Humidity is the amount of water vapour in the air. In daily language the term "humidity" is normally taken to mean
relative humidityRelative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.- Definition :...
. Relative humidity is defined as the ratio of the
partial pressureIn a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of water vapour in a
parcelIn fluid dynamics, within the framework of continuum mechanics, a fluid parcel is a very small amount of fluid, identifiable throughout its dynamic history while moving with the fluid flow. As it moves, the mass of a fluid parcel remains constant, while—in a compressible flow—its volume may...
of air to the saturated vapour pressure of water vapour at a prescribed
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
. Humidity may also be expressed as absolute humidity and specific humidity. Relative humidity is an important
metricIn mathematics, a metric or distance function is a function which defines a distance between elements of a set. A set with a metric is called a metric space. A metric induces a topology on a set but not all topologies can be generated by a metric...
used in
forecasting weatherWeather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Human beings have attempted to predict the weather informally for millennia, and formally since at least the nineteenth century...
. Humidity indicates the likelihood of
precipitationIn meteorology, precipitation is any product of the condensation of atmospheric water vapor that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel...
,
dewDew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets.When...
, or
fogFog is a cloud that is in contact with the ground. A cloud may be considered partly fog; for example, the part of a cloud that is suspended in the air above the ground is not considered fog, whereas the part of the cloud that comes in contact with higher ground is considered fog...
. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of
sweatingPerspiration is the production of a fluid, consisting primarily of water as well as various dissolved solids , that is excreted by the sweat glands in the skin of mammals...
to cool the body by reducing the
evaporationEvaporation is the vaporization of a liquid and the reverse, of condensation. A type of phase transition, it is the process by which molecules in a liquid state spontaneously become gaseous . Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed...
of perspiration from the skin. This effect is calculated in a
heat indexThe heat index is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries...
table.
Absolute humidity
Absolute humidity 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 vapor in that cube of air is the absolute humidity of that cubic meter of air. More technically: the mass of water vapor , per cubic meter of air, .
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 vapour is saturated at 30 °C.
http://www.britannica.com/eb/article-53259/climate#292984.hook
(See also
Absolute Humidity table)
The absolute humidity changes as air
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
changes. This is very inconvenient for
chemical engineeringChemical engineering is the branch of engineering that deals with the application of physical science , and life sciences with mathematics, to the process of converting raw materials or chemicals into more useful or valuable forms...
calculations, e.g. for
dryerA clothes dryer or tumble dryer is a household appliance that is used to remove moisture from a load of clothing and other textiles, generally shortly after they are cleaned in a washing machine....
s, where
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
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
PsychrometricsPsychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures...
.
Mixing ratio or humidity ratio
Mixing or
humidity ratio is expressed as a ratio of water vapour mass, , per kilogram of dry air, , at a given pressure. 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
psychrometricsPsychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures...
calculations because it does not change with temperature except when the air cools below dewpoint.
That ratio can be given as:
Mixing ratio can also be expressed with the
partial pressureIn a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of water vapor :
where
is the ratio of molecular weights of water vapour and dry air
partial pressure of water vapor in moist air
atmospheric pressure of moist air
Technically speaking, this is a
dimensionless quantityIn dimensional analysis, a dimensionless quantity is a quantity without any physical units and thus a pure number. Such a number is typically defined as a product or ratio of quantities which do have units, in such a way that all the units cancel out....
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
gThe gram , ; symbol g, is a unit of mass.Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice" , a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or...
/Kg which is Kg/Kg.
Relative humidity
Relative humidity is defined as the ratio of the partial pressure of water vapor (in a
gasThis page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...
eous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. Relative humidity is expressed as a percentage and is calculated in the following manner:
where is the
partial pressureIn a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of water vapor in the gas mixture; is the saturation vapor pressure of water at the temperature of the gas mixture; and is the relative humidity of the gas mixture being considered.
Relative humidity is often mentioned in
weather forecastsWeather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Human beings have attempted to predict the weather informally for millennia, and formally since at least the nineteenth century...
and reports, as it is an indicator of the likelihood of
precipitationIn meteorology, precipitation is any product of the condensation of atmospheric water vapor that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel...
