Measured with a Dry & Wet bulb Hygrometer
Humidity is a term used to describe the amount of water vapour in air. Absolute humidity, specific humidity, relative humidity, and etc.are different ways to express the water content in a parcel of air. Relative humidity is the most frequently used of these expressions because of its importance in weather forecasting. Humidity figures provided for public use in newspapers are usually relative humidity.
"Relative humidity is an important part of weather forecasting because it indicates the likelihood of precipitation, dew, or fog. High relative humidity makes it feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by preventing the evaporation of perspiration from the skin. This effect is calculated in a heat index table."
Above text is taken from http://en.wikipedia.org/wiki/Humidity
Please also visit discussion on humidity at How Stuff Works
The mass of water vapour in a unit volume of air. It is a measure of the actual water vapour content of the air.
The mass of water vapour per unit mass of air (including the water vapour). It is another measure of the actual water vapour content of the air.
Relative humidity (RH)
The ratio of the actual amount of water vapour in the air to the amount it could hold when saturated expressed as a percentage OR the ratio of the actual vapour pressure to the saturation vapour pressure expressed as a percentage.
The amount of water vapour the air can hold increases with temperature. Relative humidity therefore decreases with increasing temperature if the actual amount of water vapour stays the same.
Relative humidity varies significantly when the temperature changes,
even when the actual amount of water vapour in the air remains the same.
Relative humidity =
The actual amount of water
vapour in the air x 100%
The amount of water vapour required to saturate the air at that temperature
The actual vapour pressure x 100%
The saturated vapour pressure at that temperature
Measuring relative humidity: Relative humidity can measured by comparing the air temperature with the lowest temperature to which can be cooled by evaporating water.
When water evaporates it cools the surface from where it is evaporating. We use this phenomenon to cool the water in earthenware pots. The rate of evaporation depends on the moisture content in the surrounding. Dryer the air (that is less humidity) faster the evaporation and cooler temperature. Water also rises in a narrow tub or spacing due to what is called capillary action.
These two properties of water are used for measuring relative humidity. Two thermometers are suspended or placed side by side. A fine cloth is which water can rise due to capillary action is tied around the bulb of one thermometer and is soaked in water. As the water evaporates (depending upon the temperature and humidity) the temperature in this thermometer drops.
Now taking the temperature of this thermometer (called wet bulb thermometer) and the other thermometer (called dry bulb thermometer) the humidity can be calculated. But it is not a straightforward calculation for one has to take atmospheric pressure also into consideration.
However, scientists have simplified the job of amateur scientists by making a table that can be used for recording the relative humidity, which is, within a few per cent is good enough for our purpose.
Check out so DIY projects and click her for the RH table.