Humidity: Difference between revisions

The relative humidity <math>(RH</math> or <math>\phi)</math> of an air-water mixture is defined as the ratio of the [[partial pressure]] of water vapor <math>(p_{H_2O})</math> in the mixture to the [[equilibrium vapor pressure]] of water <math>(p^*_{H_2O})</math> over a flat surface of pure water<ref>{{cite web|url=http://www.atmos.umd.edu/~stevenb/vapor/|title=Water Vapor Myths: A Brief Tutorial}}</ref> at a given temperature:<ref name="Perry">Perry, R.H. and Green, D.W, ''[[Perry's Chemical Engineers' Handbook]]'' (7th Edition), [[McGraw-Hill]], {{ISBN|0-07-049841-5}}, Eqn 12-7</ref><ref>{{cite book |last=Lide |first=David |date=2005 |title=CRC Handbook of Chemistry and Physics |edition=85 |url=https://archive.org/details/crchandbookofche81lide/page/15 |publisher=CRC Press |pages=[https://archive.org/details/crchandbookofche81lide/page/15 15–25] |isbn=0-8493-0485-7 }}</ref><ref name="umd.edu" />

In other words, relative humidity is the ratio of ''how much water vapour is in the air'' and ''how much water vapour the air could potentially contain'' at a given temperature. It varies with the temperature of the air: colder air can hold less vapour. So changing the temperature of air can change the relative humidity, even when the absolute humidity remains constant.

Relative humidity only considers the invisible water vapour. Mists, clouds, fogs and aerosols of water do not count towards the measure of relative humidity of the air, although their presence is an indication that a body of air may be close to the [[dew point]].  

While humidity itself is a climate variable, it also affects other climate variables. Environmental humidity is affected by winds and by rainfall.

High temperatures combine with the high dew point to create heat index in excess of {{convert|65|C|F|}}. [[Darwin, Australia|Darwin]] experiences an extremely humid wet season from December to April. [[Houston]], [[Miami]], [[San Diego]], [[Osaka]], [[Shanghai]], [[Shenzhen]] and [[Tokyo]] 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 further from 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 {{convert|35|C|0|abbr=on}}.

Electronic devices are often rated to operate only under certain humidity conditions (e.g., 10% to 90%). At the top end of the range, moisture may increase the conductivity of permeable [[Insulator (electrical)|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, 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 (i.e. the glasses become foggy).<ref>{{cite web|title=Fogging Glasses|url=http://www.newton.dep.anl.gov/askasci/gen01/gen01333.htm|access-date=2012-08-08|archive-date=2015-02-26|archive-url=https://web.archive.org/web/20150226032245/http://www.newton.dep.anl.gov/askasci/gen01/gen01333.htm|url-status=dead}}</ref> It is advisable to allow electronic equipment to acclimatise for several hours, after being brought in from the cold, before powering on. Some electronic devices can detect such a change and indicate, when plugged in and usually with a small droplet symbol, that they cannot be used until the risk from condensation has passed. In situations where time is critical, increasing air flow through the device's internals, such as removing the side panel from a PC case and directing a fan to blow into the case, will reduce significantly the time needed to acclimatise to the new environment.

===Industry===

Relative humidity can exceed 100%, in which case the air is [[supersaturated]].  Cloud formation requires [[supersaturated]] air.  [[Cloud condensation nuclei]] lower the level of supersaturation required to form fogs and clouds - in the absence of nuclei around which droplets or ice can form, a higher level of supersaturation is required for these droplets or ice crystals to form spontaneously.  In the [[Wilson cloud chamber]], which is used in nuclear physics experiments, a state of supersaturation is created within the chamber, and moving subatomic particles act as condensation nuclei so trails of fog show the paths of those particles.

Due to the increasing potential for a higher water vapor partial pressure at higher air temperatures, the water content of air at sea level can get as high as 3% by mass at {{convert|30|C}} compared to no more than about 0.5% by mass at {{convert|0|C}}. This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, resulting in dry [[Human skin|skin]], [[itch]]y [[Human eye|eye]]s, and persistence of [[static electricity|static electric]] charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity, which lowers relative humidity and increases evaporation rates from moist surfaces indoors (including human bodies and household plants.)