It's Not the Heat, It's the Humidity

Contributor: Lindsey Congalosi. Lesson ID: 13125

It'll make your outdoor activities sweatier, your damp towels damper, and your basement more mildewy. Read more to find out how a little water vapor in the air can be so powerful.


Earth Science, Physical Science, Practical Life Skills

learning style
Auditory, Visual
personality style
Beaver, Golden Retriever
Grade Level
Middle School (6-8), High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!

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  • Why does a hot, humid day feel so much more uncomfortable than a hot, dry one?
  • Why does humidity make mold happy but make you feel like you need a shower?
  • Why does your towel always stay sort of damp when it's humid outside?
  • Why is the grass soaking wet in the morning but dries quickly as the day heats up?

Believe it or not, these questions all have the same answer! Read more to find out why.

There's water in the air; water vapor, to be exact. Water vapor is water in its gaseous state. It's invisible, but you can feel it. Think about how you feel on a humid day. The air is almost 'sticky' with water.

Humidity is a measure of how much water is in the air. Relative humidity is a measure of the amount of water in the air compared to the amount of water that could be in the air, based on its current temperature.

  • Which type of air can hold more water - hot or cold?

Actually, air can't really hold water so this was sort of a trick question. A better way to ask this would be:

  • Which temperature of water can stay in the air the longest?

The answer is warm. If the air is warmer, the water vapor is warmer. Warm water vapor is made of molecules that are moving faster than cool water vapor. Check out our lesson found in the right-hand sidebar under Additional Resources to learn more about temperature's effect on molecule speed.

If the air is cooler, the water vapor within it is also cooler. This causes the water molecules to slow down and makes them more likely to condense and turn back into liquid water. If you need a review of the water cycle, watch The Water Cycle | The Dr. Binocs Show | Learn Videos for Kids from Peekaboo Kidz:

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Picture this. It's a humid summer day, and the warm air is filled with water vapor. At night, the temperature drops and so does the temperature of the air. The now-cool air has more water vapor than it can hold as gas, so it 'sets down' some of the water as liquid droplets. This is what causes dew. After a particularly humid day, you may notice a lot of dew on the grass, the cars, and even the cobwebs.


As the day heats up and the air gets warmer, the dew droplets also warm, causing their molecules to speed up until they evaporate once again into the sky as invisible water vapor.

Watch this process occur in Time lapse dew from Stan Gariomi:

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  • Why does humid weather feel so icky?

To answer this question, we have to first look at sweat. As you know, sweat is a liquid your body releases.

sweating man

  • The main purpose of sweat is to cool you off, but do you know why it works?

When you sweat, the liquid gathers on your skin. The heat from your skin then moves into the sweat, heating it. When the sweat evaporates, it takes the heat with it and cools you off.

  • How is this process different when it's humid?

When the relative humidity is high, the air is already pretty full of water vapor. There's very little room for any extra water vapor to evaporate. This causes sweat to be less effective. It doesn't evaporate but instead stays on your skin, keeping your body heat with it. This is why a cool fan feels so refreshing on hot, humid days. It moves the air across your body causing your sweat to finally evaporate into the air.

dog and fan

Watch SciShow's Why Does Humidity Make It Feel Hotter? to get more information:

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Unfortunately, this is also why many people experience dry skin in the winter. The cooler winter air lacks the moisture found in warmer air. The dry winter air then dries you out by causing more moisture to leave your skin and move into the air. The purpose of lotion is to replace that moisture.

Some people also use a humidifier, which is a machine that uses liquid water to add more water vapor into the air.


Not everyone hates humidity. Mold and mildew thrive in a humid environment. The water in the air gives them the water they need to grow and reproduce.


To remove some of the humidity from the air, you can use a different machine called a dehumidifier. A dehumidifier turns the gaseous water vapor into liquid water that then collects in a tank you need to occasionally empty.


Now that you've learned all about humidity, find out how to calculate humidity for yourself in the Got It? section.

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