The most common way that the amount of water in the air is reported is relative humidity.

RH is a comparison between the amount of water in the air with the amount of water that could be in the air when evaporation and condensation are equal. Mathematically RH is a percentage of "the maximum amount of water possible at that temp.

For example: 75% r.h. means if air in a room could have 20 liters of water in it, it really only has ________ liters.

If temp increases without adding any water, rh decreases. If temp decreases, rh increases. Why? The amount of water that COULD be in the air changes, so with the same amount of water, percent changes.

Dewpoint is the temperature at which condensation occurs. The more water vapor that is in in air, the higher this dewpoint temperature will be. In other words, high humidity will result in a higher dewpoint temperature. The dewpoint temperature of air is important in weather forecasting because it indicates how high air must rise in order for clouds to begin condensing from that air.

Use your water vapor (dewpoints) graph:

1. How much water (at most) could be in the air at 20 degrees: _________ . If it really only has 7 g/kg, then the rh is: __________ .

2. This air has a dewpoint of ________ .

3. If the air in the previous example rose into the atmosphere and cooled at the rate of 7 degrees Celcius per kilometer of altitude, at what altitude would condensation and cloud formation occur?

4. A mass of air has a temperature at the surface of 30 degrees C and a dewpoint of -20 degrees C, and it will cool off 7 degrees for each kilometer it rises. The atmosphere around this air mass has a  surface temperature of 20 degrees C, and cools at 7 degrees C for each km altitude. Will the rising warm air cool to its dewpoint?  If so, at what altitude will the clouds form? What kind of precipitation would form at that temperature? Diagram this problem.

5. A different mass of air also has a temperature at the surface of 30degrees C and a dewpoint of -20 degrees C, and cools off 7 degrees for each kilometer it rises. However the atmosphere around this air mass has a surface temperature of 25 degrees C, and cools at 5 degrees C for each km altitude. Will the rising warm air cool to its dewpoint? If not, why? If it will, at what altitude will the clouds form? What kind of precipitation would form at that temperature? Diagram this problem.