Fig. 10. A wreath of fog settled in a valley with the hilltops rising above it.
Doubtless you have seen a wreath of fog settling in a valley at night; or in the morning you may have looked out upon a fog that has gathered there during the night ([Fig. 10]). If your home happens to be upon a hillside, perhaps you have been able to look down upon the fog nestled there like a cloud on the land, which it really is. Such a fog is caused in very nearly the same way as the tiny fog made by breathing. The damp air in the valley has been chilled until the vapor has condensed to form tiny mist or fog particles. Without doubt you can tell why this fog disappears when the sun rises and the warm rays fall upon it.
On the ocean there are great fogs, covering the sea for hundreds of miles; they make sailing dangerous, because the sailors cannot see through the mist, so that two vessels may run together, or a ship may be driven upon the coast before the captain knows it. Once more, this is merely condensed vapor caused by chilling air that has become laden with vapor. This chilling is often caused when warm, damp winds blow over the cold parts of the ocean.
This leads the way to an understanding of a rain storm; but first we must learn something about the temperature of the air. The air near the ground where we live is commonly warmer than that above the ground where the clouds are. People who have gone up in balloons tell us so; and now scientific men who are studying this question are in the habit of sending up great kites, carrying thermometers and other instruments, in order to find out about the air far above the ground.
Fig. 11. Fog clouds among the valleys in the mountains, only the mountain peaks projecting above them.
It is not necessary, however, to send up a kite or a balloon to prove this. If your home is among mountains, or even among high hills, you can prove it for yourself; for often, in the late autumn, when it rains on the lower ground, it snows upon the mountain tops, so that when the clouds have cleared away the surface of the uplands is robed in white ([Fig. 12]). In the springtime, or in the winter during a thaw, people living among these highlands often start out in sleighs on a journey to a town, which is in the valley, and before they reach the valley their horses are obliged to drag the sleigh over bare ground. It is so much warmer on the lower ground that the snow melts away much more quickly than it does among the hills.
The difference in temperature is, on the average, about one degree for every three hundred feet, so that a hill top rising twelve hundred feet above a valley would have an average temperature about four degrees lower than the valley. Now some mountains, even in New York, rise thousands of feet above the surrounding country. They rise high into the regions of cold air, so that they are often covered with snow long before any snow has fallen on the lowlands; and the snow remains upon them long after it has disappeared from the lower country ([Fig. 12]).
Fig. 12. A mountain whitened by snow on the top, while there is no snow at the base.