Relative Humidity: What It Is, How to Calculate It, and Chart Guide

In this guide, we’ll explain what relative humidity really means, how to calculate it (yes, you can!), and how to read it using a helpful chart. Plus, we’ll show you how to build a simple tool called a sling psychrometer to measure it yourself. Let’s get into the science behind the stickiness.

What Is Relative Humidity?

Humidity is all about water vapor in the air. But not all humidity is the same.

• Absolute humidity is the total amount of water vapor present in the air, no matter the temperature.
• Relative humidity tells you how full the air is with moisture compared to the maximum it could hold at that temperature.

Here’s the catch: warm air can hold more moisture than cold air. That means the same amount of water vapor feels different depending on how warm or cool it is.

Imagine the air like a sponge:

If the sponge (air) is half full of water, that’s about 50% relative humidity.

If it’s fully soaked — 100% relative humidity — the air can’t hold any more moisture. That’s when dew, fog, or rain can form.

So when your weather app says it’s 80% humidity, it means the air is holding a lot of moisture — which is why it feels sticky and uncomfortable.

How Do You Measure Humidity?

Feeling sticky is one thing, but how can you get a precise reading of humidity? Enter the sling psychrometer — a simple, handheld device that uses two thermometers to figure out relative humidity based on how much water evaporates into the air.

How It Works

• One thermometer measures the dry bulb temperature — the regular air temperature.
• The other measures the wet bulb temperature — it’s wrapped in a wet cloth.

When you swing the psychrometer through the air, water evaporates from the wet cloth. Evaporation cools the wet bulb thermometer.

• The drier the air, the more evaporation occurs, and the cooler the wet bulb gets.
• The more humid the air, the less evaporation, so the wet bulb stays closer to the dry bulb temperature.

By comparing these two temperatures, you can calculate the relative humidity using a chart or calculator.

Relative Humidity Chart

Below is a chart to determine relative humidity.

1. Find the difference between the two temperatures.
2. See where the numbers intersect on the chart below.

How to Build Your Own Sling Psychrometer

You don’t need fancy tools! Here’s what you’ll need and how to put it together:

• Two identical alcohol thermometers (dual scale: °C and °F is best)
• Small piece of cotton gauze or cloth
• Clear packing tape (strong)
• String (~18–24 in) or a handle
• Robust cardboard (from a box)
• Scissors or hole punch
• Water

How To Make It:

1. Cut the cardboard box to the size shown in the photo. the cardboard needs to be very sturdy; if you need to make it sturdier, tape two pieces back-to-back.
2. Punch a hole in the top of the cardboard (leaving about 1.5 inches). Pass a string through the hole and fasten in a loop with a square knot. Test it by gently tugging the string to make sure it holds.
3. Tape the two thermometers parallel to one another onto the cardboard lengthwise, with their bulbs hanging off the bottom edge. Ensure the measurements are facing outward so you can easily read them. Put tape across the top and bottom of both thermometers to ensure they are secure.
4. Wrap a cloth or gauze tightly around the bulb (the bottom part) of one thermometer and secure it with a rubber band and the packing tape. This will be your ‘wet bulb’ thermometer.

How to Use It:

1. Wet the cloth by dipping the wrapped bulb in water (don’t soak the thermometers)
2. Swing your sling psychrometer in the air for 2 to 3 minutes or until the wet-bulb reading stops falling. (To measure the relative humidity, air needs to be moving past the wet bulb to cause evaporation. You could also use a small fan to blow on the thermometers.)
3. Immediately read both thermometers:
   • Dry bulb = air temperature (this should remain constant)
   • Wet bulb = cooler due to evaporation
4. Calculate the difference (dry – wet temperature).
5. Determine Relative Humidity: Use a psychrometric table/chart above or an online calculator
6. Run it a few times and average the dry-wet difference for accuracy!

Why This Works

Evaporation cools the wet bulb thermometer — the drier the air, the more evaporation, and the bigger the temperature drop. Humid air slows evaporation, so the wet bulb temperature stays closer to the dry bulb temperature.

Connect with the Weather Around You!

Building and using your own sling psychrometer is a fun way to understand humidity firsthand. Try it out on a summer day, and you’ll never just guess how muggy it is again!

It’s a fun, hands-on way to connect with the weather around you!