by Chris Woodford. Last updated: February 11, 2016.
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.
Before we can figure out how to measure something, we have to have an idea of what we're measuring—and what our measurements will mean. We measure most things in scientific units of one kind or another, such as kilograms, meters, or seconds; but humidity is slightly different, and we typically measure it in two quite different ways.
One possible measurement is called the specific humidity, which is the mass of water vapor present in a kilogram mass of air (including the water), written in units such as grams per kilogram. There's a very similar measurement called the mixing ratio, which is the mass of water vapor in a kilogram mass of dry air, also written in units such as grams per kilogram.
A much more common measurement is called relative humidity, which is the amount of water vapor in the air compared to the maximum amount there could possibly be at that temperature, written as a percentage (without any units). On a really wet and soggy day, the relative humidity is likely to be 90–100 percent; on a dry day, with a dry wind blowing, and little or no chance of rain, it's more likely to be 60–75 percent. When we talk about "humidity" as a percentage, we mean relative humidity.
Because specific humidities are fairly meaningless to most people, weather forecasts typically quote relative humidities—and user-friendly hygrometers are calibrated (marked with measurements on their dials or displays) that way too.
Photo: A pine cone is a simple hygrometer. It closes up tightly when it's wet (top) and opens when it's dry (bottom). Although you can build a decent little home hygrometer with a pine cone, it will take some time to respond to changes in humidity.
How can we measure changes in 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 humid it is outside. Pine-cones aren't the most accurate hygrometers, however, not least because it takes quite a while for them to open and close—but you can still make fun and interesting home hygrometers with them and they make good science fair experiments (see in the links down below).
Some humidity measuring devices aren't much more sophisticated than pine cones. 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 (or plant fiber). 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. Just as you can make a home hygrometer from a pine cone, so you can do the same thing with a chunk of your own hair—or an obliging friend's! (Again, you'll find some links below.)
Artwork: Left: A typical twisted-fiber hygrometer. Before electronic hygrometers became popular in the 20th century, most inexpensive hygrometers worked like this one, patented by Louis Ullman of Nashville, Tennessee in 1859. It has a box (open to the air—so moisture can get in and out) with a piece of twisted plant fiber (orange) inside. The fiber is connected to a pointer (red) that turns around a dial and, as the humidity changes, the fiber either tightens or loosens, moving the pointer up or down the dial. As Ullman explains in his patent, various different plant fibers can be used, including ones from Geranium erodium. Artwork from US Patent #25,457: Hygrometer courtesy of US Patent and Trademark Office.
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 the relative humidity.
Artwork: Right: A typical psychrometer (wet-dry bulb thermometer) has two thermometers side by side. One of them (left) is a dry bulb thermometer and simply measures the temperature of the surrounding air, like any conventional thermometer. The other thermometer (right) is the wet bulb: its bulb is immersed in a bottle or reservoir of liquid (green) at the base. You measure the humidity by comparing the readings from the two thermometers. Using the sliding pointer (blue), you can then read the humidity off the rotating chart (yellow) in the center, which is essentially a look-up table that converts temperature differences into humidity measurements. This particular version of the wet-dry bulb hygrometer was invented in the 1930s by John Leonard Schwartz of Philadelphia and the drawing comes from his US Patent #1,933,283: Hygrometer, courtesy of US Patent and Trademark Office.
Photo (left): This Holmes electronic hygrometer has an easy-to-read dial. There are many other brands available, including Honeywell and 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.
These days, you can even get hygrometer apps for cellphones, and you might wonder how a little electronic gadget with no obvious humidity sensor can make those kinds of measurement. It's a cheat! The app figures out your location from its GPS satellite receiver and sends a query to the local weather station server, which sends back the humidity measurement for your phone to display. Now that's a rough measurement of the humidity at the weather station, which may be many kilometers or miles from your home. It'll give you a rough idea of the overall humidity (if it's a dry or a wet day), but not of the exact local humidity right where you are.
Photo (right): The ceramic sensing membrane from an electronic hygrometer. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).