Wet-bulb temperature
Encyclopedia
The wet-bulb temperature is a type of temperature
measurement that reflects the physical
properties of a system
with a mixture of a gas
and a vapor
, usually air and water vapor
. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one's skin is wet and is exposed to moving air. Unlike dry bulb temperature, wet bulb temperature is an indication of the amount of moisture in the air. Wet-bulb temperature can have several technical meanings:
For a given parcel air at a known pressure and dry-bulb temperature
, the thermodynamic wet-bulb temperature corresponds to unique values of relative humidity
, dew point temperature, and other properties. The relationships between these values are illustrated in a psychrometric chart.
For "dry" air, 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 due to evaporative cooling. The greater the difference between the wet and dry bulb temperatures, the drier the air and lower the relative humidity. The dew point temperature is the temperature at which the ambient air must cool to reach 100% relative humidity where condensate and rain form; and conversely, the wet bulb temperature rises to converge on the dry bulb temperature.
Cooling of the human body through perspiration is inhibited as the wet-bulb temperature (and relative humidity) of the surrounding air increases in summer. Other mechanisms may be at work in winter if there is validity to the notion of a "humid" or "damp cold."
Lower wet-bulb temperatures that correspond with drier air in summer can translate to energy savings in air-conditioned buildings due to:
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—it 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".
It is the thermodynamic wet-bulb temperature that is plotted on a psychrometric chart.
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."
that has its bulb wrapped in cloth—called a sock—that is kept wet with water via wicking
action. Such an instrument is called a wet-bulb thermometer.
An actual wet-bulb thermometer reads a slightly different temperature than 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
happens to be ~1,although for systems other than air and water they might not be close.
To understand why this is, first consider the calculation of the thermodynamic wet-bulb temperature: in this case, a stream of air with less than 100% relative humidity 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 reaches 100% saturation (and has cooled to the thermodynamic wet-bulb temperature). In this case we can write the following:
where
is the initial water content of the air on a mass basis, 
is the saturated water content of the air, 
is the latent heat of water, 
is the initial air temperature, 
is the saturated air temperature and 
is the heat capacity of the air.
For the case of the wet-bulb thermometer, imagine a drop of water with air of less than 100% relative humidity blowing over it. As long as the vapor pressure of water in the drop is more than the partial pressure of water 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 drop, so the remaining water molecules will have a lower average speed and therefore a lower temperature. If this were the only thing that happened, then the drop would cool until the following was true:
where
is the saturation pressure of the water in the drop and is a function of the drop temperature and 
is the partial pressure of water in the vapor phase. If the air started bone dry and was blowing sufficiently fast then 
would be 0 and the drop could get infinitely cold. Clearly this doesn't happen. It turns out that as the drop cools, convective heat transfer begins to occur between the warmer air and the colder water. In addition, the evaporation does not occur instantly, but instead depends on the rate of convective mass transfer between the water and the air. At a certain point the water cools to a point where the heat carried away in evaporation is equal to the heat gain through convective heat transfer. At this point the following is true:
where
is now the driving force for mass transfer, k' is the mass transfer coefficient (with English units of lb/(h⋅ft2)), 
is the heat transfer coefficient and 
is the temperature driving force.
Now if this equation is compared to the thermodynamic wet-bulb equation, we can see that if the quantity
(known as the psychrometric ratio) then 
Due to a coincidence, for air this is the case and the ratio is very close to 1.
Experimentally, the wet-bulb thermometer reads closest to the thermodynamic wet-bulb temperature if:
In practice the value reported by a wet-bulb thermometer differs slightly from the thermodynamic wet-bulb temperature because:
At relative humidities
below 100 percent, water evaporates from the bulb which cools the bulb 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 of wet-bulb versus dry-bulb temperatures, or by calculation.
Psychrometers are instruments with both a wet-bulb and a dry-bulb thermometer.
A wet-bulb thermometer can also be used in combination with a globe thermometer (which is affected by the radiant temperature
of the surroundings) in the calculation of the wet bulb globe temperature
.
.
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 stated in another section, 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."
- a potentially fatal condition - many agencies use the measurement, together with other factors, to calculate wet bulb globe temperature
, which in turn serves as the basis for heat stress prevention guidelines.
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
measurement that reflects the physical
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
properties of a system
Physical system
In physics, the word system has a technical meaning, namely, it is the portion of the physical universe chosen for analysis. Everything outside the system is known as the environment, which in analysis is ignored except for its effects on the system. The cut between system and the world is a free...
with a mixture of a gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
and a vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
, usually air and water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one's skin is wet and is exposed to moving air. Unlike dry bulb temperature, wet bulb temperature is an indication of the amount of moisture in the air. Wet-bulb temperature can have several technical meanings:
- Thermodynamic wet-bulb temperature: the temperature a volume of air would have if cooled adiabaticallyAdiabatic processIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the volume of air. - The temperature read from a wet bulb thermometerThermometerDeveloped during the 16th and 17th centuries, a 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 Developed during the 16th and 17th centuries, a thermometer (from the...
- Adiabatic wet-bulb temperature: 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).
Practical considerations
The thermodynamic wet-bulb temperature is the minimum temperature which may be achieved by purely evaporative cooling of a water-wetted (or even ice-covered), ventilated surface.For a given parcel air at a known pressure and dry-bulb temperature
Dry-bulb temperature
The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the...
, the thermodynamic wet-bulb temperature corresponds to unique values of relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
, dew point temperature, and other properties. The relationships between these values are illustrated in a psychrometric chart.
