Humidity

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Humidity is the amount of water vapor in the air. It is measured in three ways: absolute humidity, relative humidity, and specific humidity. Relative humidity is the most frequently encountered measurement of humidity because it is regularly used in weather forecasts. It is an important part of weather forecasts because it indicates the likelihood of precipitation, dew, or fog. Higher relative humidity also 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. Warmer air has more thermal energy than cooler air; thus more water molecules can evaporate and stay in the air in a vapour state rather than a liquid state. This may be why people say that warmer air "holds" more moisture — in warmer air, there is more energy for more water molecules to hold themselves in the air (and overcome hydrogen bonds which seek to pull water molecules together).

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[edit] Absolute humidity

Absolute humidity refers to the mass of water in a particular volume of air. Absolute humidity is expressed as the number of kilograms of water vapor per cubic meter of air. Imagine a cube of air that is one meter wide, one meter high, and one meter deep. If we could “squeeze” all the water out of that cube of air into a container, we could weigh the container and see how many kilograms of water it contains. The amount of vapor in that cube of air is the absolute humidity of that cubic meter of air. More technically: mw, per cubic meter of air, va .

AH = {m_w \over v_a}

As such, its value changes as the 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 vapour per unit mass of dry air, also known as the mixing ratio (see below), which is much more rigorous for heat and mass balance calculations. Mass of water per unit volume 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.

[edit] Relative Humidity

Main article: 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. That is, a ratio of how much energy has been used to free water from liquid to vapor form to how much energy is left. Relative humidity is expressed as a percentage and is calculated in the following manner:

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

where

{p_{(H_2O)}} is the partial pressure of water vapor in the gas mixture;
{p^*_{(H_2O)}} is the saturation vapor pressure of water at the temperature of the gas mixture; and
RH_{\,_\,} is the relative humidity of the gas mixture being considered.

[edit] Mixing ratio

Mixing ratio is the ratio of water vapor to wet air in a particular height of air. Mixing ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of dry air, md .

That ratio can be given as:

MRi = {m_w \over m_d}

Partial pressure of water vapor and air can also be used to express the ratio.

[edit] Specific humidity

Specific humidty is the ratio of water vapor to air (dry air plus water vapor) in a particular volume of air. Specific humidity ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of air, ma .

That ratio can be given as:

SH = {m_w \over m_a}

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

SH = {MR \over 1+MR}
MR = {SH \over 1-SH}

[edit] Measuring and regulating humidity

A hygrometer used to measure the relative humidity
A hygrometer used to measure the relative humidity

There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or hygrometer. A humidistat is used to regulate the humidity of a building with a de-humidifier. These can be analogous to a thermometer and thermostat for temperature control.

Humidity is also measured on a global scale using remotely placed satellites. These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 kilometers. Satellites that can measure water vapor have sensors that are sensitive to infrared radiation. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of future weather forecasts.

[edit] Why humidity can be less than 100% when it's raining

Humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens 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 rarely enough to bring the humidity to 100%. Indeed, falling rain may be cold enough to condense water vapor from warm, humid air, thus lowering the relative humidity.

[edit] Humidity and air density

Humid air is lighter and thus less dense than dry air. This is due to the fact that a molecule of water, H2O, weighs less than molecules of Nitrogen (N2) and Oxygen (O2). Isaac Newton discovered this phenomenon and wrote about it in his book Opticks.[1] From Avogadro's ideal gas law, a fixed volume of gas at a given temperature and pressure always contains the same number of molecules regardless of what type of gas it is. Consider a cubic meter of perfectly dry air. It contains about 78% nitrogen molecules, which each have an atomic weight of 28. Another 21% of the air is oxygen, with each molecule having an atomic weight of 32. The final one percent is a mixture of other gases. If molecules of water vapour replace the diatomic nitrogen or oxygen molecules in this fixed volume, then the weight of the air decreases, and hence the density decreases.

This may seem counterintuitive as water is commonly perceived to be much heavier than air. It is true that liquid water is heavier than air. However, the water that makes the air humid is not liquid, it is water vapor, which is lighter than nitrogen or oxygen gas. Popo Temperature and pressure have much more marked effects on the density of air; however humid air is lighter than dry air at the same temperature and pressure because of this molecular difference.

[edit] Dew point and frost point

Associated with relative humidity is dew point (if the dew point is below freezing, it is referred to as the frost point). Dew point is the temperature at which water vapor saturates from an air mass into liquid or solid usually forming rain, snow, frost, or dew. Dew point normally occurs when a mass of air has a relative humidity of 100%. This happens in the atmosphere as a result of cooling through a number of different processes.

[edit] Effects on human body

The human body sheds heat by a combination of evaporation of perspiration, conduction to the surrounding air, and thermal radiation. Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot shed heat by conduction to the air, and a condition called hyperpyrexia results. With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain, and other internal organs. Physical strength declines and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected. This resulting condition is called heat stroke or hyperthermia.

[edit] Recommendations for comfort

Humans control their body temperature by sweating and shivering. The US Environmental Protection Agency cites the ASHRAE Standard 55-1992 Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%, with below 50% preferred to control dust mites. At high humidity sweating is less effective so we feel hotter; thus the desire to remove humidity from air with air conditioning in the summer. In the winter, heating cold outdoor air can decrease indoor relative humidity levels to below 30%, leading to discomfort such as dry skin and excessive thirst.

[edit] Popular culture

First spoken by Warren Hymer in the 1939 movie Mr. Moto on Danger Island, the expression "It's not the heat, it's the humidity" refers to unpleasantly muggy weather; that is, the fact that humid air can be significantly less comfortable than drier air at the same temperature.

[edit] References

  1. ^ "Opticks", Dover, 1704.
Meteorological data and variables

Atmospheric pressure | Baroclinity | Cloud | Convection | CAPE | CIN | Dew point | Heat index | Humidex | Humidity | Lightning | Pot T | Precipitation | Sea surface temperature | Surface solar radiation | Surface weather analysis | Temperature | Theta-e | Visibility | Vorticity | Wind chill | Water vapor | Wind

[edit] External links

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