What is humidity?

Humidity readings tell us how much water is in the air. The humidity can affect the way we feel. It is recorded as a percentage (%). The humidity can never be more that 100%. On a dry, sunny day the humidity may go as low as 43%. If the humidity is low it will be a good day for drying the washing outside. If the humidity is high and it is hot it will be uncomfortable.

This is one of the more complicated types of weather reading. There is more humidity information on the More About Humidity page.

Digital Humidity Sensor

There are many fairly inexpensive devices which can measure humidity as well as temperature. These give a number between 0 and 100. If the air is very wet the number will be very high. The drier the air the lower the number. The device in the picture can have up to three wireless remote sensors. You can see the humidity reading with the % sign.

Wet-bulb and Dry-bulb Hygrometer

This instrument is made up of two thermometers. One is an ordinary one, which is dry. The other is kept wet by having wet fabric wrapped round it. It is kept wet by being linked by a wick to a container of pure water.

We compare the two temperature readings. If the difference between the two readings is large, then the air must be dry. If the readings are close together, the air is wetter.

When the data is recorded it is worth checking that the dry reading is not lower than the wet reading. If it is look again and check that something is not wrong. If they are the same, has the water run out in the reservoir?

Working out the humidity

To work out the humidity you need to know the dry temperature and the wet temperature. You then look these up on special tables. This will give you a percentage.

You could use an online calculator. This one needs the air pressure reading as well.

See a chart on this page. You need to scroll down to the Celsius chart.