National Geographic : 1925 Aug
©Herbert G. Ponting SUCH STUFF AS CLOUDS ARE MADE OF Countless billions of billions of tiny molecules of water are constantly jumping out of the sea and into the air, through the process ofevaporation, to form invisible water vapor. Normally the vapor that hangs over the sea is whisked away by the winds, still unseen, and iscarried aloft byheat to build clouds. When the air near the sea becomes chilled, however, one of Nature's secrets is disclosed. The cloud stuff isseen floating off into the air just as steam rises, at a much higher temperature, from the boiling water in a teakettle.