Psychrometer

View Larger Chart. Source: Psychrometric Chart | WikiMedia

Pictured above: Sling Psychrometer. Source: Sling Psychrometer | WikiMedia

A psychrometer consists of two thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer. These different temperatures are then compared on a Psychrometric Chart.