrary, enlarge: merge with others or condense additional vapor on its surface and then it will still fall to the ground. This is how precipitation falls. We can even say that precipitation (rain or snow) is the fall of clouds to the ground, only in fact, raindrops or snowflakes are too large and heavy to be components of clouds.