![Sling psychrometer](https://upload.wikimedia.org/wikipedia/commons/0/09/Sling_psychrometer.JPG)
The wet-bulb temperature (WBT) is the
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
read by a
thermometer
A thermometer is a device that temperature measurement, measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a merc ...
covered in water-soaked (water at ambient temperature) cloth (a wet-bulb thermometer) over which air is passed.
At 100%
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
, the wet-bulb temperature is equal to the air temperature (
dry-bulb temperature
The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation and moisture. DBT is the temperature that is usually thought of as air temperature, and it is the true ther ...
); at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of
evaporative cooling
An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...
.
The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the
evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
of water into it, with the
latent heat
Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition.
Latent heat can be understo ...
supplied by the parcel. A wet-bulb thermometer indicates a temperature close to the true (thermodynamic) wet-bulb temperature. The wet-bulb temperature is the lowest temperature that can be reached under current
ambient conditions by the evaporation of water only.
Even
heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of , equivalent to a
heat index
The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...
of . The theoretical limit to human survival for more than a few hours in the shade, even with unlimited water, is a wet-bulb temperature of – equivalent to a heat index of .
Intuition
If a thermometer is wrapped in a water-moistened cloth, it will behave differently. The drier and less humid the air is, the faster the water will evaporate. The faster water evaporates, the lower the thermometer's temperature will be relative to air temperature.
Water can evaporate only if the air around it can absorb more water. This is measured by comparing how much water is in the air to the maximum that could be in the air—the
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
. 0% means the air is completely dry, and 100% means the air contains all the water it can hold in the present circumstances and it cannot absorb any more water (from any source).
This is part of the cause of
apparent temperature
Apparent temperature, also known as feels like, is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoo ...
in humans. The drier the air, the more moisture it can take up beyond what is already in it, and the easier it is for extra water to evaporate. The result is that
sweat
Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammals.
Two types of sweat glands can be found in humans: eccrine glands and apocrine glands. The eccrine sweat glands are distribut ...
evaporates more quickly in drier air, cooling down the skin faster. If the relative humidity is 100%, no water can evaporate, and cooling by sweating or evaporation is not possible.
When relative humidity is 100%, a wet-bulb thermometer can also no longer be cooled by evaporation, so it will read the same as an unwrapped thermometer.
General
The wet-bulb temperature is the lowest temperature that may be achieved by
evaporative cooling
An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...
of a water-wetted, ventilated surface.
By contrast, the
dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
is the temperature to which the ambient air must be cooled to reach 100%
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
assuming there is no further evaporation into the air; it is the temperature where condensation (dew) and clouds would form.
For a parcel of air that is less than saturated (i.e., air with less than 100 percent relative humidity), the wet-bulb temperature is lower than the
dry-bulb temperature
The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation and moisture. DBT is the temperature that is usually thought of as air temperature, and it is the true ther ...
, but higher than the dew point temperature. The lower the relative humidity (the drier the air), the greater the gaps between each pair of these three temperatures. Conversely, when the relative humidity rises to 100%, the three figures coincide.
For air at a known pressure and dry-bulb temperature, the thermodynamic wet-bulb temperature corresponds to unique values of the relative humidity and the dew point temperature. It therefore may be used for the practical determination of these values. The relationships between these values are illustrated in a
psychrometric chart
Psychrometrics (or psychrometry, ; also called hygrometry) is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.
Common applications
Although the principles of psychrometry apply to any p ...
.
Lower wet-bulb temperatures that correspond with drier air in summer can translate to energy savings in air-conditioned buildings due to:
#Reduced
dehumidification
A dehumidifier is an air conditioning device which reduces and maintains the level of humidity in the air. This is done usually for health or thermal comfort reasons, or to eliminate musty odor and to prevent the growth of mildew by extracting w ...
load for ventilation air
#Increased efficiency of
cooling towers
A cooling tower is a device that rejects waste heat to the atmosphere through the cooling of a coolant stream, usually a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and ...
#increased efficiency of
evaporative cooler
An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning s ...
s
Thermodynamic wet-bulb temperature
The thermodynamic wet-bulb temperature is the temperature a volume of air would have if cooled
adiabatically to saturation by evaporation of water into it, all latent heat being supplied by the volume of air.
The temperature of an air sample that has passed over a large surface of liquid water in an insulated channel is the thermodynamic wet-bulb temperature—the air has become saturated by passing through a constant-pressure, ideal, adiabatic saturation chamber.
