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QA INTERNATIONAL / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY QA INTERNATIONAL / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY
Specific humidity measures the exact weight of water vapour contained in an air mass. It is a very stable unit of measurement, since the weight of a substance does not vary with temperature or pressure. On the other hand, specific humidity cannot indicate the relationship between temperature and humidity, which is behind many weather phenomena. Relative humidity expresses the relationship between the quantity of water vapour contained in an air mass and the quantity necessary to saturate that air mass. Saturated air has 100% relative humidity, while totally dry air (which does not exist) would have a relative humidity of 0%. Since saturation depends on the temperature, the relative humidity is higher near the poles than in tropical regions.
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