Humidity is a measure of amount of water vapor or moisture in the air at a certain temperature. At higher temperature air can hold more humidity. The amount of water vapor in the air varies.
Relative Humidity
The percentage of water vapor in the air compared to what the air can hold (at saturation) at that certain temperature is called the relative humidity (RH):
RH = (amount of water vapor in the air) / (amount of water vapor in the air at saturation)
When air is saturated, its RH is 100%. It contains the maximum amount of water vapor at the current air temperature and pressure where water droplets or ice particles begin to form.
Relative humidity is important because it
- influences rates of evaporation and evapotranspiration
- influences the height of cumulus clouds
- determines how cool it will get before dew forms
- is a key element of local climate
Measurement
Humidity is measured using an instrument called a hygrometer. The standard hygrometer measures the difference in temperature of the air and that of a thermometer attached to a wet wick known as a wet and dry buld thermometer. If the relative humidity of the air is low there is a lot of evaporation from the web bulb. The resulting cooling causes the web-bulb temperature to be much lower than that of the dry-bulb temperature. However, if the air is very moist there will be little difference between the web- and dry-bulb tempetures. The larger the difference between the web- and dry-bulb temperature, the lower the relative humidity.
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