Meteorologists and others may use the term "isobaric wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature". It is also called the "adiabatic saturation temperature", though meteorologists also use "adiabatic saturation temperature" to mean "temperature at the saturation level", i.e. the temperature the parcel would achieve if it expanded adiabatically until saturated.
The thermodynamic wet-bulb temperature is a
thermodynamic property of a mixture of air and water vapor. The value indicated by a simple wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.
For an accurate wet-bulb thermometer, "the wet-bulb temperature and the adiabatic saturation temperature are approximately equal for air-water vapor mixtures at atmospheric temperature and pressure. This is not necessarily true at temperatures and pressures that deviate significantly from ordinary atmospheric conditions, or for other gas–vapor mixtures."
Temperature reading of wet-bulb thermometer
![wetdryhygrometer](https://upload.wikimedia.org/wikipedia/commons/6/6d/Wetdryhygrometer.JPG)
Wet-bulb temperature is measured using a
thermometer
A thermometer is a device that temperature measurement, measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a merc ...
that has its bulb wrapped in cloth—called a ''sock''—that is kept wet with distilled water via
wicking action. Such an instrument is called a ''wet-bulb thermometer.'' A widely used device for measuring wet- and dry-bulb temperature is a ''sling psychrometer'', which consists of a pair of mercury bulb thermometers, one with a wet "sock" to measure the wet-bulb temperature and the other with the bulb exposed and dry for the dry-bulb temperature. The thermometers are attached to a swivelling handle, which allows them to be whirled around so that water evaporates from the sock and cools the wet bulb until it reaches
thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
.
An actual wet-bulb thermometer reads a temperature that is slightly different from the thermodynamic wet-bulb temperature, but they are very close in value. This is due to a coincidence: for a water-air system the psychrometric ratio (see below) happens to be close to 1, although for systems other than air and water they might not be close.
To understand why this is so, first consider the calculation of the thermodynamic wet-bulb temperature.
Experiment 1
In this case, a stream of unsaturated air is cooled. The heat from cooling that air is used to evaporate some water which increases the humidity of the air. At some point the air becomes saturated with water vapor (and has cooled to the thermodynamic wet-bulb temperature). In this case we can write the following balance of energy per mass of dry air:
*
saturated water content of the air (kg
H2O/kg
dry air)
*
initial water content of the air (same unit as above)
*
latent heat of water (J/kg
H2O)
*
initial air temperature (K)
*
saturated air temperature (K)
*
specific heat of air (J/kg·K)
Experiment 2
For the case of the wet-bulb thermometer, imagine a drop of water with unsaturated air blowing over it. As long as the vapor pressure of water in the drop (function of its temperature) is greater than the partial pressure of water vapor in the air stream, evaporation will take place. Initially, the heat required for the evaporation will come from the drop itself since the fastest moving water molecules are most likely to escape the surface of the drop, so the remaining water molecules will have a lower average speed and therefore a lower temperature.
Instead, as the drop starts cooling, it is now colder than the air, so convective heat transfer begins to occur from the air to the drop. Furthermore, the evaporation rate depends on the difference of concentration of water vapor between the drop-stream interface and the distant stream (i.e. the "original" stream, unaffected by the drop), and on a convective mass transfer coefficient, which is a function of the components of the mixture (i.e. water and air).
After a certain period, an equilibrium is reached: the drop has cooled to a point where the rate of heat carried away in evaporation is equal to the heat gain through convection. At this point, the following balance of energy per interface area is true:
*
water content of interface at equilibrium (kg
H2O/kg
dry air) (note that the air in this region is and has always been saturated)
*
water content of the distant air (same unit as above)
*
mass transfer coefficient (kg/m
2⋅s)
*
air temperature at distance (K)
*
water drop temperature at equilibrium (K)
*
convective heat transfer coefficient (W/m
2·K)
Note that:
*
is the ''driving force for mass transfer'' (constantly equal to
throughout the entire experiment)
*
is the ''driving force for heat transfer'' (when
reaches
, the equilibrium is reached)
Let us rearrange that equation into:
Now let's go back to our original "thermodynamic wet-bulb" experiment, Experiment 1. If the air stream is the same in both experiments (i.e.
and
are the same), then we can equate the right-hand sides of both equations:
Rearranging:
If
then the temperature of the drop in Experiment 2 is the same as the wet-bulb temperature in Experiment 1. Due to a coincidence, for the mixture of air and water vapor this is the case, the ratio (called ''psychrometric ratio'') being close to 1.
Experiment 2 is what happens in a common wet-bulb thermometer, meaning that its reading is fairly close to the thermodynamic ("real") wet-bulb temperature.
