Whats the difference between humidity, and relative humidity?
A. (1) Absolute Humidity, often just referred to as ‘the humidity’, is a measure of the actual amount of water vapour in a particular sample of air: measured as a partial pressure (vapour pressure/hPa or millibars); a mixing ratio (gm water vapour/kg of dry air), dew point etc. (2) Relative Humidity – expressed commonly as a percentage value, is the ratio of the actual amount of water vapour present in a sample (the Absolute Humidity) to that amount that would be needed to saturate that particular sample. The two terms are not interchangeable and can lead to confusion; e.g. on a cold, raw winter’s day close to the east coast of England, the dew point might be 1 degC and an air temperature of just 2 degC…this would give a RH of=93%; a ‘high’ Relative Humidity, yet few would refer to such conditions as ‘humid’. Conversely, on a hot summer’s day, with a dew point of 18 degC, and an afternoon temperature of 30 degC, that’s a RH=49%; a ‘low’ Relative Humidity, but high Absolute Humidity.
Here is part of a National Weather Service discussion about Water Vapor, Humidity, Dew point, and relationship to Precipitation http://www.crh.noaa.gov/lmk/soo/docu/hum… WATER VAPOR: Water is a unique substance. It can exist as a liquid, solid (ice), and gas (water vapor). A primary way water vapor increases in the atmosphere is through evaporation. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain. A lot or a little water vapor can be present in the air. Winds in the atmosphere then transport the water vapor from one place to another. A major source of water vapor in Kentucky is the Gulf of Mexico. Most of the water vapor in the atmosphere is contained within the first 10,000 feet or so above the earth’s surface. Water vapor also is called moisture. ABSOLUTE HUMIDITY: Absolute humidity (expressed as grams of water vapor per cubic meter volume of air) is a measure of the
