To complete our view of the process of atmospheric precipitation we must take into account the action of mountains.  Imagine a south-west wind blowing across the Atlantic towards Ireland.  In its passage it charges itself with aqueous vapor.  In the south of Ireland it encounters the mountains of Kerry:  the highest of these is Magillicuddy’s Reeks, near Killarney.  Now the lowest stratum of this Atlantic wind is that which is most fully charged with vapor.  When it encounters the base of the Kerry Mountains it is tilted up and flows bodily over them.  Its load of vapor is therefore carried to a height, it expands on reaching the height, it is chilled in consequence of the expansion, and comes down in copious showers of rain.  From this, in fact, arises the luxuriant vegetation of Killarney; to this, indeed, the lakes owe their water supply.  The cold crests of the mountains also aid in the work of condensation.

Note the consequence.  There is a town called Cahirciveen to the south-west of Magillicuddy’s Reeks, at which observations of the rainfall have been made, and a good distance farther to the north-east, right in the course of the south-west wind there is another town, called Portarlington, at which observations of rainfall have also been made.  But before the wind reaches the latter station it has passed over the mountains of Kerry and left a great portion of its moisture behind it.  What is the result?  At Cahirciveen, as shown by Dr. Lloyd, the rainfall amounts to fifty-nine inches in a year, while at Portarlington it is only twenty-one inches.

Again, you may sometimes descend from the Alps when the fall of rain and snow is heavy and incessant, into Italy, and find the sky over the plains of Lombardy blue and cloudless, the wind at the same time blowing over the plain towards the Alps.  Below the wind is hot enough to keep its vapor in a perfectly transparent state; but it meets the mountains, is tilted up, expanded, and chilled.  The cold of the higher summits also helps the chill.  The consequence is that the vapor is precipitated as rain or snow, thus producing bad weather upon the heights, while the plains below, flooded with the same air, enjoy the aspect of the unclouded summer sun.  Clouds blowing from the Alps are also sometimes dissolved over the plains of Lombardy.

In connection with the formation of clouds by mountains, one particularly instructive effect may be here noticed.  You frequently see a streamer of cloud many hundred yards in length drawn out from an Alpine peak.  Its steadiness appears perfect, though a strong wind may be blowing at the same time over the mountain head.  Why is the cloud not blown away?  It is blown away; its permanence is only apparent.  At one end it is incessantly dissolved; at the other end it is incessantly renewed:  supply and consumption being thus equalized, the cloud appears as changeless as the mountain to which it seems to cling.  When the red sun of the evening shines upon these cloud-streamers they resemble vast torches with their flames blown through the air.