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Dew Point and Relative Humidity Explained
Relative Humidity and Dew Point are closely related, and are based upon the amount of water vapor in the air and the total amount of water vapor that the air can hold at a given temperature. The warmer the air, the more water vapor it can contain, and the cooler the air, the less vapor it can contain. (This is the reason moisture will condense on a cold glass or other cold object: as the air near the object cools, it reaches the point where it can not contain all the water which it held at a warmer temperature, and some moisture "falls out" of the air.)
The Relative Humidity expresses how much moisture is in the air, as a percentage of the total moisture the air can contain at the current temperature. For example, if the air has all the water vapor that it can contain at a given temperature, the relative humidity is 100%; if the air has only half of the vapor that it can contain at a given temperature, the relative humidity is 50%. Therefore, for a given amount of water vapor in the air, the relative humidity will change inversely with temperature: as the temperature drops, the air becomes relatively more saturated, and the relative humidity goes up.
The dew point temperature may be thought of as the temperature at which water would condense from the air, if the air mass were cooled. In general, when the relative humidity is high (the air is nearly saturated with moisture) the dew point will be close to the air temperature. At 100% RH, the dew point is equal to the air temperature. When the relative humidity is low, the dew point is much lower than the air temperature. If the amount of water in the air remains constant, the dew point will be steady, even as the temperature changes. Conversely, if the dew point is rising, more moisture is being absorbed by the air.
In RainWise weather stations, relative humidity is measured directly, with a sensor element that changes capacitance in proportion to humidity. Dew point is computed via a table lookup. The weather station contains a table of saturation vapor pressures for water in air, indexed by temperature. This value is a standard physical property, which gives the total amount of water that the air can contain at a given temperature. The table value for the current temperature is scaled by the current relative humidity to give the amount of water vapor actually contained in the air. Next, the table is scanned to find the temperature for which this amount of water vapor would fully saturate the air. This temperature is the dew point.