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.
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.
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.