How Do Clouds Float?

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?

Actually, saying a cloud is floating on air is a bit of a misnomer. When we think of floating in a technical sense, we think of an entity kept up within the medium by buoyancy forces due to density differences such as how a piece of wood floats on a lake, or how a hot-air balloon floats in the air. The cloud does not "float" in the air, because its water droplets are heavier than the surrounding air. The second question in the preceding paragraph is in fact where the error lies. Gravity does pull the cloud down to earth. The reason the cloud we are watching does not seem to fall to the ground is the result of two other mechanisms acting on those cloud droplets.

The most important of these is that the cloud, particularly our afternoon cumulus, has been formed within a rising current of air. There are many causes for this rising air, I wrote about several elsewhere (making clouds and updrafts). A rising air current is one of the important steps in the formation of a cloud. Rising air in a cloud core continues until the cloud reaches old age and begins to dissipate. Cloud droplets within this rising air are thus continually pushed upward at a greater rate than the rate at which gravity pulls them downward. The net result is that the cloud droplets are rising within the updraft, which you can see by watching the tops of that cumulus. If the cloud is not dissipating, you will see growing and changing bumps of white at the top of the cloud, an indication of updrafts and continued growth.

Now put aside thoughts of the updrafts for a moment. We next look at what happens to those cloud droplets under the influence of gravity. Now we know that gravity wants to bring everything down to earth, and as Galileo showed us, the pull of gravity is independent of the object's mass. That is mostly true, because the objects he dropped off the Tower of Pisa were of approximately equal size and shape though not of equal mass. As a result, the forces of air resistance on the two objects were about the same, and the distance of fall not that great. Galileo's experiment works to perfection when objects fall through a vacuum, but we do not live in one nor do clouds.

Our cloud droplets are falling through air which puts up some resistence to any object falling through it. This is technically known as aerodynamic drag, and its value depends on the object's mass, shape and size. Cloud droplets are more or less spherical (ice crystals have a wide variety of shapes that may increase their drag effect) and thus have a smaller drag effect than a less symmetric shape — say a feather. They also have a very small size, 12 micrometres or less in diameter.

When any object falls through the air (or through water, for that matter), the aerodynamic drag force counters the force of gravity, and after a time/distance, the falling object reaches its terminal velocity. This is the rate at which the falling object thereafter descends toward the surface. The time it takes to reach the ground can be calculated from the terminal velocity.

For the typical cloud droplet of 10 micrometres (it takes about 15 million cloud droplets to form the typical raindrop), its terminal velocity is around 0.3 cm/s or about 10 metres per hour (30 ft/h). To fall from the cloud base at 3500 m at this rate would take 350 hours! An object moving that slow would appear to us as being stationary unless we observe it closely for some length of time. Only when the droplets congregate to form rain drops, 300 or more times larger, does the fall time drop to minutes.

This small fall velocity is countered by the rising air surrounding it so that many cloud droplets are actually rising at a net rate of perhaps tens of centimetres per second. Of course, a droplet caught in a downdraft will descend at the rate of the downdraft plus its terminal velocity. Now should a cloud droplet be in a region of descending air heading toward the surface, it must pass through the cloud base on its way to the ground. In passing, nature does a bit of a magical trick and makes the droplet disappear.

Ah, but no hocus-pocus here, just old reliable physics. You see, the cloud base is also at the condensation level for the atmosphere surrounding the cloud and its neighbours. Above this level (height), rising air will have cooled to its condensation point, and liquid water will be produced from the rising water vapour within the air. That is why all cloud bases are more or less at the same altitude. But reverse the process and the condensation level becomes the evaporation level, and liquid water within the descending air will return to the vapour state...and that makes it disappear to our sight.

The combination of slow descent of cloud droplets and their quick evaporation below the cloud base gives us the appearance that the cloud is floating gently on air. Ah the tricks of Mother Nature!

Learn More From These Relevant Books
Chosen by The Weather Doctor

Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
Day, John A.: The Book of Clouds , Sterling, 2005, paperback, ISBN 0760735360.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
November 1, 2005
A version of this material was previously published by Keith Heidorn on Suite 101: Science of the Sky, 2004