by Chris Woodford. Last updated: January 1, 2020.
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: A traditional Black-Forest-style weather house is built around a very simple hygrometer. The woman (left) and man (right) stand on a rotating turntable supported by a twisted fiber (orange). When the humidity is high in damp weather, the hair loosens and the turntable rotates clockwise, so the man comes out with his umbrella. When it's drier, the hair tightens, the turntable rotates the other way, and the woman comes out into the sun!
What is humidity?
Photo: You can measure humidity with your smartphone, but only if it has a built-in moisture detector (or one connected to it). This retro-looking humidity app for Android is a screenshot from "Hygrometer" by Borce Trajkovski.
Humidity is the "wetness" in the air around us. It's definitely something we can feel,
but we can't always see it... so how can we measure it accurately?
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?
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.
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
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: 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
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: 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: 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.
Photo The ceramic sensing membrane from an electronic hygrometer. Photo by courtesy of
NASA Langley Research Center (NASA-LaRC).
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
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
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.
Hygrometers in weather stations
Photo: The main parts of a portable, military weather station. This one can send its readings automatically
using a solar-powered transmitter. Photo by Maynelinne De La Cruz courtesy of
US Air Force.
A typical electronic weather station contains a
thermometer (measures temperature),
a barometer (measures maximum and minimum air pressure),
a pecipitation (rainfall) gauge,
and our old friend, the hygrometer!
Modern electronic weather stations usually have an LCD display that
shows all the measurements automatically and updates them instantaneously,
saving the need to read the instruments one by one. Using the
measurements, a microchip inside will also figure out and display something called
the tendency (a rough forecast for the next day's weather,
summarized with a simple picture such as a sun (fine day), partly
obscured sun (dull day), or raincloud (wet day). Electronic stations
also generally have a memory so they can record hundreds of separate
measurements covering the last few months. Some stations can be
connected to a computer
with a USB cable so you can upload your data and draw proper weather
and climate charts.
Photo: This traditional weather station contains a hygrometer and other weather forecasting equipment. The white louvered box, called a Stevenson screen, protects the instruments from the direct heat of the Sun but allows air to circulate inside, so giving more reliable measurements.
Inventing your own weather station
It's easy enough to collect together a few basic weather-measuring instruments—a thermometer, hygrometer,
rainfall gauge, and so on—to make your own local records and forecasts, but what about doing it
automatically? Is there any alternative to buying a readymade electronic weather station? Of course!
Thanks to the widespread availability of electronic microcontrollers such as the
(and the Raspberry Pi configured to work in a similar way) it's
relatively easy to turn your PC into a DIY weather station that can receive data from electronic
sensors and compile weather forecasts and climate charts. I've added a few links to Arduino-type
weather-station projects at the bottom of the further reading below.
Now it's pretty cool and cutting-edge to build your own weather station, but how about if you'd been
trying to do that a half century ago before computers and microelectronics revolutionized the world.
Impossible? Not a bit of it! Flick back through the patents on record at the US Patent and Trademark
Office and you'll find quite a few people have attempted to make mechanical, electrical, and
electronic instruments that can automatically record data from weather stations.
In 1942, Harry Diamond and Wilbur Hinman, Jr. of the United States National Bureau of Standards (NBS) built the
wonderful automatic weather recording equipment you can see here. Using a cunning mixture of mechanical
devices (levers, gears, and
clockwork), simple electrical circuits, and a radio transmitter, it collected data about pressure, temperature, humidity, wind direction, wind velocity, and rainfall and automatically beamed it to a receiving office using coded radio signals.
Eat your heart out, Arduino!
Artwork: How Diamond and Hinman's automatic weather station worked: First, they developed a generic mechanism (1, green) that could convert movements caused by various kinds of mechanical sensors into movements of a variable resistor—in other words, turning mechanical movements into measurable electric currents. Next, they produced simple mechanical weather sensors that would drive this mechanism in different ways. Three of them are shown here. 2 is an aneroid barometer in which an expanding and contracting
aneroid cell moves a lever up and down with changes in pressure; 3 is a humidity sensor, which measures humidity using the tension of those wires, which presumably tighten or loosen according to how wet or dry the air is; 4 is a rainfall gauge in which a bucket moves downward and turns wheels as it fills with rain. Finally, they devised a way of turning the resistance measurements into codes that could be transmitted by radio signals. Read more about it in US Patent 2,287,786: Automatic Weather Station by Harry Diamond and Wilbur Hinman, Jr, assignees to the US government, patented June 30, 1942. Artwork by courtesy of US Patent and Trademark Office with a bit of minor editing and coloring of the original to improve clarity.
Find out more
On this website
Books – for older readers
- Haynes Meteorology Manual: The practical guide to the weather by Storm Dunlop. Haynes, 2014. A readable introduction laid out in accessible Haynes style.
- Weather for Dummies by John D. Cox. John Wiley & Sons, 2011. A simple guide written in the no-nonsense dummies format, including clouds, storms, local and global weather, and freak weather such as hurricanes.
- 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 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
- Everything Weather by Kathy Furgang. National Geographic Kids, 2018: An engaging, colorful, 64-page overview packed with photos and activities (ages 8–10.)
- Eyewitness: Weather by Brian Cosgrove. Dorling Kindersley Children's, 2016: A simple, 72-page, illustrated introduction. (Ages 9–12.)
- Weather by Michael Allaby. Dorling Kindersley Children's, 2001: A clear, simple introduction to the workings of our weather. (Ages 9–12.)
- Teacher's Weather Sourcebook by Tom Konvicka. Libraries Unlimited, 1999. This contains some good, simple weather activities for children, including details of how to build several different types of hygrometer (in chapter 6).
How to make a hygrometer
Building your own Arduino-type weather station
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