Hygrometers

Last updated: May 1, 2009.

If you've ever been in the dry of a desert or the sopping steamy heat of a rainforest, you'll certainly remember it. What makes these extreme environments so different from one another is their humidity: the amount of water vapor in the atmosphere. Deserts, obviously, contain little or no water, while trecking through a rainforest can feel just like walking through a shower. Measuring humidity is an essential part of weather forecasting and it's also very helpful for gardeners with glasshouses and people who run saunas. We can do it simply and efficiently with cunning instruments called hygrometers. Let's take a look at how they work!

Photo: This Stevenson screen contains a hygrometer and other weather forecasting equipment. The white louvred box protects the instruments from the direct heat of the Sun but allows air to circulate inside, so giving more reliable measurements.

Sensing humidity

Lots of plants react to changes in humidity. Pine cones open their spines when it's dry (to release seeds) and close them tight when it's wet. That's why (as most children know) you can use a fallen pine cone to figure out how wet it feels outside. Pine-cones aren't actually that useful as hydrogometers, however, largely because it needs to be very (and fairly obviously) wet or dry for them to work properly.

Photo: A pine cone is a simple hygrometer. It closes up tightly when it's wet (left) and opens when it's dry (right).

Some humidity measuring devices aren't much more sophisticated than this. In a weather "house," a little man and a little woman stand in two doorways of a closed wooden box. When it's going to rain, the man comes out of his door with an umbrella; when it's dry, the man goes inside and woman pops out of her door instead. Inside the weather house, the two figures are mounted on a turntable and suspended from a piece of tightly twisted hair. When it's dry, the hair tightens up and twists the turntable one way. In wet conditions, the hair loosens and the turntable rotates the other way instead.

Psychrometers

Pine cones and weather houses give a fairly vague indication of humidity, at best. How can we put some numbers to humidity and measure it more accurately? One way is to use an instrument called a psychrometer (also known as a wet- and dry-bulb thermometer). It uses a pair of thermometers standing side by side. One has a bulb open to the air; the other has a bulb covered in a wet cloth. The water on the cloth causes evaporation and loss of heat from the bulb, making its reading lower than that on the dry-bulb thermometer. The amount of evaporation (and the lowering of the temperature) depends on how much water vapor there is in the atmosphere already. Measuring the temperature difference between the two thermometers lets you measure what's called the relative humidity: how much water vapor there is in the air compared to how much there would be if the air were totally saturated, written as a percentage.

Electronic hygrometers

Photo (left): This Holmes electronic hygrometer has an easy-to-read dial. There are many other brands available, including Honeywell, GE Panametrics Photo by courtesy of Ben Winslow, published on Flickr in 2008 under a Creative Commons Licence.

In an age where virtually everything is measured for us, instantly and electronically, the last thing many of us want to do is fiddle about with thermometers and wet cloths. Thank heavens, then, for electronic hygrometers. Typically, they measure the capacitance or resistance of a sample of air and calculate the humidity from that. In a capacitive hygrometer, there are two metal plates with air in between them. The more water there is in the air, the more it affects the plates' capacitance (ability to store a static electric charge). By measuring how much charge can be stored, it's possible to measure the humidity quickly and accurately. In a resistive sensor, electricity flows through a piece of ceramic material exposed to the air. The higher the humidity, the more water vapor condenses inside the ceramic, changing its resistance. Measuring how much current flows through the ceramic gives an accurate measurement of the humidity.

Photo (right): The ceramic sensing membrane from an electronic hygrometer. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).