For "dry" air, 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 due to evaporative cooling. The greater the difference between the wet and dry bulb temperatures, the drier the air and lower the relative humidity. The dew point temperature is the temperature at which the ambient air must cool to reach 100% relative humidity where condensate and rain form; and conversely, the wet bulb temperature rises to converge on the dry bulb temperature.
Cooling of the human body through perspiration is inhibited as the wet-bulb temperature (and relative humidity) of the surrounding air increases in summer. Other mechanisms may be at work in winter if there is validity to the notion of a "humid" or "damp cold."
Lower wet-bulb temperatures that correspond with drier air in summer can translate to energy savings in air-conditioned buildings due to:
- Reduced dehumidification load for ventilation air
- Increased efficiency of cooling towers
Thermodynamic wet-bulb temperature (adiabatic saturation 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—it 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".
It is the thermodynamic wet-bulb temperature that is plotted on a psychrometric chart.
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
Wet-bulb temperature is measured using a thermometerThermometer
Developed during the 16th and 17th centuries, a 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 Developed during the 16th and 17th centuries, a thermometer (from the...
that has its bulb wrapped in cloth—called a sock—that is kept wet with water via wicking
Capillary action
Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...
action. Such an instrument is called a wet-bulb thermometer.
An actual wet-bulb thermometer reads a slightly different temperature than 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
Psychrometrics
Psychrometrics or psychrometry or Hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures...
happens to be ~1,although for systems other than air and water they might not be close.
To understand why this is, first consider the calculation of the thermodynamic wet-bulb temperature: in this case, a stream of air with less than 100% relative humidity 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 reaches 100% saturation (and has cooled to the thermodynamic wet-bulb temperature). In this case we can write the following:
where
is the initial water content of the air on a mass basis, is the saturated water content of the air, is the latent heat of water, is the initial air temperature, is the saturated air temperature and is the heat capacity of the air.For the case of the wet-bulb thermometer, imagine a drop of water with air of less than 100% relative humidity blowing over it. As long as the vapor pressure of water in the drop is more than the partial pressure of water 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 drop, so the remaining water molecules will have a lower average speed and therefore a lower temperature. If this were the only thing that happened, then the drop would cool until the following was true:
where
is the saturation pressure of the water in the drop and is a function of the drop temperature and is the partial pressure of water in the vapor phase. If the air started bone dry and was blowing sufficiently fast then would be 0 and the drop could get infinitely cold. Clearly this doesn't happen. It turns out that as the drop cools, convective heat transfer begins to occur between the warmer air and the colder water. In addition, the evaporation does not occur instantly, but instead depends on the rate of convective mass transfer between the water and the air. At a certain point the water cools to a point where the heat carried away in evaporation is equal to the heat gain through convective heat transfer. At this point the following is true:where
is now the driving force for mass transfer, k' is the mass transfer coefficient (with English units of lb/(h⋅ft2)), is the heat transfer coefficient and is the temperature driving force.Now if this equation is compared to the thermodynamic wet-bulb equation, we can see that if the quantity
(known as the psychrometric ratio) then Due to a coincidence, for air this is the case and the ratio is very close to 1.
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
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
below 100 percent, water evaporates from the bulb which cools the bulb 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
Dry-bulb temperature
The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the...
. 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 of wet-bulb versus dry-bulb temperatures, or by calculation.
Psychrometers are instruments with both a wet-bulb and a dry-bulb thermometer.
A wet-bulb thermometer can also be used in combination with a globe thermometer (which is affected by the radiant temperature
Mean radiant temperature
The Mean Radiant Temperature is a concept arising from the fact that the net exchange of radiant energy between two objects is approximately proportional to their temperature difference multiplied by their ability to emit and absorb heat . Mean radiant temperature is simply the area weighted mean...
of the surroundings) in the calculation of the wet bulb globe temperature
Wet Bulb Globe Temperature
The Wet Bulb Globe Temperature is a composite temperature used to estimate the effect of temperature, humidity, wind speed and solar radiation on humans. It is used by industrial hygienists, athletes, and the military to determine appropriate exposure levels to high temperatures...
.
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 levelLifted condensation level
The lifted condensation level or lifting condensation level is formally defined as the height at which the relative humidity of an air parcel will reach 100% when it is cooled by dry adiabatic lifting...
.
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 stated in another section, 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 Human Health
Because excessive wet bulb temperatures can impede evaporative cooling necessary to prevent hyperthermiaHyperthermia
Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate...
- a potentially fatal condition - many agencies use the measurement, together with other factors, to calculate wet bulb globe temperature
Wet Bulb Globe Temperature
The Wet Bulb Globe Temperature is a composite temperature used to estimate the effect of temperature, humidity, wind speed and solar radiation on humans. It is used by industrial hygienists, athletes, and the military to determine appropriate exposure levels to high temperatures...
, which in turn serves as the basis for heat stress prevention guidelines.
See also
- Wet-bulb potential temperatureWet-bulb potential temperatureWet-bulb potential temperature, sometimes referred to as pseudo wet-bulb potential temperature, is the temperature attained by mass of air brought adiabatically to saturation and then carried along moist-adiabat to 1000 mb ....
- Dry-bulb temperatureDry-bulb temperatureThe dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the...
- Dew pointDew pointThe dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
- Atmospheric thermodynamicsAtmospheric thermodynamicsAtmospheric thermodynamics is the study of heat to work transformations in the earth’s atmospheric system in relation to weather or climate...