Lesson 4: Invisible Water in the Air: Humidity

Getting Started

There is always water in the air, even when we can't see it. Water can appear in the atmosphere as a gas (in the form of water vapor) or as a liquid (in the form of clouds or precipitation). We can see clouds and precipitation, but we can't see water vapor because the water molecules are so small. This water vapor is called humidity. It affects the weather in your environment, but it also affects your body. One of the substances that is most affected by humidity is human hair! Humidity makes human hair get curlier or frizzier.

NOTE: You will fill in your weather journal for today during Activity 2.

Stuff You Need

  • Eyewitness Weather by Brian Cosgrove (revised edition)
  • clear tape
  • empty 16 oz. water bottle with cap
  • scissors
  • shoelace (kit)
  • thermometers (kit)

Ideas to Think About

  • How is the water cycle related to the weather we experience?
  • How does humidity affect the temperature?

Things to Know

  • Humidity means the amount of water vapor in the air. Humid air feels warmer on human skin than air with less water vapor.
  • Relative humidity describes how much water vapor is currently in the air, compared to the maximum amount of water that could be in the air at that temperature. It is expressed as a percentage.
  • A hygrometer is a tool that measures the amount of water vapor in the air, usually using a material that behaves differently in wet weather and dry weather.
  • The heat index is a measurement of how warm the air feels in the summer due to humidity in the air.


  • Discuss and determine how the following are interrelated: temperature, relative humidity, heat index. (S)
  • Know that when liquid water evaporates, it turns into water vapor in the air. (S)
  • Measure relative humidity using a hygrometer. (S)

Introducing the Lesson

Humidity can be a difficult concept because it's impossible to see. It has to be measured by observing materials that are affected by humidity. In this lesson, your child will explore what humidity is, the difference it makes in her environment, and some materials that are used to measure humidity. NOTE: Your child will fill in her weather journal for today during Activity 2.
Reading and Questions
Materials: Eyewitness Weather by Brian Cosgrove (revised edition)
Read pages 22-23 in Eyewitness Weather by Brian Cosgrove. There are no questions for this reading.