How Can a Million-Pound Cloud Stay in the Sky?
That harmless-looking cloud overhead may contain hundreds of tons of water, sometimes more than a MILLION POUNDS. So why doesn’t it come crashing down? The answer begins with droplets so incredibly small that even gentle air currents can keep them aloft.
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A cloud is not water vapor. Water vapor is an invisible gas. The cloud we can see is made of countless microscopic liquid-water droplets, ice crystals, or a mixture of both.
So how can a cloud seem to appear out of nowhere? The water was already there. Moist air rises into lower-pressure air, expands, and cools. When it cools to its dew point, water begins condensing onto microscopic airborne particles such as dust, smoke, pollen, and sea salt. Billions of tiny droplets or ice crystals then scatter sunlight. Invisible water suddenly becomes visible.
HOW MUCH DOES A CLOUD WEIGH?
The amount of condensed water in a typical fair-weather cumulus cloud may be only about 0.5 gram per cubic meter of cloud, roughly 0.4 gram per cubic yard. That sounds like almost nothing. But spread that tiny concentration throughout a cloud measuring one cubic kilometer and it can add up to about 500 metric tons, or roughly 1.1 MILLION LBS! That’s around 80–100 elephants’ worth of water.
SO WHY DOESN’T IT FALL?
Technically, the droplets ARE falling, but extremely slowly. A typical cloud droplet is so tiny and light that air resistance greatly limits its falling speed. Gentle updrafts and ordinary atmospheric turbulence can keep the droplets suspended or carry them upward. Rain begins only after droplets or ice crystals grow large and heavy enough for gravity to overcome the surrounding air movement.
FOUR BASIC CLOUD FORMS
Clouds come in thousands of variations, but four basic forms provide an easy way to understand what you’re seeing:
• CIRRUS — High, wispy clouds composed mostly of ice crystals. Fair weather may be present now, although a weather change may be approaching.
• CUMULUS — Familiar puffy “heaps.” Small cumulus clouds generally accompany fair weather, while rapidly growing towers can signal increasing atmospheric instability.
• STRATUS — Broad, flat gray layers associated with overcast skies, mist, drizzle, or light precipitation.
• CUMULONIMBUS — Towering storm clouds capable of producing heavy rain, lightning, hail, strong winds, and severe weather.
WHY ARE SOME CLOUDS WHITE AND OTHERS DARK?
Cloud droplets scatter sunlight, which makes a well-lit cloud appear bright white. But in a thick cloud, sunlight is scattered repeatedly before it can reach the bottom. Much less light reaches the cloud base, making the underside appear gray or nearly black. It isn’t “dark water.” It’s largely a matter of cloud thickness, lighting, shadows, and where you are standing.
HOW DO MOUNTAINS MAKE CLOUDS?
Moist air can be lifted by convection, weather fronts, converging winds, or mountains. When moving air encounters a mountain, it may be forced upward into lower-pressure air. As it rises, it expands and cools. If it cools enough to reach saturation, water condenses and a cloud forms.
This is also why some clouds can appear almost stationary even while air is continuously moving through them. Cap clouds and lenticular clouds can continually form on one side while evaporating on the other, somewhat like a standing wave in moving water.
THE EDGE OF A CLOUD
Here’s one of the strangest things about clouds: a cloud does not really have a solid edge. Inside the cloud, the air is near saturation and droplets can survive. At the boundary, slightly warmer or drier air mixes in and droplets can evaporate back into invisible water vapor. Meanwhile, nearby cool, humid air that reaches saturation can form brand-new droplets.
A CLOUD’S EDGE ISN’T WHERE THE WATER STOPS. IT’S WHERE THE WATER STOPS BEING VISIBLE.
Those beautifully sharp cauliflower-shaped edges on a cumulus cloud are actually active zones where droplets are constantly forming, evaporating, and reforming.
And have you ever noticed the remarkably flat bottom of many cumulus clouds? That flat base often marks approximately the altitude where rising air first cools enough to reach saturation and condensation begins.
One more Hidden in Plain Sight fact: FOG IS SIMPLY A CLOUD AT GROUND LEVEL!
Clouds are so familiar that most of us rarely stop to question them. But look a little closer and they reveal an extraordinary story involving water, temperature, pressure, gravity, sunlight, and air that is constantly moving. It was hiding in plain sight all along.
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