What is a Psychrometer?

It's not uncommon to hear “what is a psychrometer?” for those who aren’t familiar with how weather meters and other atmospheric tracking devices work.  While it sounds like a complicated tool, a psychrometer is actually a rather simple type of hygrometer, an instrument that is used to measure the amount of humidity that is present in the atmosphere.

A psychrometer measures the relative humidity in the atmosphere through the use of two thermometers: 

  1. a dry bulb thermometer, is used to measure the temperature by being exposed to the air. 
  2. a wet bulb thermometer, measures temperature by having the bulb dipped in a liquid. 

Through the comparison of both temperatures, individuals determine the relative humidity of the surrounding area by calculating the difference between the temperatures.

Now that you know the answer to “what is a psychrometer”, you may be curious about what a psychrometer chart is.  Widely available online or on the back of a psychrometer device or packaging, a psychrometer chart makes it easy to find the relative humidity once a reading has been taken.  This reduces the need for on-the-fly calculations which may be difficult to perform when out in the field. 

Kestrel showing relative humidty

For those who really want to know what makes up a psychrometer, it’s fairly easy to make one at home in order to see how it works for yourself.  All you need is a pair of thermometers and a muslin wrap for one of the bulbs to wick water up to the bulk for evaporation.  It’s a simple science project that can be completed with just a few items from the hardware store. 

A variation on the basic psychrometer, which is a device that is swung through the air in order to increase the evaporation process.  There are other devices, include ventilated and aspirated psychromters, which are designed to work with fans that ventilate the wet bulb thermometer.  The process also increased evaporation rates evenly, which produces a more accurate reading.

When using a psychrometer, caution must be taken to avoid heating the thermometer or placing it somewhere too hot or too cold which could lead to an inaccurate reading.  Be sure that you take care to use a psychrometer carefully in order to prevent any damage to yourself or to the device.

Using a psychrometer to calibrate a hygrometer ensures the most accurate readings when it comes to determining relative humidity.  Those who rely on accurate atmospheric readings for their jobs, including those work or train outdoors, typically utilize psychrometer technology as part of a handheld weather meter device that is designed for use in the field.  The handy devices can be used to track a number of different atmospheric conditions, providing highly accurate data when it’s needed the most.

Watch a demo of Firefighter / Smoke Jumper explaining how to properly use a Psychrometer and then compare to using a Kestrel Meter.


Compare Kestrel Fire Weather Meters to get accurate RH readings and more.

  • moon says...

    this was so helpful for my school project thanks guys

    On February 07, 2020

  • sonia says...

    thanks for the info guys, very helpful

    On November 08, 2019

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