Date: Fall 2011
I know water is liquid and air is gas - so the bottom of clouds are sometimes seen universally flat - so are clouds floating on a higher pressure gradient?
Water exists as a gas (water vapor) within "air." "Air" is the term that we use for the combination of gases that our atmosphere is made of. Air consists of many gases, although primarily nitrogen and oxygen, with lesser
amounts of water vapor, plus trace gases such as the greenhouse gases (carbon dioxide, methane, ozone, etc.).
The bottoms of clouds are fairly flat because they are at the altitude where the saturation of water vapor has occurred. Saturation occurs when the combination of high enough absolute humidity and low enough temperature exist, in other words a relative humidity of about 100%.
The clouds are not floating on a higher pressure gradient. The term gradient
implies a difference in pressure with altitude. Air pressure decreases with
altitude and does contribute to the general decrease of temperature with
altitude (unless a temperature inversion exists for some reason).
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012