write the mathematical relationship for relative humidity
Answers
We've recently received several reader requests asking us how to calculate relative humidity if the temperature and dew point are known. Relative humidity is the ratio (expressed as a percentage) of the amount of moisture actually in the air to the maximum amount that can be present at that temperature. The dew point is the temperature at which a given sample of air will have a relative humidity of 100 percent; hence, the saturation temperature. All three -- relative humidity, temperature and dew point -- are bound together in the mathematical relationship below.
A source of confusion
Relative humidity changes when temperatures change. Because warm air can hold more water vapor than cool air, relative humidity falls when the temperature rises if no moisture is added to the air.
To calculate the relative humidity you divide the specific humidity by the maximum amount of water vapor the air can hold. For example if the air currently holds 30 grams of water vapor and the maximum it can hold is 50 grams of water vapor, then the relative humidity is 60% (30 divided by 50 equals 0.6 or 60%).
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