Dew
a quiet deposit of atmospheric water on the surface of the earth. The process of dew making may be seen quite frequently on the surface of a water pitcher. A pitcher of iced water, placed on the table on a hot summer day, is apt to gather moisture from the air. Dew making is closely connected with the theory of saturation. Hot air holds more moisture than cold air. If air has all the water it can hold, it is said to be saturated. If saturated air be cooled it deposits the water it cannot retain. The point of coolness at which air is unable to retain all its moisture is called the dew point. If the heated air around the ice pitcher be saturated, and the pitcher cools the adjacent air below what is called the dew point, the moisture which the air can no longer hold settles on the surface of the pitcher.
During the summer season the earth cools off after nightfall, and then begins to absorb heat from the layer of saturated air lying next to it. If there be no wind to carry away the cooled air, it precipitates the water which it can no longer hold, and globules collect, not only on the surface of the earth, but especially on blades of grass or other vegetation, which cool much more rapidly than the earth itself. To understand the cooling of the earth, it must be kept in mind that the heat is radiated into space, not absorbed by the atmosphere. Clouds hovering over the earth's surface act like a blanket to catch this radiated heat and throw it back to the earth's surface again. For this reason dew forms chiefly on clear nights. The philosophy of dew making rests on the fact that the earth readily absorbs heat from the atmosphere, but gives it off into space, not into the adjacent air.