Water in the atmosphere is the only reason we have clouds, fog, rain, snow?and warm, sticky days. The atmosphere carries water in three different forms: water vapor, liquid water, and solid water. A hot humid day is different from a hot dry day only because of water vapor. The amount of water vapor in the atmosphere is called humidity. When we say ?relative humidity? is 80 percent, we mean that the air is holding 80 percent of the water vapor it could hold. When air is at 100 percent relative humidity, it is holding all the water vapor it possibly can, and we call it saturated air. Warm air can hold more water vapor than cold air. The instrument used to measure the amount of water vapor in a hygrometer. The most accurate kind is called the wet and dry grometer. Two thermometers are mounted side by side on a base. of one is covered with muslin or some other coarse material which is The bulb of the other is left bare and dry. Evaporation of water on an object cools it. If the air contains much water vapor, the moisture on the wet bulb evaporates slowly, and the wet bulb thermometer doesn?t show much lowering in temperature. If the air is dry, the moisture on the wet bulb evaporates rapidly, and the wet-bulb thermometer shows a much lower temperature than the dry-bulb thermometer. By consulting a prepared table, the relative humidity can be learned by comparing the two temperatures. There are other types of hygrometers that use currents of air, or chemicals, or a hair, to measure the increase or loss of moisture in the air,the air is bulb hyThe bulb kept wet.