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Today's groundwork background topic looks at humidity, and from the questions I have received over the years, there seems to be a lot of confusion about humidity.
Humidity, according to meteorologists, can be absolute or relative, giving us the terms absolute humidity and relative humidity. (A third type specific humidity is also defined by meteorologists as a mass ratio, expressed as grams of water vapour per kilogram of humid air. There are good technical reasons for using this expression, but it is not popularly used.) In our high-paced media world, the adjective is often dropped, and the speaker (or writer) just says "humidity," expecting us to know which they are speaking of. Weathercasters, including those formally trained in meteorology, are often the worst offenders (mia culpa, too). Absolute humidity is defined as the mass of water vapour contained in a volume of air, that is, the density of water vapour, generally expressed as grams per cubic metre. It is like saying there are X grams of cashews in a jar of mixed nuts (too darn few!). Absolute humidity is commonly used by meteorologists because it varies only moderately through an air mass, being added mainly by evaporation or subtracted by condensation, or altered by mixing along the air mass edges. Relative humidity is the dimensionless ratio between the absolute humidity and the saturation absolute humidity for the ambient temperature. (Technically, relative humidity is the ratio between the water vapour pressure in the air and the saturation vapour pressure for the air temperature, but under normal surface conditions, it is well represented by other expressions of absolute humidity.) While I will distinguish here between relative humidity and absolute humidity, I will take one shortcut and refer to air containing water vapour simply as air rather than as humid air. Relative humidity is usually what the media mean when they say "humidity." Relative humidity has uses in human comfort determination and many areas of biometeorology, but it can be a confusing measure because its value varies through the day with the air temperature. For example, the morning may have a relative humidity of 78 percent which by afternoon drops to 53 percent as the air temperature rises. The absolute humidity for that day, however, remained
The copyright of the article Laying Some Groundwork-2: Humidity in Meteorology is owned by Keith C. Heidorn. Permission to republish Laying Some Groundwork-2: Humidity in print or online must be granted by the author in writing.
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