Dr. Sue Ann Bowling is an Associate Professor of Physics at the Institute. Cumulus clouds building over high ground near Fairbanks. Although winter clouds over most of Alaska are usually found in rather dreary sheets, in summer the puffy cumulus clouds punctuate the sky. These clouds may vary from snowy wisps of cotton candy to towering hard-edged thunderheads, and indeed may change from one extreme to the other in a startlingly short time. On other days, they may remain like flattened heaps of whipped cream, or march in ordered rows away from areas of high ground.
Cumulus clouds owe their existence to the heating of the earth's surface by the sun.
If a particular area is warmer than another area close by, the rising air will be concentrated over the warmer area. Air, like everything else, has to do work to go upwards. The energy for this work comes from the heat energy of the air, so the air cools as it rises. The height the air must rise for this to happen depends on the relative humidity of the air near the ground, so if that is fairly uniform, the clouds will all have their bases flat and at about the same height, which frequently happens with cumulus. How high the tops of the clouds can rise depends on how fast the temperature drops with height in the air through which the parcel of ground-warmed air is rising.