– How can humidity in the air be measured as a percentage? How much humidity is 100%?
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Relative humidity is given as a percentage. Absolute humidity is a density – grams of water per cubic meter of air or its equivalent.
A given absolute humidity will reflect a different relative humidity for different temperatures and air pressures. If the pressure remains constant, reducing the temperature raises the relative humidity and raising the temperature lowers the relative humidity. The idea is that the relative humidity somehow describes the ability of the air to accept added water molecules. 100% relative humidity describes air that cannot accept more water.
Consider a closed container of air (with some water molecules moving around among the air molecules -O2 and N2). Imagine a smaller container inside it, a smaller container holding just water. Everything is at the same temperature.
Open the container of water so that water inside is free to evaporate and escape into the container of air. Whenever a water molecule escapes the reservoir of liquid, it is free to move out among them. Whenever a water molecule already in the air hits the surface of the liquid water it is free to enter the container of water. There is a continuous exchange of water molecules into and out of the liquid water, crossing the surface separating the liquid from the air.
Depending on the temperature, the number of water molecules moving from the liquid to the air may be larger or smaller than the number of water molecules from the air into the liquid. If the movement is outward, we say the air is accepting the evaporating water. If the inward movement is greater, we say that water is condensing out of the air. There is a temperature at which the two are equal, and over time, no water enters or leaves the air; the air holds all the water it can hold; the air is saturated.
If all the water evaporates out of the inner container it means that water molecules were still moving outward and the air still had the ability to receive more. The air is not saturated. The relative humidity of the air describes what fraction of the available capacity to absorb has been filled up.
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