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What Causes Dew?
Have you ever woken up after a clear summer night to find the grass is wet, even though you know it didn’t rain overnight? The moisture on the grass is called dew, which forms when water in the form of a gas, which scientists call water vapor, comes in contact with cool surfaces, like grass or the glass on a car parked outside. The term for the amount of water vapor in the air is called humidity. If the air contains a lot of water vapor, like in a rain forest, we say the humidity is high. If there is little water vapor in the air, like in a desert, the humidity is low.
When a mass of air contains as much water vapor as it can hold, the air is saturated. Warm air can hold more water vapor than cold air, so if a mass of warm air is cooled, it can no longer hold some of its water vapor. Some of the water vapor condenses, which means the water vapor changes back to a liquid. The liquid water can fog up your bathroom mirror, or make the morning dew. The exact temperature at which water begins to condense from a mass of air is called the dew point.
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