Unless the air is saturated with water vapor, the evaporation from the surface of the wet-bulb thermometer will lower the temperature indicated by that instrument below that shown by the dry-bulb thermometer next to it, from which there is no evaporation. The drier the air, the greater the evaporation, and therefore the greater the difference between the readings of the two thermometers. By means of tables, constructed on the basis of laboratory experiments, we may, knowing the readings of the wet and dry-bulb thermometers, easily determine the _dew-point_ and the _relative humidity_ of the air——important factors in meteorological observations (see Chapter XXVI). In winter, when the temperature is below freezing, the muslin of the wet-bulb thermometer should be moistened with water a little while before a reading is to be made. The amount of water vapor which air can contain depends on the temperature of the air. The higher the temperature, the greater is the capacity of the air for water vapor. Hence it follows that, if the temperature is lowered when air is saturated, the capacity of the air is diminished. This means that the air can no longer contain the same amount of moisture (invisible water vapor) as before. Part of this moisture is therefore changed, _condensed_, as it is said, from the condition of water vapor into that of cloud, fog, rain, or snow. The temperature at which this change begins is called the _dew-point_ of the air. The _relative humidity_ of the air is the ratio between the amount of water vapor which the air contains at any particular time and the total amount which it could contain at the temperature it then has. Relative humidity is expressed in percentages. Thus, air with a relative humidity of 50% has just half as much water vapor in it as it _could_ hold. It is found that the readings of the wet-bulb thermometer are considerably affected by the amount of air movement past the bulb, and that in a light breeze, or in a calm, the reading does not give accurate results as to the humidity of the general body of air outside the shelter. To overcome this difficulty another form of psychrometer has been devised.