But long before the air and vapor from the equator reach the poles, precipitation occurs.  Wherever a humid warm wind mixes with a cold dry one, rain falls.  Indeed the heaviest rains occur at those places where the sun is vertically overhead.  We must enquire a little more closely into their origin.

Fill a bladder about two-thirds full of air at the sea level, and take it to the summit of Mount Blanc.  As you ascend, the bladder becomes more and more distended; at the top of the mountain it is fully distended, and has evidently to bear a pressure from within.  Returning to the sea level you find that the tightness disappears, the bladder finally appearing as flaccid as at first.

The reason is plain.  At the sea level the air within the bladder has to bear the pressure of the whole atmosphere, being thereby squeezed into a comparatively small volume.  In ascending the mountain, you leave more and more of the atmosphere behind; the pressure becomes less and less, and by its expansive force the air within the bladder swells as the outside pressure is diminished.  At the top of the mountain the expansion is quite sufficient to render the bladder tight, the pressure within being then actually greater than the pressure without.  By means of an air-pump we can show the expansion of a balloon partly filled with air, when the external pressure has been in part removed.

But why do I dwell upon this?  Simply to make plain to you that the unconfined air, heated at the earth’s surface, and ascending by its lightness, must expand more and more the higher it rises in the atmosphere.

And now I have to introduce to you a new fact, towards the statement of which I have been working for some time.  It is this:  The ascending air is chilled by its expansion.  Indeed this chilling is one source of the coldness of the higher atmospheric regions.  And now fix your eye upon those mixed currents of air and aqueous vapor which rise from the warm tropical ocean.  They start with plenty of heat to preserve the vapor as vapor; but as they rise they come into regions already chilled, and they are still further chilled by their own expansion.  The consequence might be foreseen.  The load of vapor is in great part precipitated, dense clouds are formed, their particles coalesce to rain-drops, which descend daily in gushes so profuse that the word “torrential” is used to express the copiousness of the rainfall.  I could show you this chilling by expansion, and also the consequent precipitation of clouds.

Thus long before the air from the equator reaches the poles its vapor is in great part removed from it, having redescended to the earth as rain.  Still a good quantity of the vapor is carried forward, which yields hail, rain, and snow in northern and southern lands.

Mountain Condensers.