SERVING THE DFW METROPLEX “Family owned and operated since 1972” St. Lic. # TACLA019484E 214-742-7252 Glossary Absolute humidity (Volume basis) Absolute humidity on a volume basis is the quantity of water in a particular volume of air. The most common units are grams per cubic meter, although any mass unit and any volume unit could be used. Pounds per cubic foot is common in the U.S., and occasionally even other units mixing the Imperial and metric systems are used. If all the water in one cubic meter of air were condensed into a container, the mass of the water in the container could be measured with a scale to determine absolute humidity. The amount of water vapor in that cube of air is the absolute humidity of that cubic meter of air. More technically, absolute humidity on a volume basis is the mass of dissolved water vapor, mw, per cubic meter of total moist air, Vnet: Absolute humidity ranges from 0 grams per cubic meter in dry air to 30 grams per cubic meter (0.03 ounce per cubic foot) when the vapor is saturated at 30 °C. Climate/humidity table The table shows the "absolute humidity" in g/m3 (upper line) and the "dew point temperature" of the air in°C (lower line) for certain air temperatures as a function of "relative humidity. Example: At an air temperature of 50°C and a relative humidity of 70%, the absolute humidity is 58.1 g/m3 and the dew point temperature is 43°C. The absolute humidity changes as air pressure changes. This is very inconvenient for chemical engineering calculations, e.g. for clothes dryers, where temperature can vary considerably. As a result, absolute humidity is generally defined in chemical engineering as mass of water vapor per unit mass of dry air, also known as the mass mixing ratio (see below), which is much more rigorous for heat and mass balance calculations. Mass of water per unit volume as in the equation above would then be defined as volumetric humidity. Because of the potential confusion, British Standard BS 1339 (revised 2002) suggests avoiding the term "absolute humidity". Units should always be carefully checked. Most humidity charts are given in g/kg or kg/kg, but any mass units may be used. The field concerned with the study of physical and thermodynamic properties of gas-vapor mixtures is named Psychrometrics. Relative humidity Relative humidity is defined as the ratio of the partial pressure of water vapor (in a gaseous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. In other words, relative humidity is the amount of water vapor in the air at a specific temperature compared to the maximum water vapor that the air is able to hold without it condensing, at that given temperature. Relative humidity is expressed as a percentage and is calculated in the following manner:   where is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. For example, according to the Heat Index, a relative humidity of 75% at 80°F (27°C) would feel like 83.574°F ±1.3 °F (28.652°C ±1.7 °C) at ~44% relative humidity. Specific humidity Specific humidity is the ratio of water vapor to air (including water vapor and dry air) in a particular mass. Specific humidity ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of total moist air mt . That ratio can be shown as:    Specific humidity is related to mixing ratio (and vice versa) by: Humidity during rain Humidity is a measure of the amount of water vapor carried in the air, not including any liquid water or ice falling through the air. For clouds to form, and rain to start, the air does not have to reach 100% relative humidity at the Earth's surface, but only where the clouds and raindrops form. This normally occurs when the air rises and cools. Typically, rain falls into air with less than saturated humidity. Some water from the rain may evaporate into the air as it falls, increasing the humidity, but not necessarily enough to raise the humidity to 100%. It is even possible for rain falling through warm, humid air to be cold enough to lower the air temperature to the dew point, thus condensing water vapor out of the air. Although that would indeed raise the relative humidity to 100%, the water lost from the air (as dew) would also lower the absolute humidity. φ = 100% Pν = Pg = Psaturateda@T Absolute Humidity Sala Air Conditioning has been serving the DFW area since 1972, providing commercial and residential heating and air conditioning. (Parts, Sales, Service, Installations, and Repairs) Home | Products | Services | Coupons | Contracting  | Contact  | Employment  | About Us   | Sitemap Go Back: Glossary Terms “A” A Absolute Humidity Go Back: Glossary Terms “A” A