Water Vapor and Humidity

1. Water Vapor and Humidity

2. Humidity: Two types! • Specific Humidity • The actual amount of moisture in the air. Expressed as g/kg (grams of H2O per kilogram of air). • Relative Humidity • The ratio of the amount of moisture actually in the atmosphere compared to how much moisture could be in the atmosphere at a given temperature.

3. Think of the atmosphere as a sponge. . .

4. For example: Air has 11.2 g/m3 of moisture. It can hold 14.7 g/m3 at this temperature. 11.2 g/m3 _________ X 100% = 76.2 % humidity 14.7 g/m3

5. How do we measure relative humidity? • With a sling psychrometer. • Has two thermometers: one dry, one wet. • Dry bulb measures actual air temperature. • Wet bulb measures temperature with evaporation. • The actual temperature and the difference between wet and dry bulb temperatures are used to look the relative humidity up on a chart.

6. Some Questions: • What would evaporation do to the wet bulb temperature? • Does evaporation increase or decrease with increased humidity?

7. In general: • More moisture = less evaporation • Less evaporation = smaller difference between wet and dry bulb temperatures.

8. Finally, the DEW POINT! • The temperature to which the atmosphere would have to be cooled to reach the saturation point (when the atmosphere is holding all the moisture it can possible hold). • Think of a sponge decreasing in size, but not water content. • At dew point, dew forms (imagine that!)

9. Questions?