[AS.] The name given to the drops of water which are seen on the leaves of plants on bright mornings, more especially in the spring and autumn. The air contains aqueous vapor, and the amount of vapor which the air will carry increases as the temperature of the air rises, and diminishes as it falls. When the air contains as much vapor as it is capable of taking up at any particular temperature, it is said to be saturated; and when it is cooled below the point of saturation, condensation takes place, and the moisture deposited in this way from the atmosphere is termed dew. The temperature at which dew begins to be deposited is termed the dew-point. When the sky is clouded, the greater part of the heat radiated by the earth is reflected back from the clouds, and the temperature of the air does not sink to the dew-point. It will be found that on cloudy nights there is no deposition of dew. The air should be still, otherwise no air remains long enough in contact with the ground to be cooled to the dew-point.