page_title: Wet-bulb temperature
section_title: Temperature reading of wet-bulb thermometer
hierarchical_section_title: Wet-bulb temperature / Temperature reading of wet-bulb thermometer
context_page_description: The wet-bulb temperature is the temperature read by a thermometer covered in water-soaked cloth over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature; at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling.
  The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation by the evaporation of water into it, with the latent heat supplied by the parcel. A wet-bulb thermometer indicates a temperature close to the true 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 32 °C, equivalent to a heat index of 55 °C. The theoretical limit to human survival for more than a few hours in the shade, even with unlimited water, is 35 °C – theoretically equivalent to a heat index of 70 °C, though the heat index doesn't go that high.
context_section_description: Wet-bulb temperature is measured using a thermometer 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.
  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 vapour (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 (kgH₂O/kgdry air)
   initial water content of the air (same unit as above)
   latent heat of water (J/kgH₂O)
   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. If this were the only thing that happened and the air started bone dry, if the air blew sufficiently fast then its partial pressure of water vapor would remain constantly zero and the drop would get infinitely cold.
  Clearly this doesn't happen. It turns out that, as the drop starts cooling, it's now colder than the air, so convective heat transfer begins to occur from the air to the drop. Also, understand that 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 (kgH₂O/kgdry 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/m²⋅s)
   air temperature at distance (K)
   water drop temperature at equilibrium (K)
   convective heat transfer coefficient (W/m²·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 a little bi
caption: A sling psychrometer. The sock is wet with distilled water and whirled around for a minute or more before taking the readings.
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