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

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!

Step 2: Collecting the Materials

As stated in the intro, you will need only a few household items.
1. A stand-alone non-digital thermometer (Fahrenheit or Celsius)
2. A roll of duck-tape
3. One paper towel
4. One cotton ball
5. Roughly four feet of string
6. A piece of scrap paper
7. A writing utensil (pencil, pen, crayon, or sidewalk chalk depending on age and preference)
8. A Pocket Calculator (or beastly graphing calculator, as shown below)
9. Scissors

Because I am your guide for this instructable, I have gone around and collected these items as well. You can see them in the image below.

Step 3: Collecting Data Part 1 - Dry Bulb Temperature

Nothing unusual yet. You will only need the thermometer, scrap paper, and writing utensil here.

The first step towards upsetting the balance of weather predications is to measure the temperature of the environment in which you want to figure out the humidity. 
For most people this environment is going to be outside.

1. Place your thermometer in a dry place out of the sun.

2. Wait for five minutes or however long it takes for the thermometer to stop changing temperature.

3. Record what the thermometer reads as the "Dry Bulb Temperature." If possible, record the temperature in degrees Celsius. If your thermometer does not give the temperature in Celsius that is fine; in step seven I will show you how to convert your temperature from Fahrenheit to Celsius.

I will be measuring the humidity inside of my apartment because it is snowing outside. My thermometer reads 65 degrees Fahrenheit.

This is pretty easy, huh? It's time to move on to step four, where the fun starts (literally).


Step 4: Assembly of the "Sling Psychrometer" Part 1 - the Sling

Our goal here is to create our own sling psychrometer. Gather the cotton ball, string, duck tape, paper towel, and the thermometer. Set them nearby, you’re going to need them.
1. SEE IMAGE ONE - Line up an inch of string with the top inch of the thermometer (opposite end of the bulb).

2. SEE IMAGE TWO - Tape the string to the thermometer securely with the duck tape such that it covers the overlapping inch of the string and thermometer. It should also cover about another half an inch of the thermometer.

Do not tape below whatever the dry bulb temperature reading was. If you do, you might have trouble reading the thermometer. As an example, let's say you had a dry bulb reading of 80 degrees Fahrenheit. If you taped the string all the way down to the 70 degree Fahrenheit mark you wouldn't be able to read the temperature! That would defeat the purpose of the thermometer.

Step 5: Assembly of the "Sling Psychrometer" Part 2 - the Wetted Bulb

Now that the thermometer has the string secured on one end, the bulb needs to be wetted. You can't just run the bulb underwater though; you need something that will keep the thermometer bulb wetted for over a minute.

1. SEE IMAGE ONE - Cut out a 2 x 2 inch square from one corner of a paper towel. 

2. SEE IMAGE TWO - Take a cotton ball and pull it apart so it's a little separated but not so much that it is falling apart.

3. SEE IMAGE THREE - Place the cotton ball in the middle of the 2 x 2 inch paper towel square. It should cover up the paper towel square.

4. SEE IMAGES FOUR AND FIVE - Take the bulb of the thermometer and place it in the center of the cotton and paper towel. Fold up the sides of the paper towel around the thermometer bulb such that it is covered by the cotton and paper towel.

5. SEE IMAGES SIX AND SEVEN - There should be some paper towel above the bulb and on the actual thermometer. Use a thin piece of duck tape (about 1/2 an inch in width) and tape the paper towel that is above the bulb to the thermometer. This will secure the cotton and paper towel to the thermometer. Do not tape over the paper towel portion that is covering the thermometer bulb. The bulb segment needs to be exposed to the environment.

You're done with the sling psychrometer! See image seven for a picture of my finished product. Perhaps a little anti-climatic, but science works like that sometimes. Now to take the wet bulb temperature!

Step 6: Collecting Data Part 2 - Wet Bulb Temperature

With the sling psychrometer built it's time to take the wet bulb temperature.

1. Run the bulb of the thermometer covered with the cotton and paper towel under water for about 10 seconds such that it is thoroughly soaked.

2. Go to the place where you took your dry bulb temperature and try to find a nice open space. If people are around it might be best to tell them "STAND BACK, SCIENCE IS GOING TO HAPPEN!"

3. SEE IMAGE ONE - Once you have a good six feet of clearance around you, take the string of the sling psychrometer in your hand and wrap it around your hand one or two times so you have a secure grip on it. Begin to spin the thermometer around your head (like a lasso). You need to spin it fast enough that the thermometer is off the ground and above your head. You can spin it as fast as you want, as long as you're able to keep it up for a good minute.

