Humidity is the amount of water vapour present in air. Water vapour, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The amount of water vapour needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C.
Three primary measurements of humidity are widely employed: absolute, relative and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metreWyer, S.S., "A treatise on producer-gas and gas-producers", (1906) The Engineering and Mining Journal, London, p.23 or grams per kilogram.Perry, R.H. and Green, D.W, (2007) Perry's Chemical Engineers' Handbook (8th Edition), Section 12, Psychrometry, Evaporative Cooling and Solids Drying McGraw-Hill, Relative humidity, expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass.
Humidity plays an important role for surface life. For animal life dependent on perspiration (sweating) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces. This effect can be calculated using a heat index table, also known as a humidex.
Absolute humidity is the mass of the water vapor , divided by the volume of the air and water vapor mixture , which can be expressed as:
The absolute humidity changes as air temperature or pressure changes, if the volume is not fixed. This makes it unsuitable for chemical engineering calculations, e.g. in drying, where temperature can vary considerably. As a result, absolute humidity in chemical engineering may refer to mass of water vapor per unit mass of dry air, also known as the humidity ratio or mass mixing ratio (see "specific humidity" below), which is better suited for heat and mass balance calculations. Mass of water per unit volume as in the equation above is also defined as volumetric humidity. Because of the potential confusion, British Standard BS 1339 British Standard BS 1339 (revised), Humidity and Dewpoint, Parts 1-3 (2002-2007) suggests avoiding the term "absolute humidity". Units should always be carefully checked. Many 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 is normally expressed as a percentage; a higher percentage means that the air-water mixture is more humid.
Relative humidity 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, a rise in relative humidity increases the heat index to (and other ) by hindering the evaporation of perspiration from the skin. For example, according to the Heat Index, a relative humidity of 75% at air temperature of 80.0 °F (26.7 °C) would feel like 83.6 °F ±1.3 °F (28.7 °C ±0.7 °C).Lans P. Rothfusz. "The Heat Index 'Equation' (or, More Than You Ever Wanted to Know About Heat Index)", Scientific Services Division (NWS Southern Region Headquarters), 1 July 1990
There are various devices used to measure and regulate humidity. Calibration standards for the most accurate measurement include the gravimetric hygrometer, chilled mirror hygrometer, and electrolytic hygrometer. The gravimetric method, while the most accurate, is very cumbersome. For fast and very accurate measurement the chilled mirror method is effective.Pieter R. Wiederhold. 1997. Water Vapor Measurement, Methods and Instrumentation. Marcel Dekker, New York, NY For process on-line measurements, the most commonly used sensors nowadays are based on capacitance measurements to measure relative humidity"BS1339" Part 3, frequently with internal conversions to display absolute humidity as well. These are cheap, simple, generally accurate and relatively robust. All humidity sensors face problems in measuring dust-laden gas, such as exhaust streams from Clothes dryer.
Humidity is also measured on a global scale using remotely placed . These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 kilometres. Satellites that can measure water vapor have sensors that are sensitive to infrared. 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 weather forecasts.
The most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in South Asia and Southeast Asia are among the most humid. Kuala Lumpur, Manila, Jakarta, and Singapore have very high humidity all year round because of their proximity to water bodies and the equator and often overcast weather. Some places experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm sauna, such as Kolkata, Chennai and Cochin in India, and Lahore in Pakistan. Sukkur city located on the Indus River in Pakistan has some of the highest and most uncomfortable in the country, frequently exceeding in the Monsoon season. High temperatures combine with the high dew point to create heat index in excess of . Darwin, Australia experiences an extremely humid wet season from December to April. Shanghai and Hong Kong also have an extreme humid period in their summer months. 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 .
The highest dew points consistently found 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. However, Houston lacks the coastal breeze present in Key West, and, as a much larger city, it suffers from the urban heat island effect.
For reasons speculated to include evapotranspiration from crops with broad leaves (e.g. maize) and, on some occasions, bright sunshine beating down on saturated soil, the upper Mississippi River valley can on occasion have extreme dew points, and in fact the record for the Western Hemisphere has been set in the region and has remained somewhere in the region for many years. A dew point of was recorded in Moorhead Minnesota on July 19, 2011, with a heat index of 133.5, although dew points over are rare there. Appleton, Wisconsin registered a dew point of 90 degrees F on 13 July 1995 at 17.00 local time (UTC -5.00) with an air temperature of resulting in a heat index of degrees; the dew point record and ostensibly the simultaneous possible heat index (for western hemisphere), specific humidity and absolute humidity world records have apparently held and in fact the highest dew point measured in the country bounced amongst or was tied by locations in Wisconsin, Minnesota, and Iowa during the preceding 90 years or more with locations in northern Illinois also coming close. The record was set during the 1995 Midwest heat wave disaster which was characterised by a very strong temperature inversion which caused cloudless skies during this in spite of an apparent world record surface-based CAPE set at the same time."The Heat" Northside Wednesday Evening Observer (Madison and Waunakee, Wisconsin) 25. July 1995, pp 1-3, 6
Given the extreme lack of moisture reaching the area, as well as low temperatures, the lowest dewpoints as well as absolute and specific humidity, as well as instances of virtually 0% relative humidity or at least too small a fraction of a percent to calculate are found in Antarctica in places such as the South Pole and the Russian Vostok scientific station.
The US city with the lowest annual humidity is Las Vegas, Nevada, averaging 39% for a high and 21% as a low. The lowest recorded, and a record for North America, was set in April 2016 and given as 1 per cent and was in fact a fraction of that in a case of a temperature of with a dew point of .
Second, water vapor is the most abundant of all greenhouse gases. Water vapor, like a green lens that allows green light to pass through it but absorbs red light, is a "selective absorber". Along with other greenhouse gases, water vapor is transparent to most solar energy, as one can literally see. But it absorbs the infrared energy emitted (radiated) upward by the earth's surface, which is the reason that humid areas experience very little nocturnal cooling but dry desert regions cool considerably at night. This selective absorption causes the greenhouse effect. It raises the surface temperature substantially above its theoretical radiative equilibrium temperature with the sun, and water vapor is the cause of more of this warming than any other greenhouse gas.
Unlike most other greenhouse gases, however, water is not merely below its boiling point in all regions of the Earth, but below its freezing point at many altitudes. As a condensible greenhouse gas, it precipitates, with a much lower scale height and shorter atmospheric lifetime- weeks instead of decades. Without other greenhouse gases, Earth's blackbody temperature, below the freezing point of water, would cause water vapor to be removed from the atmosphere. Water vapor is thus a "slave" to the non-condensible greenhouse gases.
Humid air is less dense than dry air because a molecule of water (molecular mass ≈ 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.
The human body dissipates heat through perspiration and its evaporation. Heat convection, to the surrounding air, and thermal radiation are the primary modes of heat transport from the body. Under conditions of high humidity, the rate of evaporation of sweat from the skin decreases. 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 dissipate heat by conduction to the air. With so much blood going to the external surface of the body, less goes to the active , the Human 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.
Some people experience difficulty breathing in humid environments. Some cases may possibly be related to respiratory conditions such as asthma, while others may be the product of anxiety. Sufferers will often hyperventilation in response, causing sensations of numbness, faintness, and loss of concentration, among others.
Air conditioning reduces discomfort by reducing not just temperature but humidity as well. Heating cold outdoor air can decrease relative humidity levels indoors to below 30%, leading to ailments such as dry skin, cracked lips, dry eyes and excessive thirst.
In contrast, a very low humidity level favors the build-up 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. often monitor relative humidity levels for these reasons.