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Humidity

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Humidity

The amount of Water vapour that may be present in the air at a given time varies widely from place to place. It ranges from almost nothing in the cold, dry air of arctic regions in winter to as much as 4 or 5 percent of a given volume of the atmosphere in the humid equatorial zone.

The term humidity simply refer to the degree to which water vapour is present in the air. For any specified temperature there is a definite limit to the quantity of moisture that can be held by the air.

This limit is known as the saturation point. The proportion of water vapour present relative to the maximum quantity is the relative humidity, expressed as a percentage. At the saturation point, relative humidity is 100 percent; when half of the total possible quantity of vapour is present, relative humidity is 50 percent and so on.

A change in relative humidity of the atmosphere can be caused in one of two ways. If an exposed water surface is present, the humidity can be increased by evaporation. This is a slow process, requiring that the water vapour diffuse upward through the air. The other way is through a change of temperature. Even though no water vapour is added a lowering of temperature results in a rise of relative humidity. This is automatic and is a logical consequence of the fact that the capacity of the air to hold water vapour has been lowered by cooling; thus the existing amount of vapour represents a higher percentage of the total capacity of the air. Similarly, a rise of air temperature results in decreased relative humidity, even though no water vapour has been taken away. The principle of relative humidity change caused by temperature change is illustrated by a graph of these two properties throughout the day. As air temperature rises, relative humidity falls, and vice versa.

Next: Hygrometers

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