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Name: Sadie
Status: student
Grade: K-3
Location: MO
Country: USA
Date: N/A 


Question:
How much air is in a cloud?


Replies:
Sadie

A cloud is all air. More precisely, a cloud is a volume of air that has a high water vapor content. The water vapor is like when you see your breath on a cold day. That is a cloud.

Sincere regards,
Mike Stewart


Hi Sadie,

Good question! A cloud forms because the rising warm air cools as it rises. As it cools, the amount of invisible water vapor (water in the form of a gas) that can co-exist with the dry air becomes smaller and smaller. At some temperature, the water vapor cannot all stay in the vapor phase anymore and some of it changes into tiny liquid water drops that you can now see. This is the formation of a cloud. This point where phase change from vapor to liquid occurs can be described different ways: it is the point where the air is said to be "saturated" with water vapor; it is the point of 100% relative humidity; and it is the point where the dewpoint temperature = the air temperature.

The relative amounts of air and water vapor at the point where the air becomes saturated depends on the temperature. To put some typical numbers on this, let us assume that we have a cloud that has begun to form at an altitude of 5000 ft where the air temperature is 0 deg Celsius. We can read from something called a psychrometric chart (a chart telling you how temperature, water vapor amount, heat energy and volume relate to each other) that at this point that there is about 5 g of water per 1000 g of dry air. So another way to put this is that this cloud is 99.5% dry air, and only 0.5% water, by mass. That does not seem like much water! But consider this -- a typical small puffy cumulus cloud might have a volume of about 1 billion cubic meters (equivalent to a cube that has a side length of 1000 meters). Moist air has a density very close to 1 kg per cubic meter, so the mass of that cloud is about 1 billion kg or about 20,000 tons! That is equal to the weight of about fifty 747 jet airliners! 0.5% of that is water and comes out to be about 100 tons. The other 19,900 tons is the dry air.

So when you look at a cloud, remember that even though 99.5% of the mass of that cloud is dry air, it may still be holding hundreds of tons of water.

John C. Strong


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