, dew, or fog. In hot summer
weatherWeather is a set of all the phenomena occurring in a given atmosphere at a given time. Weather phenomena lie in the troposphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods...
, it also increases the
apparent temperatureThe heat index is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries...
to
humanHumans are bipedal primates belonging to the species Homo sapiens in Hominidae, the great ape family. They are the only surviving member of the genus Homo. Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving...
s (and other
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s) by hindering the
evaporationEvaporation is the vaporization of a liquid and the reverse, of condensation. A type of phase transition, it is the process by which molecules in a liquid state spontaneously become gaseous . Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed...
of perspiration from the skin as the relative humidity rises. For example, if it was 80 degrees in Portland, Oregon; with a relative humidity of approximately 75%, it would feel about 110 degrees.
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, , per kilogram of air (including water vapor), .
That ratio can be shown as:
Specific humidity is related to mixing ratio (and vice versa) by:
Humidity during rain
Humidity is a measure of the amount of water vapor dissolved in the air, not including any liquid water or ice falling through the air. For
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
s to form, and
rainRain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface...
to start, the air doesn't have to reach 100% relative humidity at the Earth's surface, but only where the clouds and rain drops 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 pointThe dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point...
, thus condensing water vapor out of the air. Although that would indeed raise the
relative humidityRelative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.- Definition :...
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 pointThe dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point...
(If the dew point is below freezing, it is referred to as the frost point). Dew point is the
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
at which water vapor saturates from an air mass into liquid or solid usually forming
rainRain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface...
,
snowSnow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snowflakes...
,
frostFrost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air. Frost crystals' size differ depending on time and water vapor available. Frost is also usually translucent in appearance. There are many types of...
, or
dewDew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets.When...
. Dew point normally occurs when a mass of air has a relative humidity of 100%. This happens in the
atmosphereThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
as a result of cooling through a number of different processes.
Measuring and regulating humidity
There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or
hygrometerHygrometers are instruments used for measuring relative humidity. A simple form of a hygrometer is specifically known as a psychrometer and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature...
. A humidistat is used to regulate the humidity of a building with a de-humidifier. These can be analogous to a
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
and
thermostatA thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The name is derived from the Greek words thermos "hot" and statos "a standing"...
for temperature control.
During coating applications Humidity and air temperature are measured in combination with surface temperature to determine the dewpoint. This is done by use of a
dewcheckA dewcheck is a dewpoint gauge intended for the coating industry, a device used to measure air temperature, surface temperature and relative humidity. Dewpoint is calculated from air temperature and relative humidity, and the difference between dewpoint and surface temperature is determined and can...
dewpoint gauge.
Humidity is also measured on a global scale using remotely placed
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s. These satellites are able to detect the concentration of water in the
troposphereThe troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75 percent of the atmosphere's mass and 99 percent of its water vapor and aerosols....
at altitudes between 4 and 12 kilometers. Satellites that can measure water vapor have sensors that are sensitive to
infrared radiationInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
. 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 (
mThe molecular mass of a substance, frequently referred to by the older term molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u...
= 18) is less dense than a molecule of
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
(m = 28) and a molecule of
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
(m = 32). About 78% of the molecules in dry air are nitrogen (N
2). Another 21% of the molecules in dry air are oxygen (O
2). 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 is constant for a particular volume - see
ideal gas lawThe Ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behaviour of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...
. So when water molecules (vapor) are introduced to the dry air, the number of air molecules must reduce by the same number in a given volume, without the pressure or temperature increasing. Hence the mass per unit volume of the gas (its density) decreases.
Isaac NewtonSir Isaac Newton FRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian who is perceived and considered by a substantial number of scholars and the general public as one of the most influential men in history...
discovered this phenomenon and wrote about it in his book
OpticksOpticks is a book written by English physicist Isaac Newton that was released to the public in 1704. It is about optics and the refraction of light, and is considered one of the great works of science in history...
.
Effects on human body
The human body sheds heat by a combination of evaporation of perspiration, heat convection in the surrounding air, and
thermal radiationThermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. An example of thermal radiation is the infrared radiation emitted by a common household radiator or electric heater. A person near a raging bonfire will feel the radiated...
