Quiz #3 Study Guide

Humidity variables (35 pts) - Water vapor in the atmosphere. 
mixing ratio ( r ) - the actual amount of water vapor in air expressed as grams of water vapor per kilogram of dry air (the name and the units are helpful, think  about what they mean). This variable is not affected by changes in air temperature (unless you cool air below the dew point and cause water vapor to start to condense),  it only changes when water vapor is added to or removed from the air. 

saturation mixing ratio ( rs ) - the maximum amount of water vapor that can be found in the air (again in grams of water vapor per kilogram of air). This value of this variable depends on the air temperature; you can look the value of rs in a chart or on a graph.

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 potentially be a lot more water vapor in warm air than in cold air. When air is saturated with water vapor, RH = 100%.
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 tells you whether the air is close to be filled to capacity (close to being saturated) or not.  RH does not really tell you how much water vapor is actually in the air.  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 to review this material on humidity variables.  There's also a review at the start of the Thu., Oct. 24 Lecture Notes

Miscellaneous (25 pts). Cooling moist air to below its dew point and then warming it back up (what effect will that have on the air?).   Rain shadow effect. Why is the relative humidity 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, what happens to it when brought indoors)?   Measuring relative humidity and dew point with a sling psychrometer. Dry and wet bulb temperatures. Heat index. 

What role do cloud condensation nuclei (particles) play in cloud formation.  Dry & wet haze, fog.  Cloud in a bottle demonstration. 

Sample Questions (from the online quiz packet):   Quiz #3: 1, 6, 8 - 10, 12, 14, 16, EC3, EC4          Final Exam: 22, 27, 52
                                                                                   see also the Tue., Oct. 22 and the Thu., Oct. 24 Optional Assignments

 Cloud identification and classification (60 pts). Ten cloud types. Clouds are classified according to altitude and appearance; what key words are used? You should be able to identify each of the 10 cloud types from pictures (hand drawn) or from a written description (eg. high altitude cloud with a filamentary appearance). How would you distinguish between Cc, Ac, and Sc or between Cs and As? What cloud type could produce a halo? Common features on thunderstorm clouds: anvil, mammatus, shelf cloud.  Here's a link to a hand drawn cloud chart shown in class (abbreviations are used instead of the full cloud names)

Sample Questions:  Quiz #3: 2, 4, 11, 17, EC1, EC2     Final Exam: 18, 25. 43

Formation of precipitation (30 pts). Approximate sizes of cloud condensation nuclei, cloud droplets, and raindrops. It is relatively easy to form cloud droplets (condensation); what about precipitation? Which of the two processes below is the most important precipitation producing process in the US?

Collision coalescence process. Produces rain in warm clouds (clouds in the tropics which contain water droplets only). Falling droplets collide (why?) and stick together. Effects of cloud thickness and updraft speed on raindrop size. Which cloud type produces the largest raindrops and the heaviest precipitation? About how large can raindrops get (why don't they get any larger)?

Ice crystal process. Structure of a cold cloud. What is supercooled water? Are there more water droplets or ice crystals in the mixed phase region in a cold cloud? Are ice crystal nuclei abundant or scarce in the atmosphere? Where does precipitation begin to form in a cold cloud? Why are ice crystals able to grow while supercooled water droplets do not? Riming (accretion) and graupel. Can the ice crystal process produce rain or just frozen forms of precipitation?

Types of precipitation (25 pts). Rain, virga, snow (snowflakes), drizzle, sleet (ice pellets), hail, freezing rain, graupel ("soft hail" or snow pellets). What type of cloud and special cloud characteristics are needed for hail formation?

Sample Questions:    Quiz #3: 3, 5, 13, 18, 19     Final Exam: 5, 11, 29      

Satellite Photographs of clouds (20 pts).  Infrared and visible photographs.  What do white and grey on these two types of photographs represent?   Thunderstorms can produce severe weather; how would a thunderstorm appear on VIS and IR photographs?  How can satellites view clouds at night?  The ground will change from light grey to dark gray or black during the day while the ocean nearby hardly changes at all.  Why is this?  Here is a sample satellite photograph question.

Sample Questions     Quiz #3: 7, 15     Final Exam: 31, 35

Reviews
Tue., Nov. 5 4 - 4:50 pm ECE 102
Wed., Nov. 6
 4 - 4:50 pm MLNG 310
ECE is Electrical and Computer Engineering, MLNG is Modern Languages