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Humidity Terms: Humidity Hygrometer Dewpoint Humidity Humidity refers to the amount of moisture in the air. You might be surprised to know that the air in our atmosphere has water in it. Lots of water! Water in the atmosphere exists in 3 main states: Water Vapor, Clouds Droplets (sometimes frozen ice crystals) and Liquid Rain Drops (sometimes frozen). Important to know is that warm air can hold more water vapor than colder air. Air near the ground or ocean usually has more water in it than the colder air up high. The relative humidity tells how much water the air is holding compared to how much it could hold at a certain temperature. You can mesure humidity with a Hygrometer, it will be indicated in % The first Hygrometer, made by Leonardo da Vinci >>>>> Hygrometer A hygrometer is an instrument used to measure the moisture content or the humidity of air or any gas. The best known type of hygrometer is the "dry and wet-bulb psychrometer", best described as two mercury thermometers, one with a wetted base, one with a dry base. The water from the wet base evaporates and absorbs heat causing the thermometer reading to drop. Using a calculation table, the reading from the dry thermometer and the reading drop from the wet thermometer are used to determine the relative humidity. Other kinds of hygrometers use human hair (blond) to determine moisture content. These are called mechanical hygrometers, based on the principle that organic substances ( human hair) contract and expand in response to the relative humidity. The contraction and expansion moves a needle guage. In 1783, Swiss physicist and geologist, Horace Bénédict de Saussure built the first hygrometer using a human hair to measure humidity. Some hygrometers use the measurements of changes in electrical resistance, using a thin piece of lithium chloride or other semiconductor devices and measuring the resistance which is affected by humidity. Leonardo da Vinci built the first crude hygrometer in the 1400s, see above. Francesco Folli invented a more practical hygrometer in 1664. Dewpoint: The dewpoint, or more precisely the dewpoint temperature, is the temperature at which the liquid and gaseous phases of a material present in a gas, such as water in air, are in equilibrium at a given gas pressure. In other words, the dewpoint temperature, or dewpoint, is the temperature at which the liquid water, or dew, evaporates at the same rate at which it condenses. Measurements of dewpoint and related humidity conditions represent a significant use of temperature sensors, usually integral to the device or instrument that reports the dewpoint temperature or humidity conditions. © Copyright 2005 - Alpha Informatica & Webdesign, all rights reserved www.worldweatherweb.com www.worldweatherweb.co.uk www.wereldweerweb.nl www.weltwetterweb.de