A Cloud Can Weigh More Than a Million Pounds

Quick Answer

A typical cumulus cloud -- the fluffy, white kind you see on a pleasant afternoon -- contains roughly 500,000 kilograms (1.1 million pounds) of water. That is the equivalent of about 100 elephants floating overhead. Clouds stay aloft despite their enormous weight because the water is dispersed across billions of tiny droplets, each small enough to be supported by rising air currents. The droplets are so small that their terminal velocity is slower than the speed of the updrafts holding them up.

One Million Pounds of Water, Floating

Look up at a cumulus cloud on a sunny day. That cheerful puff of white weighing about as much as a Boeing 747 at maximum takeoff weight. Some of them weigh considerably more.

The calculation, first widely popularized by National Center for Atmospheric Research scientist Peggy LeMone, goes like this:

A typical cumulus cloud has a volume of roughly one cubic kilometer (1 billion cubic meters). The liquid water content of a cumulus cloud is approximately 0.3 grams per cubic meter. Multiply those together: 1,000,000,000 cubic meters times 0.3 grams equals 300,000,000 grams, or 300,000 kilograms (about 660,000 pounds).

More recent estimates using denser, more mature cumulus clouds put the figure higher -- around 500,000 kilograms or 1.1 million pounds. A large cumulonimbus thunderstorm cloud, which can be 10 kilometers tall and several kilometers across, can contain billions of kilograms of water.

These are real, measurable quantities. Satellites and aircraft routinely measure cloud liquid water content, and the numbers are well established. A million pounds of water is not an exaggeration or an estimate with wide error bars. It is what is up there.

Why Clouds Do Not Fall

This is the obvious question, and the answer is beautifully simple once you understand the physics.

A cloud is not a solid or liquid mass hanging in the sky. It is a collection of billions upon billions of tiny water droplets (or ice crystals at higher altitudes), each measuring roughly 10 to 20 micrometers in diameter. That is about one-fifth the width of a human hair.

At that size, physics works differently than our daily experience suggests. Each droplet is so small and so light that air resistance dominates over gravity. The terminal velocity of a 10-micrometer cloud droplet -- the speed at which air resistance exactly balances gravitational pull -- is approximately 1 centimeter per second, or about 0.03 feet per second.

That is absurdly slow. At that rate, a droplet would take over 24 hours to fall just one kilometer.

But cloud droplets do not even fall that fast in practice, because the air inside and around clouds is not static. Cumulus clouds form above rising columns of warm air called updrafts. These updrafts typically move at 1 to 5 meters per second -- hundreds of times faster than the droplet's falling speed. The rising air pushes the droplets upward faster than they can fall.

The cloud, in essence, is continuously being held aloft by the same warm air that created it. The droplets are not defying gravity -- they are falling, just very slowly, and the air beneath them is rising faster.

How Clouds Form

Understanding cloud formation explains why the weight makes sense. Warm, moist air near the ground absorbs water vapor through evaporation from oceans, lakes, rivers, soil, and plants. This air rises (because warm air is less dense than cool air), and as it ascends, it cools at a rate of about 10 degrees Celsius per kilometer.

At some altitude -- called the condensation level -- the air cools enough that the water vapor exceeds saturation. The vapor begins condensing around tiny particles called cloud condensation nuclei (CCN): dust, pollen, sea salt, and pollution particles. Each droplet forms around a single nucleus.

The result is a cloud: a visible collection of water droplets suspended in air. And because a cubic kilometer of air at ground level can contain thousands of kilograms of water vapor, a cubic kilometer of cloud containing a few hundred thousand kilograms of liquid water is perfectly consistent with the available moisture.

Tip

Clouds appear white because the water droplets scatter all wavelengths of visible light equally (Mie scattering). When clouds become thick enough that light cannot penetrate through them, they appear gray or dark from below -- this is why storm clouds look threatening. The cloud has not changed color; it is simply blocking more light.

Cloud Types and Their Weights

Not all clouds are created equal. Here is how different types compare:

Cumulus (fair weather) -- The classic puffy clouds. Roughly 500,000 kg per cloud. Relatively small and short-lived.

Stratocumulus -- Low, lumpy blanket clouds that cover much of the ocean surface. Individual cells are lighter than cumulus, but the total sheet can cover hundreds of square kilometers.

Cumulonimbus (thunderstorm) -- The heavyweights. A mature thunderstorm cloud can extend from 2,000 feet to over 40,000 feet, with a volume of hundreds of cubic kilometers. These contain billions of kilograms of water and produce precipitation rates measured in inches per hour.

Cirrus -- High-altitude wispy clouds made of ice crystals. They are much less dense than cumulus clouds, but they can still contain tens of thousands of kilograms of ice per cloud.

The total mass of water suspended in Earth's atmosphere at any given moment is estimated at around 12,900 cubic kilometers -- roughly 12.9 trillion kilograms. About 0.001 percent of the world's total water supply is floating over your head right now.

When Clouds Finally Fall

Cloud droplets eventually become rain when they grow large enough that updrafts can no longer support them. This happens through two main processes:

Collision and coalescence -- Larger droplets fall slightly faster than smaller ones, colliding with and absorbing them. A raindrop is roughly 1 to 2 millimeters in diameter, about 100 times larger than a cloud droplet and roughly a million times its volume. It takes about a million cloud droplets to form a single raindrop.

Ice crystal process -- In clouds that extend above the freezing level, ice crystals and supercooled water droplets coexist. Ice crystals grow at the expense of the surrounding droplets (because the saturation vapor pressure over ice is lower than over liquid water), eventually becoming large enough to fall as snow, which may melt into rain on the way down.

Once a droplet reaches raindrop size, its terminal velocity jumps to 5 to 9 meters per second -- far too fast for updrafts to counteract. The million pounds comes down.

When you watch a thunderstorm roll in, you are watching millions of kilograms of water finally losing their battle with gravity. The cloud that looked weightless five minutes ago is dumping a few hundred thousand tons of water on your neighborhood.

It was never weightless. You just could not tell.