4. Spin the thermometer around your head as described in step three for a minute.

5. After a minute has passed, immediately look at the thermometer and record the temperature it reads as the "Wet Bulb Temperature."

In my case the thermometer reads 48 degrees Fahrenheit after a minute of spinning.

If you have your temperatures in degrees Celsius, you can skip step seven and go directly to step eight. If the temperatures are in Fahrenheit, go to step seven which will help you convert your temperatures to Celsius.

Step 7: Temperature Conversion to Celsius

If your dry and wet bulb temperatures are already in degrees Celsius then you can proceed to step eight. If they are in Fahrenheit, please read on to learn how to convert the temperatures to degrees Celsius.

The formula for temperature conversion is:

[ Temperature in Degrees Celsius ] = ( [ Temperature in Fahrenheit ] - 32 ) x 5 / 9

Take your temperature in Fahrenheit, subtract 32 from it, and then multiple that number by 5/9. The resulting number is the temperature in degrees Celsius. Convert both your dry and wet bulb temperatures into Celsius.

In my situation this comes out to be:

Dry bulb in Celsius = ( 65 - 32 ) x 5 / 9 = 18.33 Degrees Celsius
Wet bulb in Celsius = ( 48 - 32 ) x 5 / 9 = 8.88  Degrees Celsius

Now it's time to find the humidity and dew point. On to step eight!

Step 8: Reading the Psychrometric Chart

In the first image below I have included a copy of Psychrometric chart. It might be hard to read due to the image restrictions placed by instructables. If this is the case, open up in a new window. This is an enlarged version of the chart.

The chart looks complicated at first glance, but it's not hard to read with a little help

1. Open up in a separate window if you have not already. It might be a good idea to print it off because you will be referencing it often.

2. SEE IMAGE TWO - The bottom axis of the graph represents the dry bulb temperature. There are several temperatures recorded on the axis. The light green lines extending from this axis are guidance lines. Find the dry bulb temperature you recorded in degrees Celsius and draw a line straight up through the graph, parallel with the guidance lines. This is illustrated by the bold black line I have drawn for my dry bulb temperature of 18.33 degrees Celsius in image two.

3. SEE IMAGE THREE - The top-most concave line of the graph represents the wet bulb temperature axis. The guidance lines for the wet bulb temperature are at an angle and are labeled on the graph in baby blue writing. Find your wet bulb temperature on the wet bulb axis and draw a line parallel with the wet bulb guidance lines, similar to what you did in step one. As seen in the third image, I have added my wet bulb temperature line to the dry bulb temperature.

4. SEE IMAGE FOUR - The maroon lines on the graph are labeled as "Relative Humidity." They run down the map in a concave manner similar to the wet bulb temperature axis. I have labeled these labeled in the fourth picture. You need to draw a "humidity line" of similar curvature starting from your dry and wet bulb intersection and ending at the labeled relative humidity points. I have done this for my points in the fourth picture.

To find your humidity, see where your humidity line falls between the closest two humidity lines and estimate what percent it is. Since the guidance lines are in ten percent increments it should be easy to find your relative humidity, give or take one percent. For me, the relative humidity is about 27%.

As promised, you can also find the dew point.

5. SEE IMAGE FIVE - Find your dry and wet bulb intersection again. Make a horizontal line from the intersection to the left until it hits the wet bulb axis.

The point where the line hits the axis will correspond to a temperature, as seen in the fifth image. This temperature is the dew point temperature. It looks like my dew point is -1 degrees Celsius.

Step 9: The Results!

After reading the psychrometric chart you have successfully found the relative humidity and dew point!

Now... what good did this do for you? Well, if the relative humidity is reaching 100% then that means the air is becoming saturated with water and it's probably going to rain soon. If the temperature drops below the dew point temperature then it’s probably going to rain as well. Meteorologists stand back, science is at hand!

The next time your friend asks you if it's going to rain tell them to wait for ten minutes... then pull out your sling psychrometer and start swinging! If your friends are chemical engineers you can tell them that you're measuring the relative humidity using a sling psychrometer. They will admire you and possibly buy you dinner for your ingenuity.

If they aren't chemical engineers then they might just look at you awkwardly and then go inside to look up the weather on the internet. If this is the case, they just don't understand.