A Cloud Can Weigh 1.1 Million Pounds. Here's Why It Floats

On a summer afternoon, a fair-weather cumulus cloud looks like it weighs nothing. It drifts, holds its shape and drags a soft shadow across the ground. Yet the U.S. Geological Survey estimates that an ordinary cloud of this kind holds about 1.1 million pounds of water. The usual explanation for why it doesn’t come crashing down is that clouds are “lighter than air.” That answer is only half right. The water in a cloud is far denser than air, and every droplet in it is falling. It falls so slowly, though, that the air rising beneath it wins.

How the USGS gets 1.1 million pounds

The USGS Water Science School starts with an idealized cloud: a cube about 1 kilometer wide and about as tall as it is wide. Those dimensions come from Peggy LeMone of the National Center for Atmospheric Research, by way of a ScienceAlert article the agency cites. Inside the cube, it assumes about 0.5 gram of liquid water in every cubic meter of air.

The arithmetic is short. One cubic kilometer is 1 billion cubic meters. At 0.5 gram each, that comes to 500,000,000 grams, or 500,000 kilograms. The USGS converts that to about 1.1 million pounds, or roughly 551 US (short) tons.

The more revealing step is to weigh the air in the same box.

Basis: USGS cloud estimate (2019); air at a standard sea-level density of 1.225 kg per cubic meter. Air totals are our calculation.

Why a cloud is not lighter than air

The table shows the problem with the “lighter than air” explanation. The same cubic kilometer holds about 1.225 billion kilograms of air, roughly 2,450 times the mass of the cloud’s water. The liquid makes up only about 0.04% of the mass in that space. Air at cumulus height, 1 to 2 kilometers up, is somewhat thinner than at sea level, but it still outweighs the water by thousands of times.

So the cloud’s 1.1 million pounds is not hanging in empty sky. It is scattered as tiny specks through a far heavier body of air.

The USGS page itself says clouds float because moist air is less dense than dry air. That is true of the air, and it helps explain why cloudy air tends to rise. It does not explain the water. A drop of liquid water is far denser than the air around it, and left alone it would sink. The useful question is how fast it sinks.

How fast cloud droplets actually fall

Every cloud droplet falls relative to the air around it. Its size sets the speed. A droplet’s weight grows with the cube of its radius, while the area pushing against the air grows only with the square. Shrink a drop and its weight falls off much faster than its air resistance, so the smallest drops barely move.

Basis: still air at sea level and 20°C. Speeds for 0.1 mm and larger are Gunn and Kinzer’s 1949 measurements; the 0.02 mm value is a Stokes-law estimate from Roland Stull’s Practical Meteorology. Fall times are 1,000 feet divided by fall speed and ignore updrafts and evaporation.

A typical raindrop is 100 times wider than a cloud droplet and falls hundreds of times faster (6.49 m/s versus about 0.012 m/s). In practice, a cloud droplet almost never finishes a seven-hour fall; it drifts into drier air and evaporates first. Douglas Wesley, a meteorologist with UCAR’s COMET program, put it plainly to Scientific American: “The vast majority of clouds you see contain droplets and/or crystals that are too small to have any appreciable fall velocity. So the particles continue to float with the surrounding air.”

Rising air wins the tug-of-war

A droplet falling at about 1 cm per second holds its height if the air around it rises at the same speed, and climbs if the air rises faster. Clouds form where air is rising. According to Wesley, layered (stratiform) clouds form in widespread upward motion of only a few centimeters per second. Convective clouds such as cumulus have updrafts that exceed a few meters per second.

That is a lopsided contest. For example, if a cumulus updraft runs at 3 meters per second, it rises 250 times faster than a cloud droplet falls. In a growing thunderstorm the gap is wider still. The FAA’s Aviation Weather Handbook says updrafts in the towering cumulus stage can exceed 3,000 feet per minute, or 50 feet per second, and supercell updrafts may reach 9,000 feet per minute. Those speeds beat even the 29.8 ft/s of a large raindrop, which is why strong cumulus updrafts can hold up rain-size drops and let them keep growing.

The cloud is also not a fixed object. Rising air keeps cooling and forming new droplets inside it, while droplets at the edges evaporate as cloudy air mixes with drier surroundings. Steven Ackerman of the University of Wisconsin-Madison notes that this steady evaporation is what keeps a cloud’s boundary sharp. Drops that sink out of the bottom fall into drier air below the base and usually evaporate there. A cumulus that looks still is closer to a standing wave: the shape stays while the water passing through it keeps changing.

What it takes to turn droplets into rain

For rain, the balance has to flip: drops must grow until they fall faster than the air rises. That takes a lot of growth. Going from a 0.02 mm cloud droplet to a typical 2 mm raindrop multiplies the diameter by 100 and the volume by a million. NOAA’s JetStream puts it the same way: about one million cloud droplets supply the water for one raindrop.

Condensation alone cannot get there. Vapor collecting directly on a droplet grows it to only about 50 micrometers, according to a meteorology lesson hosted by the National Weather Service, which is too small to fall as precipitation. Clouds use one of two routes instead:

1. Collision-coalescence. Some droplets end up larger than the rest, at least about 20 micrometers. They fall faster, overtake smaller droplets and absorb them. This is the main rain-making process in the tropics and in low stratus clouds.
2. The ice-crystal (Bergeron) process. In clouds colder than freezing, ice crystals sit among supercooled droplets, which are still liquid below 0°C. The crystals grow at the droplets’ expense until they are heavy enough to fall. This is the main mechanism in mid-latitude storms.

Growth has a ceiling, too. Drops much larger than about 5 mm across become unstable and break into smaller drops, which caps how fast rain can fall.

What these numbers can’t tell you

• The cloud’s weight is an order of magnitude, not a measurement. USGS took its 0.5 g per cubic meter from a ScienceAlert article rather than a research paper. Real liquid water content varies widely by cloud type and stage, and a 1-kilometer cube is an idealized shape.
• The air comparison is a sea-level figure. Thinner air at cloud height lowers the 2,450-to-1 ratio, though it stays in the thousands.
• The fall times are simplified. They assume still air and no evaporation, so they show the scale of the difference, not how long any real drop takes.
• Breakup size is disputed. The NWS lesson says drops become unstable above about 4 mm, while Stull puts the limit near 5 mm. Treat “about 5 mm” as a rough boundary.
• The 3-meter-per-second updraft is an illustration. Fair-weather cumulus updrafts vary from cloud to cloud, and published sources describe them only as exceeding a few meters per second.