Cloud on Blue sky
in Tech

How Do Clouds Float?

You look up at the sky and the views are always beautiful. It’s the privilege of having that blue dome above our heads, brightening our day.

Sometimes it’s gray and fine threads of rain fall, it’s just as beautiful, and other times it’s the clouds that are suspended up there like feather castles in the air. We don’t ask ourselves many questions, beyond imagining that they have many more shapes than they actually do, but… are they really floating? And how?

According to a recent article from Live Science, it is simply a kind of illusion. It’s not as if there were a cushion or something magically floating in the air. A cloud is a collection of water droplets and ice crystals. These droplets form around a cloud condensation nucleus, which could be a speck of dust or salt.

Even before the rain, these droplets are heading towards the Earth, although at a slow pace

When a water-laden cloud grows too large, the precipitation falls in the form of rain, snow, or hail. But even before the rain, these droplets are heading towards the Earth, albeit at a slow pace. They fall very (but very, very) slowly and everything that falls reaches what is known as terminal velocity, or its maximum possible speed when falling freely.

Terminal velocity occurs when the drag force of the air perfectly counteracts gravity. Water droplets are so light that their terminal velocity is super slow: between 18 and 36 meters per hour for a droplet with a radius of 5 to 10 microns. Since clouds are typically thousands of feet high in the atmosphere, this slight downward shift is not noticeable to the naked eye.

Interestingly, specks of dust that swirl in a ray of sunlight also fall, but because they are tiny, they fall slowly. The average size of a water droplet in a cloud is smaller than the radius of a human hair. But something counteracts this slow descent, and that’s where the illusion comes in. Upward air currents keep the merged droplets suspended, even as they gradually fall.

The average size of a water droplet in a cloud is smaller than the radius of a human hair

They appear to float because they are essentially falling at a rate equal to or less than the speed of the upward current in the cloud. The water droplets become “markers of air movement”. That is, the upward air pushes millions of tiny droplets along its path, forming the visible cloud. But it’s not just this simultaneous fall and lift at play; while clouds appear at a relatively fixed height, they fluctuate as the upward air mixes with the droplets as they condense and evaporate. In reality, they are forming and evaporating at a rate that makes them seem somewhat stationary.

A cloud is the visible result of vertical movement and air mixing with water, while the droplets slowly fall to the ground. You don’t really see the movement of the cloud droplets at all. All you really see is the tracer of large-scale movement in the atmosphere.

A cloud is the visible result of vertical movement and air mixing with water, while the droplets slowly fall to the ground

Cloud formation requires warm, humid air. Warm air is more buoyant than cold air, so it rises into the atmosphere and then condenses into a cloud as it cools. The cloud is less dense than the air beneath it. While some clouds look light and fluffy, a cumulus or storm cloud can weigh as much as 100 elephants, although its mass and water content depend on its dimensions. In a smaller cloud of only a few tens of meters high and wide that will not precipitate imminently.

You look up at the sky and the views are always beautiful. It’s the privilege of having that blue dome above our heads, brightening our day. Sometimes it’s gray and fine threads of rain fall, it’s just as beautiful, and other times it’s the clouds that are suspended up there like feather castles in the air. We don’t ask ourselves many questions, beyond imagining that they have many more shapes than they actually do, but… are they really floating? And how?