For some reason you just don’t trust them. Maybe it was all of the years of good weather turning to floods, or perhaps the “no more than three inches of snow” that turned into twenty inches. Maybe it’s the technology they use, or the tie they wear every night in the broadcast. It could even be that a Meteorologist made fun of you when you were a small child... whatever the case is, the result is the same and you just don’t trust them.
Good news! You don’t have to! You can make your own weather predictions by measuring the humidity and dew point of the air using a handful of household items! In seven steps, I will show you how to accurately calculate the humidity and dew point in the air without the use of a computer program or any other sort of Meteorologist trickery.

A little bit about how this instructable is set up:

This instructable contains nine steps, but step one and nine are the process overview and results interpretation, respectably. The other steps are broken down within themselves into smaller "essential" steps. All of these "essential" steps are bolded within the text and any image references are made in the first line so you can easily check the image while reading the instructions. These detailed images are complete with captions; please read them!

The text that is not bolded is supplemental and important to read the first time through. This supplemental text contains tips on how to avoid commonly made mistakes as well as the project's restrictions! If you've read this instructable before then you can probably skip reading these and just read the bold text. This setup is for your convenience.

Enjoy the instructable!

Step 1: Overview and How it Works

So you want to measure the humidity and dew point so you can trump the Meteorologists? This is good, but before you get started there are a few basic concepts you should understand. Don’t worry, you don’t need to take notes and you can even skip this section if you like! Jump to step two and get to work. Just be sure to follow the pictures I've included and to read the captions! This step is for the nerds (like myself) who are curious about the theory behind the madness.


To measure the humidity you are going to take two readings from a thermometer and record them on a piece of paper. This is referred to as the data collection, and I have split it into two parts.

Part 1: The first reading is called the “dry bulb” temperature. Let's just say that Scientists like to call it “dry bulb” because it unnecessarily complicates the reading of an ordinary thermometer.

Part 2: The second reading is referred to as the “wet bulb” temperature. You’re going to have to get a little frisky to obtain this reading... you're going to use an improvised sling psychrometer. A sling psychrometer is a fancy name for some string attached to a small base which can hold and secure a wetted thermometer. The string is used to spin the base and thermometer around at high speeds, above someone’s head. Yes, you read that correctly. You’re going to spin a thermometer around your head.

How It Works:

The difference between the dry and wet bulb temperatures is the key to finding the humidity. The dry bulb temperature is the ordinary air temperature and will be used as a standard to compare the wet bulb temperature reading to. The wet bulb temperature is lower than the dry bulb temperature because it is taken using a thermometer that is wet with water. The wetted thermometer is spun around to cause the water to evaporate. This evaporation occurs because energy is pulled out of the bulb and into the water by the convective air current caused by spinning the thermometer around. Transferred energy is used by the water to change phases from the liquid phase to the gas phase. The loss of energy in the thermometer is relative to the thermometer's temperature reading in addition to the water’s heat of vaporization; the amount of water that evaporates depends on how much water is in the air (aka the air's humidity).
What does this mean? If the air is very humid less water will evaporate from the thermometer because there is already a lot of water in the air. This means the wet bulb temperature will be closer to the dry bulb temperature.

If the air isn’t very humid then more water will evaporate off of the thermometer into the air. This will cause greater loss of energy from the thermometer and a much lower temperature reading with respect to the dry bulb temperature. Thus, the difference between the dry and wet bulb temperature readings is directly proportional to how humid the air is! 
The Psychrometric Chart is a product of experimental results at varying temperatures and a constant pressure that summarizes the correlations of changing wet bulb temperatures to dry bulb temperatures as a function of humidity. In other words, a bunch of scientists performed this experiment hundreds of times at different temperatures and levels of humidity; this graph is the result of their work. This means that all we have to do is look at the chart and trace a line or two to find our humidity and dew point. Thank you, Carrier Corporation!

It should be noted that this method will not work when the temperature is below freezing. In addition, there is also a case when this method can give unreliable results. If you happen to live in the mountains or a high elevation city the values you find using the psychrometric chart will be less accurate. This is because the psychrometric chart was made from data taken at atmospheric pressure, so the chart is the most effective at sea-level elevations. However, there are charts made from data taken at higher elevations, so those can be used as a substitute for the one I provide in step eight.
Now that the basics of finding the humidity and dew point have been covered it's time to collect some materials scattered throughout the house. On to step two!
Nice piece of information. I indeed think that a software model of the psychrometric chart is even more insightful for doing first-time calculations. A free alternative to the software below which I find very useful can be found on <a href="http://www.psychrometricchart.net" rel="nofollow">www.psychrometricchart.net</a>.
Hi nrromsey, good fundamental info to plot the psychrometric chart. There is a free psychrometric chart software that I came across at<a href="http://www.practicalpsychrometric.com/Free-Software.php" rel="nofollow"> http://www.practicalpsychrometric.com/Free-Software.php</a> There is no usage time limit. It will be good for those who want to learn more about plotting the psychrometric chart. Cheers
That chart of yours is kinda like one of those hypnotizing, spiral things (even though it doesn't move). It takes like ~ 10 seconds for me to know which one's what.
A very interesting article, but please, PLEASE make an effort to tie the string to the thermometer more securely. Duct Tape simply WILL NOT stand up to that kind of stress for very long, (in this application, the forces could easily exceed 10G and they won't be smooth and even either). Perhaps you have been lucky, but sooner or later the sheer forces will over come that adhesive, and a fragile piece of glass will be flying at high speed, in a random direction, just around head height! Consider perhaps tying and gluing your string to a piece of wood or plastic and fixing the thermometer to that? That said, hat's off to you for discussing this method and the math's that goes with it, safety is important, (I do appreciate your safety visor in the pictures) but don't stop exploring and investigating, that's what makes this site and others like it so much fun.
make a knot before you tape with duct tape. and make sure the knot is lower than the tape so that it will &quot;catch&quot; on the tape.
<br> As a mining engineer, we use these underground to determine the quality of the ventilating air. Very good instructable! You even cover how to read the psychrometric charts!<br>
&nbsp;Cool! I love the idea combining high tech and low tech and creating something like this that is mechanical but when it comes to rest there is a web cam with OCR enable to bring it back to digital. &nbsp;Yeah, I am strange. &nbsp;Probably looking at too much &quot;steampunk&quot; items on the web.

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More by nrromsey: Who Needs a Meteorologist? Measure Humidity the Old-School Way, With a Sling Psychrometer!
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