How is humidity percentage determined??

Answer 1

There are a couple of instruments that measure relative humidity. One is a hygrometer. Dewpoint hygrometers use a chilled mirror and optoelectronics to detect condensation on the mirror surface.

The other is a psychrometer, a wet bulb/dry bulb thermometer with a look-up chart. The thermometers are attached together to a handle and are manually spun in the air. The two thermometers will read differently depending on the humidity. A chart gives the humidity from the different temperature readings.

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Answer 2

Actual moisture in the air in g Water/kg air divided by maximum amount of moisture in the same units possible at that temperature without condensation occurring. It is basically moisture present in the air divided by moisture content necessary in the same air for the humidity to be 100%.

10 g water/kg air of moisture in the air with a maximum of 30 g water/kg air possible, means about 33% humidity.

A psychrometer is used to determine the dry-bulb and wet-bubl temperatures. The dry-bulb temperature is the regular temperature and the wet bulb temperature is the temperature of the air after cooling though evaporation of a wet thing on a thermometer. If the air has 100% humidity, then the wet-bulb will not experience any evaporation and the dry and wet bulb tempuratures will be the same. If there is lower humidity, then there will be more evaporative cooling of the wet-bulb and the wet-bulb temperature will be lower. So, the larger the difference between the dry and wet bulb temperatures, the lower the humidity.

It can also be calculated using the Clausius-Clapeyron equation:
http://www.theweatherprediction.com/habyhints/186/

Answer 3

It is usually determined either by one of four thermodynamic diagrams. These can have 5 or more axis, and you basically take knowns like temperature, pressure, altitude, etc and plot them. Where they intersect you can find out about humidity.