Quiz #3 Study Guide Pt. 1
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Saturation (10 pts). Saturation is an upper limit to the amount of water vapor that can be found in the air. The saturation mixing ratio is a property of air and depends on the air temperature - there can be a lot more water vapor in warm air than in cold air. When air is saturated with water vapor, condensation balances evaporation. Click here to test yourself on this topic.

Humidity variables (35 pts).  Ways of measuring or specifying the amount of water vapor in the air.
mixing ratio (r) - the actual amount of water vapor in air expressed as grams of water vapor per kilogram of dry air (think about what the units mean). This variable is not affected by changes in air temperature (unless you cool air below its dew point temperature) or pressure, it changes only when water vapor is added to or removed from the air.

saturation mixing ratio (rs) - the water vapor capacity of the air in grams of water vapor per kilogram of air. This property of air depends on temperature; you can look the value of rs in a chart or on a graph.

relative humidity (RH) - the amount of water vapor expressed as a percentage of the maximum amount (the saturation amount):
RH = 100% x r /rs
RH does not really tell you how much water vapor is in the air. The saturation amount, rs, depends on the air temperature and you may not know what that is. How can you change the RH? How would you expect the RH to change during the day?

dew point temperature (Td) - the temperature to which you must cool air in order for it to become saturated (RH becomes 100%). If you know Td, you can determine the mixing ratio (and vice versa), thus Td is a good measure of the actual amount of water vapor in the air. A large difference between the air temperature and the dew point temperature means the relative humidity is low. What is the RH when the difference is small? When the difference is zero?  Click here if you want to review this material on humidity variables.

Miscellaneous (20 pts). Cooling moist air to below its dew point and then warming it back up. Rain shadow effect. Why are relative humidities indoors often very low in the wintertime (where did that indoors air originate, did that air contain a lot, or not so much water vapor)? Measuring relative humidity and dew point with a sling psychrometer. Dry and wet bulb temperatures. Heat index (p. 142).

Sample Questions from the Fall 2000 Quiz Packet     Quiz #3: 3, 12, EC1, EC3     Final Exam: 1, 49

Dew, frozen dew, frost (10 pts). How do these differ (Is the nighttime minimum temperature, Tmin, above or below freezing; is the dew point temperature, Td, above or below freezing; is Tmin warmer or colder than Td)?  What weather conditions favor their formation?

Cloud condensation nuclei (CCN) (5 pts). Do CCN make it harder or easier for clouds to form? Typical concentrations. Hygroscopic nuclei. Dry haze, wet haze, and fog. Radiation fog and steam fog (aka evaporation or mixing fog).  "Cloud in a bottle" demonstration. Clouds clean the atmosphere.