# WEATHER U: How hygrometers work

Now that we are “staying the blazes home” I’ve received a few interesting questions about weather instruments.

Perhaps we're clearing out closets and coming across a few treasures. This letter is from John Cross:

“Can you explain to me what is being measured by the hygrometer? I am a faithful weather watcher with daily records going back many years of temperature, barometric pressure, clouds, precipitation, wind and even changes in my ‘storm glass.’ I have never understood why the reading does not correspond to humidity as reported?”

That’s a great question.  Let me begin by saying that the hygrometer is my favourite weather instrument.  It was invented by Cardinal Nicholas of Cusa – a German philosopher, theologian, jurist, and astronomer. He also raised sheep, and that’s what led to his discovery of the hygrometer, back in 1450. He observed the changes in the weight of bags of wool during different weather conditions. Wool and human hair expand and contract as a result of the surrounding humidity.  That explains why many of us experience our “bad hair days” when it’s humid.

Back to John’s hygrometer:

Hygrometers measure the amount of moisture or humidity in the air, but there are several different types of hygrometers, and they display their results in different ways. Most hygrometers display their readings in the amount of water in a given volume of air; the units used are millilitres per cubic centimetre. Alternatively, some hygrometers measure relative humidity, which is expressed as a percentage.

From the photo above, it appears that John’s hygrometer displays results in percentage, but to John’s point, the value doesn’t seem to correspond to the humidity of the air. I feel the instrument needs to be calibrated.

There’s an easy way to calibrate your hygrometer; it’s called the towel test. Dampen a paper towel, not dripping wet … but good and damp, then wrap the hygrometer in the towel for 30 to 45 minutes. The humidity should read close to 100 per cent. If it doesn’t, take a small screwdriver and turn the screw at the back of the dial until it reads 100 per cent. Following that, it should come down and settle to the humidity in the room. Indoor humidity levels should be between 30 to 50 per cent, with the ideal level being about 45 per cent.

Cindy Day is the chief meteorologist for SaltWire Network