Experimentally, the wet-bulb thermometer reads closest to the thermodynamic wet-bulb temperature if:
*The sock is shielded from radiant heat exchange with its surroundings
*Air flows past the sock quickly enough to prevent evaporated moisture from affecting evaporation from the sock
*The water supplied to the sock is at the same temperature as the thermodynamic wet-bulb temperature of the air
In practice the value reported by a wet-bulb thermometer differs slightly from the thermodynamic wet-bulb temperature because:
*The sock is not perfectly shielded from radiant heat exchange
*Air flow rate past the sock may be less than optimum
*The temperature of the water supplied to the sock is not controlled
At relative
humidities below 100 percent, water
evaporates
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
from the bulb, cooling it below ambient temperature. To determine relative humidity, ambient temperature is measured using an ordinary thermometer, better known in this context as a
dry-bulb thermometer. At any given ambient temperature, less relative humidity results in a greater difference between the dry-bulb and wet-bulb temperatures; the wet-bulb is colder. The precise relative humidity is determined by reading from a
psychrometric chart
Psychrometrics (or psychrometry, ; also called hygrometry) is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.
Common applications
Although the principles of psychrometry apply to any p ...
of wet-bulb versus dry-bulb temperatures, or by calculation.
Psychrometer
A hair tension dial hygrometer with a nonlinear scale.
A hygrometer is an instrument used to measure the amount of water vapor in air, in soil, or in confined spaces. Humidity measurement instruments usually rely on measurements of some other qu ...
s are instruments with both a wet-bulb and a dry-bulb thermometer.
A wet-bulb thermometer can also be used outdoors in sunlight in combination with a ''globe thermometer'' (which measures the incident
radiant temperature) to calculate the
Wet Bulb Globe Temperature
The wet-bulb globe temperature (WBGT) is a type of apparent temperature used to estimate the effect of temperature, humidity, wind speed (wind chill), and visible and infrared radiation (usually sunlight) on humans. It is used by industrial h ...
(WBGT).
Adiabatic wet-bulb temperature
The ''adiabatic wet-bulb temperature'' is the temperature a volume of air would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process (AMS Glossary). Such cooling may occur as air pressure reduces with altitude, as noted in the article on
lifted condensation level
The lifted condensation level or lifting condensation level (LCL) is formally defined as the height at which the relative humidity (RH) of an air parcel will reach 100% with respect to liquid water when it is cooled by dry adiabatic lifting. The ...
.
This term, as defined in this article, may be most prevalent in meteorology.
As the value referred to as "thermodynamic wet-bulb temperature" is also achieved via an adiabatic process, some engineers and others may use the term "adiabatic wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature". As mentioned above, meteorologists and others may use the term "isobaric wet-bulb temperature" to refer to the "thermodynamic wet-bulb temperature".
"The relationship between the isobaric and adiabatic processes is quite obscure. Comparisons indicate, however, that the two temperatures are rarely different by more than a few tenths of a degree Celsius, and the adiabatic version is always the smaller of the two for unsaturated air. Since the difference is so small, it is usually neglected in practice."
Wet-bulb depression
The wet-bulb depression is the difference between the dry-bulb temperature and the wet-bulb temperature. If there is 100% humidity, dry-bulb and wet-bulb temperatures are identical, making the wet-bulb depression equal to zero in such conditions.
Wet-bulb temperature and health
Living organisms can survive only within a certain temperature range. When the ambient temperature is excessive, many animals cool themselves to below ambient temperature by
evaporative cooling
An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning sy ...
(sweat in humans and horses,
saliva
Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be ...
and water in dogs and other mammals); this helps to prevent potentially fatal
hyperthermia
Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...
due to heat stress. The effectiveness of evaporative cooling depends upon humidity; wet-bulb temperature, or more complex calculated quantities such as
wet-bulb globe temperature
The wet-bulb globe temperature (WBGT) is a type of apparent temperature used to estimate the effect of temperature, humidity, wind speed ( wind chill), and visible and infrared radiation (usually sunlight) on humans. It is used by industrial h ...
(WBGT) which also takes account of
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
, give a useful indication of the degree of heat stress, and are used by several agencies as the basis for heat stress prevention guidelines.
A sustained wet-bulb temperature exceeding is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature human bodies switch from shedding heat to the environment, to gaining heat from it.