. 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,
bloodBlood is a specialized bodily fluid that delivers necessary substances to the body's cells — such as nutrients and oxygen — and transports waste products away from those same cells....
brought to the body surface cannot shed heat by conduction to the air, and a condition called
hyperpyrexiaIn medicine, hyperpyrexia is an excessive and unusual elevation of set body temperature greater than or equal to 41 °C . It is an extremely high fever...
results. With so much blood going to the external surface of the body, relatively less goes to the active
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s, the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
, and other internal
organsIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
.
Physical strengthPhysical strength is the ability of a person to exert force on physical objects using muscles. Increasing physical strength is the goal of strength training.-Overview:...
declines and
fatigueFatigue is a state of awareness. It can describe a range of afflictions, varying from a general state of lethargy to a specific work-induced burning sensation within one's muscles. It can be both physical and mental...
occurs sooner than it would otherwise. Alertness and mental capacity also may be affected. This resulting condition is called
heat stroke or
hyperthermiaHyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate...
.
Recommendations for comfort
Humans control their body temperature mainly by
sweatingPerspiration is the production of a fluid, consisting primarily of water as well as various dissolved solids , that is excreted by the sweat glands in the skin of mammals...
and
shiveringShivering is a bodily function in response to early hypothermia in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered. Muscle groups around the vital organs begin to shake in small movements in an attempt to create warmth by expending energy...
. The
United States Environmental Protection AgencyThe U.S. Environmental Protection Agency is an agency of the federal government of the United States charged to regulate chemicals and protect human health by safeguarding the natural environment: air, water, and land...
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 conditioningAn air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area, or provide heat to an area. The cooling is done using a simple refrigeration cycle...
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.
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
insulatorsAn insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, also called insulators or insulation, intended to support or separate...
leading to malfunction. Too low humidity may make materials brittle. A particular danger to electronic items, regardless of the stated operating humidity range, is
condensationCondensation is the change of the physical state of aggregation of matter from gaseous phase into liquid phase and the reverse of evaporation. When the transition happens from the gaseous phase into the solid phase directly, bypassing the liquid phase, the change is called deposition...
. When an electronic item is moved from a cold place (eg 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 circuitA short circuit in an electrical circuit is one that allows a current to travel along a different path from the one originally intended....
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.
Low humidity also favors the buildup of
static electricityStatic electricity refers to the buildup of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge...
, 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 centerA data center or datacenter , also called a server farm,is a facility used to house computer systems and associated components, such as telecommunications and storage systems...
s often monitor relative humidity levels for these reasons.
Humidity in construction
Traditional building designs typically had weak
insulationBuilding insulation refers broadly to any object in a building used as insulation for any purpose. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation...
, and this allowed air moisture to flow freely between the interior and exterior. The energy-efficient, heavily-sealed architecture introduced in the
20th centuryThe 20th century of the Common Era began on January 1, 1901 and ended on December 31, 2000, according to the Gregorian calendar.The British Empire, the Russian Empire, the German Empire, the Ottoman Empire, and the Austro-Hungarian Empire dissolved in the first half of the century, with all but the...
also sealed off the movement of moisture, and this has resulted in a secondary problem of
condensationCondensation is the change of the physical state of aggregation of matter from gaseous phase into liquid phase and the reverse of evaporation. When the transition happens from the gaseous phase into the solid phase directly, bypassing the liquid phase, the change is called deposition...
forming in and around walls, which encourages the development of mold and mildew. 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, formerly , is the capital of the Indian state of West Bengal. It is located in eastern India on the east bank of the River Hooghly. When referred to as Calcutta, it usually includes the suburbs, and thus its population exceeds 15 million, making it India's third-largest metropolitan area and...
and those in
KeralaKerala , is a state located in southwestern India. The state was created in 1956 on linguistc basis, bringing together those places where Malayalam formed the principal language...
in
IndiaIndia, officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
, the cities of
ManilaThe City of Manila , or simply Manila or Maynila, is the capital of the Philippines and one of the 17 cities and municipalities that make up Metro Manila. It is located on the eastern shores of Manila Bay, on the western portion of the National Capital Region, in the western side of Luzon...
in the
PhilippinesThe Philippines officially known as the Republic of the Philippines, is a country in Southeast Asia with Manila as its capital city. It comprises 7,107 islands in the western Pacific Ocean....
and
BangkokBangkok is the capital, largest urban area and primary city of Thailand. Known in Thai as Krung Thep Mahanakhon , or กรุงเทพฯ Krung Thep for short, it was a small trading post at the mouth of the Chao Phraya River during the Ayutthaya Kingdom. It came to the forefront of Siam when it was given the...
in
ThailandThe Kingdom of Thailand is an independent country that lies in the heart of Southeast Asia.It is bordered to the north by Laos and Burma, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the west by the Andaman Sea and Burma...
