So depending on the relative humidity and temperature, 100 or more water molecules diffuse out of the leaf for every molecule of carbon dioxide that diffuses in.

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Heat indices combine temperature and relative humidity to indicate how hot the body feels.

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The Met Office's "feels like temperature" takes into account expected air temperature, relative humidity and the strength of the wind at around 5 feet (1.5m) - the typical height of a person's face.

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High-temperature is a relative concept.

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Yet temperature drops are all relative: It will be cooler, for instance, for residents of Louisville, Kentucky, but they can still expect a high of 99, and a heat index of 103, on Monday.

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In water the temperature changes are a good measure of relative enthalpy changes.

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