September 3 – Castle In The Sky

Today’s factismal: A typical cloud can weigh as much as a castle.

A cumulonimbus (rain) cloud that is 3 miles across and 3 miles high can hold about 39 billion tons of water; that’s the same weight as the rock in a castle! So why do clouds float and castles, well, not? It all has to do with how the water got in the cloud in the first place.

Though most people see clouds as big puffy things floating in the air, meteorologists see them as a continual process of evaporation and condensation. As the temperature of the air increases, it can hold more moisture in the form of water vapor. As the temperature of the air decreases, the amount of water vapor it can hold decreases. So cooling the air will force the water vapor in the air to condense into small drops of liquid water; this is why water drops form on the outside of cold glasses.

Clouds are a process, not a thing to meteorologists

Clouds are a process, not a thing to meteorologists

This would be pretty boring except for the fact that the air is in constant motion. Warm air rises, taking water vapor along for the ride. As it rises, the air pressure decreases, which allows the air to expand. And expanding takes energy which comes from the heat in the air, causing it to cool. If the air rises far enough and cools off enough, then the air can no longer hold all of the water vapor and some of it condenses into little drops. Voilà! A cloud is born.

Virga falling from a cloud in Florida (My camera)

Virga falling from a cloud in Florida
(My camera)

The water droplets are usually small enough to be carried around by the air’s movement. But that movement can bump the droplets into other droplets, causing them to get larger and larger. Once the drops are large enough, then they are too heavy to ride the wind and start to fall as rain. If the rain passes through air that is dry enough on the way down, it can evaporate again, feeding the cloud from underneath; this is called “virga” by meteorologists. Some meteorologists estimate that less than half of the water in a cloud ever makes it down to the ground as rain; the rest is re-evaporated and put back into the cloud.

If you like clouds and would enjoy working with NASA as a citizen scientist, then why not go back to S’COOL? The scientists need observers to “ground truth” cloud observations by telling NASA what types of clouds they see. S’COOL also has lots of neat information for educators, so go take a look:
http://scool.larc.nasa.gov/

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