A sling psychrometer consists of two thermometers mounted side by side.  One is an ordinary thermometer, the other is covered with a wet piece of cloth.  To make a humidity measurement you swing the psychrometer around for a minute or two and then read the temperatures from the two thermometers.  The dry thermometer measures the air temperature. 

Would the wet thermometer be warmer or colder or the same as the dry thermometer?   You can check it out for yourself - go get one of your hands wet.  Does it feel the same as the dry hand?  You might blow on both hands to increase the evaporation from the wet hand.  I think you'll find the wet hand feels colder.  That's basically what happens with the wet bulb thermometer.




What could you say about the relative humidity in these two situations (you can assume the air temperature is the same in both pictures).  You would feel coldest on a dry day (the left picture indicates dry air).  The evaporative coolers that people like me use in Tucson in the summer work much better (more cooling) early in the summer when the air is dry.  Once the thunderstorm season begins in July and the air is more humid it is hard to cool your house below 80 F (but by then you're used to it and it doesn't matter too much).

You feel colder because energy is needed in order for water to evaporate.  The energy in the cases above come from your body.  When your body starts to lose energy you feel cold.

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Here are a bunch of details that you can read through if you're so inclined.  My goal is that you understand the basic principle behind a sling psychrometer.  If you'd rather not worry about the details skip to the summary a few pictures further on. 

You need to be aware of a few things to understand the details that follow:
(1a) evaporation is a cooling process
(1b) warm water evaporates more rapidly than cold water (think of a steaming glass of hot tea and a glass of iced tea)

(2a) condensation is a warming process
(2b) whenever there is any moisture in the air there will be some condensation, the rate of condensation depends on how much water vapor is in the air

(3) these two phenomena, evaporation and condensation, operate independently of each other

Here's the situation on a day with low relative humidity.

The figure shows what will happen as you start to swing the wet bulb thermometer.  Water will begin to evaporate from the wet piece of cloth.  The amount or rate of evaporation will depend on the water temperature  Warm water evaporates at a higher rate than cool water.

The evaporation is shown as blue arrows because this will cool the thermometer.   The water on the wet thermometer starts out at 80 F and evaporates fairly rapidly.

The figure at upper left also shows one arrow of condensation.  The amount or rate of condensation depends on how much water vapor is in the air surrounding the thermometer.  In this case (low relative humidity) there isn't much water vapor.  The condensation arrow is orange because the condensation will release latent heat and warm the thermometer.

Because there is more evaporation (4 arrows) than condensation (1 arrow) the wet bulb thermometer will drop.  As the thermometer cools the rate of evaporation will begin to decrease.  The thermometer will continue to cool until the evaporation has decreased enough that it balances the condensation.


The rates of evaporation and condensation are equal.  The temperature will now remain constant.

The figure below shows the situation on a day with higher relative humidity.  There's enough moisture in the air to provide 3 arrows of condensation. 


The rate of evaporation stays the same, the rate of condensation is higher.  The rate of evaporation is still higher than condensation but not by much. 


There'll only be a little cooling before the evaporation is reduced enough to be in balance with condensation.

Here's the summary