Humidity

Humidity is the amount of water vapor in the air. Relative humidity is defined as the ratio of the partial pressure of water vapor 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.

Contents

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 balloon, 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.

Mixing ratio, humidity ratio, or mass-basis absolute humidity

Mixing or humidity ratio is expressed as a ratio of water vapor mass, m_w , per kilogram of dry air, m_d , in any given parcel of air. The colloquial term moisture content is also used instead of mixing/humidity ratio. Humidity ratio is a standard axis on psychrometric charts, and is a useful parameter in psychrometrics calculations because it does not change with temperature except when the air cools below dewpoint (at which time some water vapor condenses out of the air/vapor mixture).

That ratio can be given as:

\omega = {m_w \over {m_d}}

Mixing ratio can also be expressed with the partial pressure of water vapor:[1]

\omega = { M_w p_w \over {M_d ( p_a - p_w )}}

Or in terms of the molar fraction of water vapor in the air:

\omega = { M_w X_w \over {M_d ( 1 - X_w )}}

Or, to go the other direction and calculate molar fraction or partial pressure:

X_w = {{ \omega M_d } \over {\omega M_d + M_w}}
p_w = {{ p_a \omega M_d } \over {\omega M_d + M_w}}

where

M_w = the molar mass of water vapor (18.02 kg/kmol)
M_d = the molar mass of dry air (28.97 kg/kmol)
X_w = molar fraction of water vapor in moist air
p_w = partial pressure of water vapor in moist air
p_a = bulk pressure of parcel of moist air.

Technically speaking, this is a dimensionless quantity as it is the mass of water vapor to the mass of dry air. So it is expressed as Kg/Kg. However, the mass of water vapor is much less than the value of the mass of dry air and most commonly meteorologists use gram/Kg which is 10^{-3} Kg/Kg.[2]

Effective molar mass of a moist air parcel can be calculated in terms of the mass mixing ratio \omega and the molar masses of both water vapor and dry air:

M_\text{moist} = { {M_w M_d (\omega + 1)} \over {M_w + \omega M_d}}

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. 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, at 27°C (80°F), a relative humidity of approximately 75% would feel like about 29°C (84°F).

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.

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.

Relative Humidity.png

Measuring humidity

A hygrometer is a device used for measuring the humidity of the air

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.

Humidity and air density

Humid air is less dense than dry air because a molecule of water (M ≈ 18 u ) is less massive than either a molecule of nitrogen (M ≈ 28) or a molecule of oxygen (M ≈ 32). About 78% of the molecules in dry air are nitrogen (N2). Another 21% of the molecules in dry air are oxygen (O2). The final 1% of dry air is a mixture of other gases. For any gas, at a given temperature and pressure, the number of molecules present in a particular volume is constant - see ideal gas law. So when water molecules (vapor) are introduced into that volume of dry air, the number of air molecules in the volume must decrease by the same number, if the temperature and pressure remain constant. (The addition of water molecules, or any other molecules, to a gas, without removal of an equal number of other molecules, will necessarily require a change in temperature, pressure, or total volume; that is, a change in at least one of these three parameters. If temperature and pressure remain constant, the volume increases, and the dry air molecules that were displaced will initially move out into the additional volume, after which the mixture will eventually become uniform through diffusion.) Hence the mass per unit volume of the gas—its density—decreases. Isaac Newton discovered this phenomenon and wrote about it in his book Opticks.[3]

Effects on human body

The human body sheds heat by a combination of evaporation of perspiration, heat convection in the surrounding air, and thermal radiation. Under conditions of high humidity, the evaporation of sweat from the skin decreases, 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, resulting in heat stroke or hyperthermia.

Recommendations for comfort

Humans control their body temperature mainly by sweating and shivering. The United States Environmental Protection Agency cites the ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%. At high humidity, sweating is less effective, and we feel hotter. Air conditioning works by reducing humidity in summer. In winter, heating cold outdoor air can decrease relative humidity levels indoor to below 30%, leading to discomfort such as dry skin and excessive thirst.

Effects on electronics

Many electronic devices have humidity specifications, for example, 5% to 95%. At the top end of the range, moisture may increase the conductivity of permeable insulators leading to malfunction. Too low humidity may make materials brittle. A particular danger to electronic items, regardless of the stated operating humidity range, is condensation. When an electronic item is moved from a cold place (e.g. garage, car, shed, an air conditioned space in the tropics) to a warm humid place (house, outside tropics), condensation may coat circuit boards and other insulators, leading to short circuit inside the equipment. Such short circuits may cause substantial permanent damage if the equipment is powered on before the condensation has evaporated. A similar condensation effect can often be observed when a person wearing glasses comes in from the cold. It is advisable to allow electronic equipment to acclimatise for several hours, after being brought in from the cold, before powering on. The inverse is also true. In situations where time is critical, increasing air flow through the device's internals when possible, such as removing the side panel from a PC case and directing a fan to blow into the case, can dramatically reduce the time needed to acclimatise to the new environment.

