Air in Clouds
How much air is in a cloud?
A cloud is all air.
More precisely, a cloud is a volume of air that has a high water vapor
The water vapor is like when you see your breath on a cold day.
That is a cloud.
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
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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Update: June 2012