Fun science for kids: It’s not the heat, it’s the humidity!

Ever step outside on a hot day and feel it right away? If your skin feels sticky and your hair starts to frizz before you’ve even left the porch, it’s probably humidity.

So what is humidity? It’s the invisible water vapor (it’s a little like fog that you can’t see) floating in the air all around you. In the winter, lack of humidity — dry air — can make your skin feel dry and flaky. It can even give some kids bloody noses. YUCK! 

In the summertime, high humidity makes the heat feel way hotter. It also makes your air conditioner work way harder! Today, we’re going to learn a little bit more about how humidity works and how to measure it with three super-fun experiments you can do right in your own backyard.

• Build a hygrometer or a psychrometer.
• Conduct a wet bulb test.
• Create your own backyard weather station!

Let’s build a hygrometer.

Warm air can hold more water vapor than cool air. When the air is packed with moisture, sweat doesn’t evaporate off your skin as fast as it does when the air around you is drier — so you feel hotter and stickier. 

Scientists measure humidity with a tool called a hygrometer (hi-GROM-eh-ter). The number it gives you is called relative humidity, and it’s usually shown as a percentage. So 100% means the air is completely saturated with moisture, and it probably feels pretty gross outside. On the other hand, 1% humidity is really, really dry!

Fun fact: If it’s 90°F with low humidity, it might feel like 90°F. Add in high humidity, and it can feel like 105°F! 

Meteorologists call this “feels like” difference the heat index — and it’s why people in our seven-state Tennessee Valley region complain so much about muggy summer days.

Build a pine cone hygrometer.

Pine cones are nature’s own humidity detector! Their scales — the little “petals” that form the cone open up when the air is dry and snap shut when it’s humid. Pine trees scatter their seeds by opening so the seeds can float on the breeze and fall to the ground. Their seeds have the best chance of surviving when they can travel far away from the original tree. And they can’t travel very far when it’s too wet outside. So that’s why the cones close when it’s humid and open up when it’s dry! 

What you need:

• 3 dry, fully open pine cones
• A piece of cardboard
• A marker
• String (to hang it outside)

Steps:

1. Start with fully open (dry) pine cones. (You may need to dry them out inside for a couple of days so they can open up.) 
2. Carefully tie a piece of string or yarn around the base of one of the cones.
3. Hang the pine cone with the string tied around it from a tree branch.
4. Put another pine cone on a windowsill.
5. Put the third cone in a shallow bowl of water.
6. Observe how the pine cones change each day and record your observations.

SOURCES: littlebinsforlittlehands.com, treejourney.com, science-sparks.com

Build a psychrometer.

A psychrometer (si-KROM-ih-ter) uses two thermometers. One of them should be dry, and the other one should have a wet cloth or cotton ball wrapped around it. These are the tools you’ll need to calculate relative humidity. When water evaporates from the wet one, it cools down. The bigger the gap between the two temperatures, the drier the air!

What you need:

• 2 matching thermometers
• A small piece of cotton cloth (or thin sock fabric)
• Water
• Tape and a piece of cardboard
• A notebook to record results

Steps:

1. Tape both thermometers side by side on a piece of cardboard.
2. Wrap a small piece of damp cotton gauze or a damp cotton ball snugly around the bulb of one thermometer. This is your “wet bulb.”
3. Gently wave them around for 2–3 minutes. (Waving them speeds up evaporation.)
4. Read and record both temperatures. The wet bulb will be lower!
5. Look up a psychrometric chart online: Enter your dry bulb and wet bulb temperatures to get your relative humidity percentage.

If both temperatures are nearly the same, the air is very humid. A big difference between the two? It’s a dry day!

If you have enough people and enough thermometers, use one to measure indoor humidity and another pair to measure outdoor humidity.

SOURCE: sciencebuddies.org, eol.ucar.edu, teachengineering.org

Build your own backyard weather station!

Now put all your humidity knowledge together and become your neighborhood’s youngest meteorologist! Combine your pine cone hygrometer, your psychrometer, and a basic outdoor thermometer into one station. Track temperature, humidity and sky conditions every single day.

Keep a daily log and be sure to record these data points every morning:

• Date and time
• Outdoor temperature
• Pine cone status: open, partially open, or closed?
• Wet bulb and dry bulb readings (and your calculated humidity %)
• Sky description: clear, partly cloudy, overcast, rainy?

After two weeks, review your data. Can you spot any patterns? Try making a simple bar graph showing how humidity changed through the week.

Bonus challenge: Compare your pine cone readings to the official forecast. How often was your pine cone right? 

What does this have to do with energy efficiency and staying cool?

Your air conditioner doesn’t just cool the air; it also removes moisture. Which we now know is called humidity! On super-humid days, your AC has to work extra hard, which uses more electricity and costs your family more money. Newer, high-efficiency HVAC systems are specially designed to pull humidity out of the air faster and more efficiently, keeping everyone comfortable while helping your family save money.

Now that you understand humidity, you understand one of the biggest reasons why a good AC and a well-maintained HVAC system matters! Be sure to tell your family about rebates on things like air conditioning, heat pumps and insulation!