Humidity
Humidity:
-Humidity is the general term for the amount of water vapor in the air. Absolute humidity is mass
of water vapor (g) / volume of air (cubic meters). Meteorologists employ several methods to
express the water-vapor content of the air, which are 1. mixing ratio, 2. relative humidity, and 3.
dew-point temperature
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How to determine humidity:
-Mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry
air. mixing ratio = mass of water vapor (grams) /mass of dry air (kilograms) Because the
mixing ratio is expressed in units of mass (grams and kilograms), it is not affected by changes in
pressure or temperature. However the mixing ratio is time-consuming to measure by direct
sampling.
-Relative humidity is a ratio of the air’s actual water-vapor content compared with the amount of
water vapor required for saturation at that temperature (and pressure). Relative humidity
indicates how near the air is to saturation, rather than telling the actual quantity of water vapor in
the air. Because relative humidity is based on the air’s water vapor content and the amount of
moisture required for saturation, it can be changed by the addition or removal of water, or a
change in temperature. (Saturation vapor pressure is temperature dependent, and at higher
temperatures it takes more water vapor to saturate air than at lower temperatures.) Relative
humidity and temperature have an inverse relationship. If temperature decreases, relative
humidity will increase. This is because cool air holds less water vapor (has less space) than
warmer air. Relative humidity is usually stated as a percentage.
-Dew-point temperature is the temperature when a parcel of air needs to be cooled in order to
reach saturation. Unlike relative humidity however, dew-point temperature is a measure of the
air’s actual moisture content. Because the dew-point temperature is directly related to the amount
of water vapor in the air, and because it is easy to determine, it is one of the most widely used
measures of humidity. High dew-point temperatures mean the air is saturated, while lower dewpoint temperatures mean the air is relatively dry. Based on what we have learned about vapor
pressure and saturation, we can state that for every 10 degree Celsius increase in the dew-point
temperature, the air contains about twice as much water vapor.
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Instrument that measures relative humidity:
-Relative humidity is commonly measured using a hygrometer. The main type of hygrometer,
called a psychrometer, consists of two identical thermometers mounted side by side. One
thermometer, the dry-bulb, gives the present air temperature. The other, called the wet-bulb
thermometer has a thin muslin wick tied around the end. To use the psychrometer, the cloth
sleeve is saturated with water and a continuous current of air is passed over the wick. As a result,
water evaporates from the wick, and the heat absorbed by the evaporating water makes the
temperature of the wet bulb drop. The loss of heat that was required to evaporate water from the
wet bulb lowers the thermometer reading. The amount of cooling that takes place is directly
proportional to the dryness of the air. The drier the air, the more moisture that evaporates.
Therefore, the larger the difference between the thermometer readings, the lower the relative
humidity.
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Other terms and concepts:
-For air to be saturated means it is filled to it’s capacity.
-Vapor pressure is part of the total atmospheric pressure that can be attributed to the water vapor
content in the atmosphere.
-Water vapor is the most important gas in the atmosphere when it comes to understanding
atmospheric processes.
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