: these places experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm Sauna. Darwin, Australia experiences an extremely humid wet season from December to April.
Kuala LumpurKuala Lumpur , is the capital and largest city of Malaysia. The city proper, making up an area of , has an estimated population of 1.6 million in 2006. Greater Kuala Lumpur, also known as the Klang Valley, is an urban agglomeration of 7.2 million...
and
SingaporeSingapore , officially the Republic of Singapore, is an island city-state located at the southern tip of the Malay Peninsula, lying north of the equator, south of the Malaysian state of Johor and north of Indonesia's Riau Islands. At , Singapore is a microstate and the smallest nation in Southeast...
have very high humidity all year round because of their proximity to water bodies and the
EquatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
and overcast weather; despite sunshine, perfectly clear days are rare in these locations and it is often misty. 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 humidityRelative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.- Definition :...
, are
ForksForks is a city in Clallam County, Washington, United States. The population was 3,120 at the 2000 census. It is named after the forks in the nearby Quillayute, Bogachiel, Calawah, and Sol Duc rivers....
and
Olympia, WashingtonOlympia is the capital city of the U.S. state of Washington and the county seat of Thurston County. It was incorporated on January 28, 1859. The population was 42,514 at the 2000 census...
. 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 are found in coastal
FloridaFlorida is a state located in the southeastern region of the United States, bordering Alabama to the northwest and Georgia to the north. It was the 27th state admitted to the United States...
and
TexasTexas is the second-largest U.S. state in both area and population, and the largest state in the contiguous United States.The name had wide usage among native Americans, meaning "friends" or "allies"...
. 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, Houston lacks the coastal breeze present in Key West, and, as a much larger city, it suffers from the
urban heat islandAn urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described, though not by name, by Luke Howard FRS in the 1810s. The temperature difference usually is larger at night than during the day, and is...
effect.
The US city with the lowest annual humidity is
YumaYuma is a city in and the county seat of Yuma County, Arizona, United States. The population of the city was 77,515 at the 2000 census, with a 2006 Census Bureau estimated population of 87,423....
,
ArizonaThe State of Arizona is a state located in the southwestern region of the United States. The capital and largest city is Phoenix. The second largest city is Tucson, followed in size by the four Phoenix metropolitan area cities of Mesa, Glendale, Chandler, and Scottsdale.Arizona was the 48th and...
, averaging under 50% for a high and 22% as a low. The next-lowest humidity is
Tucson, ArizonaTucson is a city in and the county seat of Pima County, Arizona, United States, located 118 miles southeast of Phoenix and 60 miles north of the U.S.-Mexico border. As of July 1, 2006, a Census Bureau estimate puts the city's population at 541,811, with a metropolitan area population at...
, average high humidity of 57% and a low of 26%. Lowest in the world is
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
.
See also
- Relative humidity
Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.- Definition :...
- Savory brittleness scale
The Savory brittleness scale is a measure of the annual distribution of humidity in a particular environment. It was developed by Allan Savory, a Zimbabwean biologist. A tropical rainforest would measure 1 on the scale, and an arid desert such as the Sahara would rate a 10.The scale reflects annual...
- Concentration
In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.To...
- Heat index
The heat index is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries...
- Dew point
The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point...
- Dew point depression
The dew point depression is the difference between the temperature and dew point temperature at a certain height in the atmosphere....
- Humidity indicator
A humidity indicator is a moisture-sensitive chemical that it will change color when the indicated relative humidity is exceeded.Some chemicals are:* Cobalt chloride * copper chloride...
- Hygrometer
Hygrometers are instruments used for measuring relative humidity. A simple form of a hygrometer is specifically known as a psychrometer and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature...
- Psychrometrics
Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures...
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