How to measure humidity?
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.
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!
Line drawing of a wet-dry bulb thermometer type of hygrometer, also known as a psychrometer.
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
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; you'll need a smartphone with a built-in humidity sensor or a standalone sensor you can connect to by USB cable or Bluetooth (wireless) connection to make them work. A few hygrometer apps also work more indirectly by figuring out your location (from "location services" or the phone's GPS satellite receiver) and sending a query to a 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.