About three-fourths of the earth's surface is covered by water, so that the air is receiving vapor all the time. In fact, every minute thousands of barrels of water-vapor are rising into the atmosphere from the surface of the ocean. The air is constantly moving about, forming winds, and this load of vapor is, therefore, drifted about by the winds, so that the air you are breathing may have in it vapor that came from the ocean hundreds or even thousands of miles away. You do not see the vapor, you are perhaps not even aware that it is there; yet in a room 10 feet high and 20 feet square there is often enough vapor, if it could all be changed back to water to fill a two-quart measure. There is a difference in the amount of vapor from time to time. Some days the air is quite free from it, and then clothes will dry rapidly. On other days the air is damp and humid; then people say it is "muggy," or that the "humidity is high." On these muggy days in summer the air is oppressive because there is so much vapor in it. Near the sea, where there is so much water to evaporate, the air is commonly more humid or moist than in the interior, away from the sea, where there is less water to evaporate. We have seen that there is some vapor in all air, and that there is more at some times than at others. We have also seen how it has come into the air, and that cold will cause it to condense to liquid water on cold window panes and on water glasses. There are other ways in which the vapor may be changed to liquid. After a summer day, even when there has been no rain, soon after the sun sinks behind the western horizon the grass becomes so damp that one's feet are wet in walking through it. The dew is "falling." During the daytime the grass is warmed by the sun; but when the sun is gone it grows cooler, much as a stove becomes cool when the fire is out. This cool grass chills the air near it and changes some of the vapor to liquid, which collects in drops on the grass, as the vapor condenses on the outside of a glass of ice water. In the opposite season of the year, on a cold winter's day, when you step out of a warm house into the chilly air, a thin cloud, or fog, forms as you expel the air from your lungs, and you say that you can "see your breath." What you really see are the little drops of water formed as the vapor-laden breath is chilled on passing from the warm body to the cold air. The vapor is condensed to form a tiny mist. [Illustration: _Fig. 10. A wreath of fog settled in a valley with the hilltops rising above it._]