in to fill spaces made by rising air wherever they occur, and so
these clouds may be made of vapour collected in the
Mediterranean, or in the Gulf of Mexico off the coast of America,
or even, if the wind is from the north, of chilly
particles gathered from the surface of Greenland ice and snow,
and brought here by the moving currents of air. Only, of one
thing we may be sure, that they come from the water of our earth.

Sometimes, if the air is warm, these water-particles may travel a
long way without ever forming into clouds; and on a hot,
cloudless day the air is often very full of invisible vapour.
Then, if a cold wind comes sweeping along, high up in the sky,
and chills this vapour, it forms into great bodies of water-dust
clouds, and the sky is overcast. At other times clouds hang
lazily in a bright sky, and these show us that just where they
are (as in Fig. 19) the air is cold and turns the invisible
vapour rising from the ground into visible water-dust, so that
exactly in those spaces we see it as clouds. Such clouds form
often on warm, still summer's day, and they are shaped like
masses of wool, ending in a straight line below. They are not
merely hanging in the sky, they are really resting upon a tall
column of invisible vapour which stretches right up from the
earth; and that straight line under the clouds marks
the place where the air becomes cold enough to turn this
invisible vapour into visible drops of water.



Week 11