Low humidity also favors the buildup of static electricity, which may result in spontaneous shutdown of computers when discharges occur. Apart from spurious erratic function, electrostatic discharges can cause dielectric breakdown in solid state devices, resulting in irreversible damage. Data centers often monitor relative humidity levels for these reasons.

Humidity in building construction

Traditional building designs typically had weak insulation, and it allowed air moisture to flow freely between the interior and exterior. The energy-efficient, heavily-sealed architecture introduced in the 20th century also sealed off the movement of moisture, and this has resulted in a secondary problem of condensation forming in and around walls, which encourages the development of mold and mildew. Additionally, buildings with foundations not properly sealed will allow water to flow through the walls due to capillary action, notably cement which is a good conductor of water. Solutions for energy-efficient buildings that avoid condensation are a current topic of architecture.

Most humid places on Earth

The most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in South and Southeast Asia are among the most humid, such as Kolkata and those in Kerala in India, the cities of Manila in the Philippines and Bangkok in Thailand: these places experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm sauna.[4] Darwin, Australia experiences an extremely humid wet season from December to April. Shanghai and Hong Kong in China also have a extreme humid period in their summer months. Kuala Lumpur and Singapore have very high humidity all year round because of their proximity to water bodies and the equator and overcast weather. Perfectly clear days are dependent largely upon the season in which one decides to travel. During the South-west and North-east Monsoon seasons (respectively, late May to September and November to March), expect heavy rains and a relatively high humidity post-rainfall. Outside the monsoon seasons, humidity is high (in comparison to countries North of the Equator), but completely sunny days abound. In cooler places such as Northern Tasmania, Australia, high humidity is experienced all year due to the ocean between mainland Australia and Tasmania. In the summer the hot dry air is absorbed by this ocean and the temperature rarely climbs above 35 degrees Celsius.

In the United States the most humid cities, strictly in terms of relative humidity, are Forks and Olympia, Washington.[5] This fact may come as a surprise to many, as the climate in this region rarely exhibits the discomfort usually associated with high humidity. Dew points are typically much lower on the West Coast than on the East. Because high dew points play a more significant role than relative humidity in the discomfort created during humid days, the air in these western cities usually does not feel "humid."

The highest dew points in the US are found in coastal Florida and Texas. When comparing Key West and Houston, two of the most humid cities from those states, coastal Florida seems to have the higher dew points on average. But, as noted by Jack Williams of USA Today,[6] Houston lacks the coastal breeze present in Key West, and, as a much larger city, it suffers from the urban heat island effect. A dew point of 86 degrees Fahrenheit was recorded in southern Minnesota on July 23, 2005, though dew points over 80 degrees Fahrenheit are rare there.[7] The US city with the lowest annual humidity is Yuma, Arizona, averaging under 50% for a high and 22% as a low. The next-lowest humidity is Tucson, Arizona, average high humidity of 57% and a low of 26%. Lowest in the world is Antarctica.

See also

References

  1. ↑ "Mixing ratio". Glossary of Meteorology. American Meteorological Society. http://amsglossary.allenpress.com/glossary/search?id=mixing-ratio1. Retrieved 2009-08-02. 
  2. ↑ Met Office. "Data processing procedure". E-AMDAR Evaluation. World Meteorological Organisation. http://www.wmo.int/pages/prog/www/IMOP/meetings/Upper-Air/ET-IOC-3/Doc5.pps#374,7,Data%20processing%20procedure. Retrieved 2009-08-02. 
  3. ↑ Newton, Isaac (1704). Opticks. Dover. http://books.google.com/books?id=iTpXLrPR2TQC&printsec=frontcover&dq=isaac+newton+optics. 
  4. ↑ BBC - Weather Centre - World Weather - Average Conditions - Bangkok
  5. ↑ What Is The Most Humid City In The U.S.? | KOMO-TV - Seattle, Washington | News Archive
  6. ↑ Answers: Is Florida or Texas more humid: September 3,2003
  7. ↑ High Dew Point Temperatures: July 23, 2005

External links

Meteorological data and variables
General
Adiabatic processes · Lapse rate · Lightning · Surface solar radiation · Surface weather analysis · Visibility · Vorticity · Wind
Condensation
Cloud · Cloud condensation nuclei · Fog · Precipitation · Water vapor
Convection

Convective available potential energy (CAPE) · Convective inhibition (CIN) · Convective instability · Convective temperature (Tc) · Helicity · Lifted index (LI) · Bulk Richardson number (BRN)
Temperature
Dew point (Td) · Equivalent temperature (Te) · Forest fire weather index · Haines Index · Heat index · Humidex · Humidity · Potential temperature (θ) · Equivalent potential temperature (θe) · Sea surface temperature (SST) · Wet-bulb temperature · Wet-bulb potential temperature · Wind chill
Pressure
Atmospheric pressure · Baroclinity · Barotropicity