Joseph Vettorel, age 8, of Tucson, Arizona, for his question:
What is humidity?
The word humidity is always in the weather report. The weatherman tells us that the day's humidity is thirty percent, sixty percent or maybe ninety percent. It may be anything from one percent to a hundred percent. Twenty percent is low humidity. It tells us that the air is dry. Ninety percent is high humidity. It tells us that the air is full of moisture.
Moisture is water and water can be invisible. You can see the liquid water in a glass. It is visible, You also can touch it and feel its wetness. But with no trouble at all, you can make this water disappear. Then you cannot see it or touch it. However, it still will be water, it still will be moisture.
All you need for this magic trick is some heat. So pour your glass of water in a saucepan and put it on a hot stove. As the water gets hot, a steamy mist rises up from the top. The liquid water is turning into an invisible gas which is called water vapor.
Water is made from tiny molecules, too small for our eyes to see. In liquid water, the little molecules cling together. They are packed closely enough to touch each other and water fills all the Space in the glass. We can see it and touch it. When liquid water is heated its molecules tend to separate from each other. And remember we cannot see a single molecule.
Vapor is made from trillions of single water molecules. The tiny midgets are light enough to fly away. They spread out and mingle with the other gases. Of the air and all these gases are invisible. This vapor adds moisture to the air and moisture in the air is humidity. When the humidity is high, there is a lot of moisture in the air. When the humidity is low, the air is dry and almost free of moisture.
The vapor in the air turns to clouds and the clouds turn to rain. All this moisture comes from liquid water on the earth. The smiling sun warms the sea and the lakes, the rivers and the little puddles. Countless water molecules get enough heat to break free and fly off in the air. The sun turns water into vapor. And vapor adds humidity to the air.
The air can hold so much water and no more. When the humidity is one hundred percent, the air can hold no more moisture. If more vapor molecules are added, the air must get rid of them. The extra vapor must be changed back to liquid water. It becomes a mass of misty droplets and a cloud is born.