In practice, such ideal conditions for humans to cool themselves will not always existhence the high fatality levels in the
2003 European and
2010 Russian heat waves, which saw wet-bulb temperatures no greater than 28 °C.
[
]
A 2015 study concluded that depending on the extent of future
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, parts of the world could become uninhabitable due to deadly wet-bulb temperatures. A 2020 study reported cases where a wet-bulb temperature had already occurred, albeit too briefly and in too small a locality to cause fatalities.
In 2018,
South Carolina
)''Animis opibusque parati'' ( for, , Latin, Prepared in mind and resources, links=no)
, anthem = " Carolina";" South Carolina On My Mind"
, Former = Province of South Carolina
, seat = Columbia
, LargestCity = Charleston
, LargestMetro = ...
implemented new regulations to protect high school students from heat-related emergencies during outdoor activities. Specific guidelines and restrictions are in place for wet-bulb globe temperatures between and ; wet-bulb globe temperatures of or greater require all outdoor activities to be canceled.
Heat waves with high humidity
*On 8 July 2003,
Dhahran, Saudi Arabia
Dhahran ( ar, الظهران, ''Al-Dhahran'') is a city located in the Eastern Province, Saudi Arabia. With a total population of 240,742 as of 2021, it is a major administrative center for the Saudi oil industry. Together with the nearby cit ...
saw the highest
heat index
The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...
ever recorded at with a temperature of and a dew point.
*The
2015 Indian heat wave
In May 2015, India was struck by a severe heat wave. , it caused the deaths of at least 2,500 people in multiple regions. The heat wave occurred during the Indian dry season, which typically lasts from March to July with peak temperatures in A ...
saw wet-bulb temperatures in
Andhra Pradesh
Andhra Pradesh (, abbr. AP) is a state in the south-eastern coastal region of India. It is the seventh-largest state by area covering an area of and tenth-most populous state with 49,386,799 inhabitants. It is bordered by Telangana to the ...
reach . A similar wet-bulb temperature was reached during the
1995 Chicago heat wave
The July 1995 Chicago heat wave led to 739 heat-related deaths in Chicago over a period of five days. Most of the victims of the heat wave were elderly poor residents of the city, who could not afford air conditioning and did not open windows or ...
.
*A heat wave in August 2015 saw temperatures of and a dew point of at
Samawah
, nickname = Samawa
, settlement_type = City
, motto =
, image_skyline = مدينة السماوة.jpg
, imagesize =
, image_caption = Samawah
, image_flag =
, fl ...
,
Iraq
Iraq,; ku, عێراق, translit=Êraq officially the Republic of Iraq, '; ku, کۆماری عێراق, translit=Komarî Êraq is a country in Western Asia. It is bordered by Turkey to Iraq–Turkey border, the north, Iran to Iran–Iraq ...
, and with a dew point of in
Bandar-e Mahshahr
Bandar-e Mahshahr ( fa, بندرماهشهر) is a city and capital of Mahshahr County, Khuzestan Province, Iran.
Mahshahr has two universities. Islamic Azad University of Mahshahr and Amirkabir University of Technology, Mahshahr campus. Both uni ...
,
Iran
Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...
. This implied wet-bulb temperatures of about and respectively. The government urged residents to stay out of the sun and drink plenty of water.
Highest recorded wet-bulb temperatures
The following locations have recorded wet-bulb temperatures of or higher.
Weather station
A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include tempera ...
s are typically at airports, so other locations in the city may have experienced higher values.
Climate change
Study results indicate that
limiting global warming to 1.5 °C would prevent most of the
tropics
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referred to ...
from reaching the wet-bulb temperature of the human physiological limit of 35 °C.
See also
*
Atmospheric thermodynamics Atmospheric thermodynamics is the study of heat-to-work transformations (and their reverse) that take place in the earth's atmosphere and manifest as weather or climate. Atmospheric thermodynamics use the laws of classical thermodynamics, to descr ...
*
Dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
*
Heat index
The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...
*
Wet-bulb conditions
The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked (water at ambient temperature) cloth (a wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal ...
*
Wet-bulb potential temperature
Wet-bulb potential temperature, sometimes referred to as pseudo wet-bulb potential temperature, is the temperature that a parcel of air at any level would have if, starting at the wet-bulb temperature, it were brought at the saturated adiabatic ...
References
External links
3 ways to get wet-bulb temperatures for engineersWet-bulb chart for snow making (Fahrenheit)Indirect evaporative cooler cools below wet-bulbWet-bulb and dew-point calculator from NOAAShortcut to calculating wet-bulbHeat Stress Index Calculation
{{Meteorological variables
Atmospheric thermodynamics
Temperature
Meteorological data and networks
es:Temperatura#Temperatura húmeda