Lesson 2 Introduction
What causes rain, hail, sleet, and snow?
It forms clouds, rains water drops, soaks the soil, flows into rivers, lakes, and oceans, and then disappears into the air as a gas called water vapour.
So where does this water vapour come from? Well, you are breathing it right now! There is always some water vapour in the air you breathe. It is in the air because of evaporation. Evaporation occurs when the sun provides heat energy. Some water molecules move so fast, they let go of their liquid form and become an invisible gas (water vapour).
Wind speeds up evaporation by pushing full air away and pushing "thirsty air" next to water molecules.
You may not be able to see water vapour in the air, but there is a way the human body can feel it. Perhaps you have heard of the word humidity.Humidity describes the amount of water vapour in the air. The more vapour, the higher the humidity. On a day with high humidity, you might notice you feel sweaty, and your clothes might stick to you. This is because of all the water vapour in the air.
Fresh grass and a nice cold glass of juice – what could they have in common? Have you noticed that in the cool morning air, grass is often covered with droplets of water? Have you noticed that when you fill a glass with an ice-cold drink, a film of water droplets forms on the glass?
The water droplets on both the grass and the soda glass form for the same reason: condensation. Condensation occurs when water changes from water vapour (a gas) to liquid water. Heat must be taken away from molecules of water so that they slow down. Water vapour needs a surface on which to condense. Vapour condenses on the cold ground to form dew and frost. Vapour condenses in the air around cold dust particles or sea salt to form fog, and clouds.
Video
Watch this BrainPop video: What is humidity?
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Condensation is very important for weather because warm air can hold more water vapour than cold air. When air cools, it can no longer hold all its water vapour, and the water vapour condenses into liquid water. In fact, a special term – dew point –refers to how much water air can hold at a given temperature. When the temperature of air drops below the dew point, water comes out of the air. In nature, we see pretty droplets of dew on flowers, on a window, or on the grass.
View this video. The Water Cycle - MacMillan McGraw-Hill Review Lesson