Humidity: Basic Principles and Measurement Techniques

Dry air is a mixture of nitrogen (about 78%) and oxygen (about 21%), with argon, carbon dioxide and other minor constituents making up the remaining 1%. Moist air is defined as a two-component mixture of dry air and water vapor. The amount of water vapor in air can be highly variable, ranging from 0 (dry air) to a maximum that depends upon temperature and pressure. The term “humidity” relates to the presence of water vapor in air but, depending upon its particular usage, may be used to express either the absolute quantity or the relative amount of water vapor in a moist air mixture.