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.