Cumulus cloud closeup
in Lifestyle

How do clouds float?

You look to the sky and the views are always magnificent. It’s a privilege to have that blue canopy overhead, 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 hang up there like feather castles in the air. We don’t ask ourselves many questions, apart from imagining that they have many more shapes than they actually do, but… do they really float? And how do they float?

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

Even before the rain, these drops are heading towards the Earth, albeit at a slow pace.

When a water-laden cloud becomes too large, precipitation falls as rain, snow or hail. But even before it rains, these drops make their way to Earth, albeit at a slow pace. They fall very (but very, very) slowly, and everything that falls reaches what is known as terminal velocity – its maximum possible speed as it falls freely.

Terminal velocity occurs when the drag force of the air perfectly counterbalances gravity. Water drops are so light that their terminal velocity is super-slow: between 18 and 36 meters per hour for a drop with a radius of 5 to 10 micrometers. As clouds are generally thousands of feet high in the atmosphere, this slight downward movement is not perceptible to the naked eye.

Curiously, dust grains swirling in a sunbeam also fall, but because they are tiny, they fall slowly. The average size of a drop of water in a cloud is smaller than the radius of a human hair. But something counterbalances this slow descent, and that’s where the illusion comes in. Rising air currents keep the merging drops in suspension, even as they gradually fall away.

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 fall, essentially, at a rate equal to or less than the speed of the updraft in the cloud. Water drops become “markers of air movement”. In other words, the rising air pushes millions of tiny drops along its path, forming the visible cloud. But it’s not just this simultaneous falling and rising that’s at play; while clouds appear at a relatively fixed height, they fluctuate as rising air mixes with the condensing and evaporating droplets. In fact, they form and evaporate at a rate that makes them appear somewhat stationary.

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

A cloud is the visible result of the vertical movement and mixing of air with water, as the drops fall slowly toward the ground.

Cloud formation requires warm, moist air. Warm air is more buoyant than cold air, so it rises in the atmosphere and condenses into a cloud as it cools. The cloud is less dense than the air below. Although some clouds appear light and fluffy, a cumulus or thundercloud can weigh as much as 100 elephants, although its mass and water content depend on its size. In a smaller cloud, just a few dozen meters high and wide, there’s no imminent precipitation.

You look to the sky and the views are always magnificent. It’s a privilege to have that blue canopy overhead, brightening our day. Sometimes it’s gray and fine threads of rain fall, which is just as beautiful, and other times it’s the clouds that hang up there like feather castles in the air. We don’t ask ourselves many questions, apart from imagining that they have many more shapes than they actually do, but… do they really float? And how do they float?