Humidity is a term that describes the amount of water vapor in the air. The humidity varies according to the temperature and pressure of the air. The warmer the air is, the more water vapor it can hold. When the air contains as much water vapor as it can hold at a certain temperature and pressure, the air is saturated.
The amount of water vapor in the air compared to the amount the air can hold at saturation is called relative humidity. If the air contains only half of the water vapor it can hold, the relative humidity is 50 percent. In clouds and fog, the air is saturated and the relative humidity is 100 percent. The lower air over oceans is almost saturated and the relative humidity is close to 100 percent. In the Sahara and other subtropical deserts, the relative humidity may be as low as 10 percent.
The relative humidity of an area may vary greatly during the day, even though the amount of water vapor in the air remains the same. In such cases, the relative humidity changes as the temperature rises and falls. The relative humidity may be high in the morning, when the temperature is low and the air cannot hold much more than its present water vapor. But as the temperature rises during the day, the air can hold more water vapor and the relative humidity becomes lower.
As air at a fixed pressure and with a constant amount of water vapor is cooled, it will reach a temperature where it becomes saturated. This temperature is called the dew point. If the temperature is lowered further, water vapor will begin to condense and form clouds, fog, or dew. The closer the temperature of the air is to its dew point, the higher the relative humidity will be.
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Humidity affects our comfort and health. When the temperature and the relative humidity are high, most people feel uncomfortable and “sticky” because their perspiration does not evaporate easily. Many people use air conditioners and dehumidifiers in the summer to take water vapor out of the air. In winter, the heated air in buildings is dry and the relative humidity indoors may be very low. These conditions can cause drying of nasal passages and other health problems. As a result, people often use humidifiers in winter to put water vapor into the air.
