Ideally the instrument should be in a well-shaded spot or in a Stevenson screen (refer to “Making your own weather station” in Section 4: Contacts and further information).

The instrument contains two alcohol-in-glass thermo-meters with a scale of minus 10 to plus 110 degrees Celsius, in 1 degree increments. The end of the ‘wet-bulb’ thermometer is covered with a damp cloth dipped in water. The other thermometer is the ‘dry-bulb’.

• Record the wet-bulb and dry-bulb temperatures.
• Calculate the wet-bulb depression – the dry-bulb temperature minus the wet-bulb temperature.
• Determine the relative humidity (%) using the table (reverse page). Read down from the wet-bulb depression and across from the dry-bulb temperature.

Do’s and Don’ts

	Be careful with the instrument. The thermometers are made of glass and can easily break.
	Empty the water bottle before returning the hygrometer to its box.

How it works

The hygrometer works on a similar principle to the whirling psychrometer. The main difference is that the hygrometer is not rotated, so evaporation from the damp cloth around the wet-bulb thermometer occurs more slowly.