• Kate

Activity of the day: Cool it down!

Today we're going to have a look at how effective evaporative cooling is using a science experiment.


There are two versions of the experiment- one using just your own arms and water, and one using a thermometer (or preferably two thermometers) to measure the difference in temperature that you feel using evaporative cooling- and to show that it doesn't really matter if the water you use is warm or cold, the cooling effect is very similar.


For the no-thermometer version you will need:

Your arms, or hands if you aren't happy to use your arms.

A dish of warm water

A dish of cold water

A cloth or flannel

A stopclock (on a phone is fine)



You need bare arms up to the elbow for this to work best. If that's not possible, you can do the experiment on the backs of your hands.


  • Sit in a room with CLOSED windows.

  • Rest your arms on a surface such as a table.

  • Wet the cloth with warm water.

  • Start the stopclock and straight away wipe the warm water on the back of ONE of your arms. Keep the other arm dry.

  • Notice what happens to the temperature of your wet arm compared with the temperature of your dry arm.


How does your wet arm feel after 30 seconds have passed? How about after 1 minute? How about after 5 minutes?


Try the experiment again:

  • Near an open window or in front of a working fan

  • In the sunshine and in the shade

  • Indoors or outdoors

  • In a humid room, such as a bathroom when someone has just had a bath or shower


What differences do you notice in each case?



For the thermometer version you will need:

A thermometer

A dish of warm water

A dish of cold water

A small piece of cloth or kitchen roll, to wrap around the thermometer bulb



To carry out the experiment:

First, check the temperature of the room by leaving the thermometer in a shady place for a while until the thermometer temperature is steady.

Then, wet a small cloth with the warm water and wrap it around the bulb of the thermometer.

ALWAYS HOLD THE THERMOMETER NEAR THE TOP- otherwise you'll just measure the temperature of your hand!

Check the temperature on the thermometer as you start, then at 30 seconds, 1 minute and 5 minutes after you begin, using the stopclock to check the time.

If the thermometer is still moving, keep going timing and measuring until the temperature on the thermometer is steady.


Try again using the cold water


Do you notice any difference?


As with the no-thermometer version, try again in places with a breeze (open vs closed window or in front of a fan), in the sunshine, or in a hot bathroom after someone has been in the shower to see how differently evaporative cooling works in different environments.


What you're looking at is called the wet-bulb temperature. Measuring the difference in temperature between a wet-bulb thermometer and a dry-bulb thermometer tells you how much water is already in the air- the humidity. As we discussed in the How to Keep Your Cool post on evaporative cooling, air can only hold so much water, meaning evaporative cooling works better the drier the air is. The closer the wet bulb thermometer reads to the dry thermometer, the more humid the air is, the harder it will be to cool down using water (including sweating) and the stickier you're likely to feel. Doing both the no-thermometer and the thermometer versions of this experiment will help you really feel the difference between environments which are good for evaporative cooling and those which are not so great- and help you adjust how to keep yourself warm or cool, depending on the weather.





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