Humidity
Humidity is a measure of the amount of water vapor in the air. It can be expressed in several ways. "Specific humidity" is the mass of water vapor per unit mass of combined dry air and water vapor, generally expressed in grams per kilogram. The specific humidity of an air parcel does not change with temperature. "Absolute humidity" is the density of water vapor, expressed as grams per cubic meter of air. "Relative humidity" is discussed below. Related terms are "saturation," which describes the condition where water vapor is at a maximum concentration for the air temperature (warm air can hold more moisture than cold); "dew point temperature," the temperature at which saturation occurs if air is cooled at constant pressure without addition or removal of water vapor; and "vapor pressure," which in meteorology is that part of the total atmospheric pressure due to water vapor content.
Relative Humidity
Relative humidity is the most familiar way to describe the air's moisture content. When "humidity" is used alone, generally relative humidity is meant. Relative humidity compares the actual concentration of water vapor in the air with the concentration of water vapor that the atmosphere could hold (if the atmosphere were at saturation). Relative humidity is usually expressed as a percentage. When the actual concentration of water vapor in air is equal to the water vapor concentration at saturation, the relative humidity is 100 percent.
The relative humidity will vary with the air temperature. Generally, the relative humidity varies inversely with air temperature such that the relative humidity is highest when the temperature is lowest, and vice versa. After sunrise, as the air warms, the relative humidity drops.
The relative humidity of air also increases when water vapor is added to the air. As relative humidity increases, clouds may begin to form. When the air temperature and dew point temperature are equal the relative humidity is 100 percent.
Measuring Humidity
Some instruments used to measure humidity are: the psychrometer, the wet-bulb thermometer, the hair hygrometer, and the dew-point hygrometer.
The psychrometer consists of one dry-bulb thermometer and one wet-bulb thermometer mounted side by side with a fan to force air past the thermometers. The dry-bulb thermometer measures the ambient air temperature. Air streaming past the wet-bulb thermometer vaporizes water from the wet muslin wick wrapped around the bulb, and causes evaporative cooling. The drier the air, the greater the evaporation, and the lower the reading will be on the wet-bulb thermometer compared the the dry-bulb reading. The temperature difference between the two thermometers, called the "wet-bulb depression," is calibrated in terms of percent relative humidity on a psychrometric table. When the temperature is below freezing, the muslin wick is removed and the wet bulb thermometer is used with a thinly coated ice bulb. Sometimes, the dry and wet bulb thermometers are mounted in an instrument shelter without aspiration; appropriate tables to determine dew-point temperature and relative humidity are available in either case.
The hair hygrometer measures changes in humidity using the fact that human hair lengthens slightly as the relative humidity increases and shrinks slightly as the relative humidity drops.
A dew-point hygrometer detects when condensation or crystallization first occur on a cooled surface to give the dew-point or frost-point temperature.
Humidity in the Arctic
In the Arctic, the prevalent air mass is characterized by low temperatures and low moisture content. The continental air of the subarctic in winter is significantly colder and dryer than arctic marine air. Steam fog which occurs when cold air moves out over a warm water surface, causing moisture to evaporate into the air near the surface, is common in arctic regions. A typical value of relative humidity at the surface is 50 to 60 percent.