Humidity

Humidity is the amount of water vapor in the air. Relative humidity is defined as the ratio of the partial pressure of water vapour in a parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature. Humidity may also be expressed as specific humidity. Relative humidity is an important metric used in forecasting weather. Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by reducing the evaporation of perspiration from the skin. This effect is calculated in a heat index table.

Types of humidity

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 container could be weighed 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,

m w

, per cubic meter of total moist air,

V net

$AH = {m_w \over V_{net}}.$

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.[1] (See also Absolute Humidity table)

The absolute humidity changes as air pressure changes. This is very inconvenient for chemical engineering calculations, e.g. for 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 engineering 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 that is in the air at a specific temperature compared to the maximum amount of water vapor that the specific temperature is able to hold without the water condensing. Relative humidity is expressed as a percentage and is calculated in the following manner:

$\phi = {p_{({\rm H_2O})} \over p^*_{({\rm H_2O)}}} \times 100%$

where

${p_{({\rm H_2O)}}}$ is the partial pressure of water vapor in the gas mixture;

${p^*_{({\rm H_2O)}}}$ is the saturation vapor pressure of water at the temperature of the gas mixture; and

$\phi {\,_\,}$ is the relative humidity of the gas mixture being considered.

Relative humidity is often mentioned 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 ±17. °C) at ~44% relative humidity..

[edit] 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,

m w

, per kilogram of total moist air

m t

That ratio can be shown as:

$SH = {m_w \over m_t}.$

Specific humidity is related to mixing ratio (and vice versa) by:

$SH = {{\omega} \over {1+\omega}}$

$\omega = {{SH }\over {1 - SH}}.$

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.

SCM - Wet & Dry

Tuesday, November 2, 2010