Hygrometers
by Chris Woodford. Last updated: March 7, 2011.
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 trekking 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 louvered 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 hygrometers, 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).
Further reading
On this website
Books – for older readers
- The Rough Guide to Weather by Robert Henson. Penguin/Rough Guides, 2007. A very clearly written, well-illustrated guide. Everything you need to know explained simply!
- Weather for Dummies by John D. Cox. John Wiley & Sons, 2000. A simple guide written in the no-nonsense dummies format, including clouds, storms, local and global weather, and freak weather such as hurricanes.
- Weather Matters by Bernard Mergen. University Press of Kansas, 2008. Why do we care so much about the weather? How has it shaped our history?
Books – for younger readers
- Weather by Michael Allaby. Dorling Kindersley Children's, 2001: A clear, simple introduction to the workings of our weather. (Ages 9–12.)
- How the Weather Works by Michael Allaby. Dorling Kindersley Children's, 1999: A more detailed introduction to weather. (Ages 9–12.)
Websites
