Water in the Atmosphere Humidity Humidity is a

Water in the Atmosphere

Humidity • Humidity is a measure of the amount of water vapor in the air. • Air’s ability to hold water vapor depends on its temperature. Warm air can hold more water vapor than cold air.

Relative Humidity • Relative Humidity is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold at a particular temperature. • Example: at 10 degrees Celsius, 1 cubic meter of air can hold at most 8 grams of water vapor. If there were actually 8 grams of water vapor in the air, then the relative humidity would be 100%. Air with a relative humidity of 100% is said to be saturated. What if there were 4 grams of water in the air?

Measuring Relative Humidity • Relative humidity can be found using an instrument called a psychrometer. • Two thermometers- a wet and a dry bulb. The wet bulb has a cloth covering it that is moistened with water. When the psychrometer is “slung” air blows over both thermometers. Because the wet bulb thermometer is cooled by evaporation, its reading drops below that of the dry bulb thermometer.

Water cycle and humidity • If the relative humidity is high, do you think the water on the wet bulb would evaporate slowly or quickly? What if the relative humidity is low? – Think about the definition of relative humidity and what is happening to the water on the wet bulb. • Why might condensation occur if air cools enough?

Calculating Relative Humidity • Example: Dry Bulb = 18 degrees C Wet Bulb = 14 degrees C Use the chart to find the relative humidity. Dry Bulb Reading 10 12 14 16 18 20 22 Difference between wet, and dry bulb reading 1 88 88 89 90 91 91 92 2 76 78 79 80 81 82 83 3 65 67 69 71 72 74 75 4 54 57 60 62 64 66 68 5 43 48 50 54 56 58 60