Why clouds are puffy

Q: Why do clouds "stick together"? I realize that clouds are condensation of evaporated moisture from the ground, but why are they "puffy"? Why don't they diffuse uniformly into the atmosphere at the altitude in which the moisture condenses, i.e. more of a uniform fog than a puffy cloud? Eric, Lincoln, Nebraska

A: Well, not all clouds are puffy. Cirrus clouds — the long, thin, high ones — are wispy. Stratus clouds look like layers (more about them later).

Puffy cumulus clouds over the Atlantic Ocean. Courtesy of Ralph F. Kresge, NOAA

You, though, are probably talking about cumulus clouds. Cumulus, by the way, means "a heap", which is how they look. Their puffiness is largely due to forming in "turbulent updrafts", says meteorologist Craig Bohren and author of Clouds in a glass of beer.

Before I explain puffiness, however, let’s consider the inside of clouds. Fog is just a cloud on the ground, and, even though it looks uniform, actually swirls in patches — forming, thickening and dissipating. Flying into a puffy cloud, in a plane, reveals a fog-like milieu. So, your question isn’t about the cloud’s inside (that’s fog) but rather about its boundaries and shape. Why are some clouds puffy?

I go outside on a warm winter’s day, and see blue sky mostly, some clouds. Three sharply-lit, tiny, puffy, white clouds float almost overhead. An amorphous, distant, grey bank largely covers the northwest sky. A warm front announces its arrival with a leading, thin, wispy cloud, attached to a huge bank of clouds filling the southern horizon. Each cloud different. Each created by different forces. Each shaped by different airflows.

That seemingly peaceful blue sky belies a seething ocean of turbulent air. Warm, cool, dry and wet air masses churn together, and create a jumble of clouds. The clouds don’t last long. "Individual clouds form and dissipate in perhaps a half hour or so," says Bohren. But then, neither do air currents last. Watch the clouds to see the patterns. "Clouds are tracers of air flows."

Puffy cumulus clouds outline air bubbles, much as smoke shows the curling updraft flowing from a cigarette’s hot tip.

A puffy cloud starts forming with air near the ground. As the Sun heats the ground, water evaporates into the nearby air, which is warmer than air at higher altitude. In the troposphere (the lower atmosphere), the higher the air, the colder.

The colder, heavier air sinks, and buoys the warmer air up. The warm moisture-laden air, pushed from below, rises, and cools as it rises. Its moisture condenses to form a cloud when the air cools to its dew point. The base of the cloud forms at that dew-point altitude. ("Just for the record, the altitude at which the air reaches the dew point is usually called the lifting condensation level," says Bohren.) That’s the shape of one cloud boundary — a flat base.

The rest of the puffy cloud gets its shape because of warm, buoyant air. When the moisture condenses to form the cloud base, the water vapor gives up heat. The heat expands the cloud, lessening its density and increasing cloud buoyancy, writes Elizabeth Wood in Science from your airplane window. So, the cloud top continues to rise, buoyed by the cooler air outside. The combination puffs the cloud into a rounded, fractal (self-similar) "cauliflower" shape.

"In any one cloud, there may be many local regions of rising and descending air as the upper surface engulfs pockets of the surrounding dry air. This gives the upper surface its many small bulges," Wood says.

If the atmosphere air is turbulent to about 3000 to 4000 feet (900 to 1200 m), then these air pockets rise like bubbles to that height, eventually defining small, puffy cumulus clouds. If the turbulent air extends to 20,000 or more feet (6100 m), then the bubbles will rise to the higher heights, and form towering, cumulus clouds called cumulonimbus clouds.

If the air is stable, the lower air can be cooler, but not necessarily. For stability, the temperature gradient must merely decrease with height more slowly than about 30 degrees Fahrenheit per mile of altitude (10 degrees C per kilometer).

Above the stratocumulus looking at many cloud layers. Courtesy of Historic NWS Collection, NOAA

When lower air is cooler than upper air, this is called a temperature inversion. Cold air is denser than warmer air at the same pressure. So, the cold air stays down, and the warmer air stays on top. Stratus (layer) clouds can still form, though, much like fog does at ground level. If the air temperature drops to the dew point, the water vapor in the whole air layer condenses, and forms a stratus cloud.

So, it’s how the moisture-laden air rises, or not, that largely shapes the resulting clouds, and gives each a distinct look. Moreover, clouds reveal how winds blow.

Winds aloft, here in Albuquerque New Mexico, frequently exceed 70 mph (110 k/h). About dawn a couple of days ago, I watched a train of clouds scooting across the sky like raggedy boxcars towards the rising sun — tracing morning winds.

Further Reading:

Craig F. Bohren, What Light through Yonder Window Breaks? New York: John Wiley & Sons, Inc., 1991.

Craig F. Bohren, Clouds in a glass of beer. New York: John Wiley & Sons, Inc., 1982.

Elizabeth A. Wood, Science from your airplane window. New York: Dover Publications, Inc., 1975.

Jack Williams, The weather book. New York: Vintage Books, 1997.

(Answered Jan. 31, 2006)