REVIEW FOR QUIZ 6

• Condensation and Freezing of water in the atmosphere
1. If only water is involved in either the process of condensation or freezing then that process is called either HOMOGENEOUS condensation or HOMOGENEOUS freezing.
2. Most condensation or freezing, however, involves more than just water.  Most often particulates (often called aerosols) serve as nuclei to initiate the process.  In this case the process is a HETEROGENEOUS process.
3. What does CCN stand for?
4. Why is it important to the formation of clouds that CCN exist in the earth's atmosphere?
5. What two complications to condensation were discussed in class and how do they affect the condensation process?
6. What is the lowest temperature at which pure water will freeze?  This is often called the homogeneous freezing point.

• Cloud Formation
1. As mentioned several times during the course, in order to form a cloud, the air must be very near saturation (with respect to water).  Three general methods were mentioned in class along with a few specific examples of how this might be done.  The general methods are:
1. Cooling the air until saturation
2. Adding water vapor to the air until it is saturated
3. Mixing cool and warm, moist air
2. The most common method of forming a cloud is lift air up and as it does so it will cool.  Why does it cool as it rises?  What are three specific examples of cloud formation via cooling of air?
3. The rate at which unsaturated air cools as it rises is 10C/km.
4. The rate of cooling for saturated air, 6 C/km, is less than the dry case.  This is due to the partial warming of the air when water vapor condenses, releasing latent heat that is absorbed by the air parcel.
5. How would the rates of cooling, both dry and saturated, compare for parcels in Northern Canada (cold) and Southern Florida (warm)?
6. If the air temperature of a parcel at the surface in Tucson is 25C and the dew point 0C, how high above the surface would this parcel need to be lifted in order for it to become saturated?  In this case what is the Condensation Level?
7. As mentioned in class, one of the means of getting air to rise and cool is flow over a mountain.  Depending upon what happens to the moisture in the air as it flows over a mountain, there can be stark differences in the climate of the windward side of the mountain and the leeward side of the mountain.  Here are two examples:
8. Air blows from the west toward the east up and over a mountain that is 3 km high in altitude.  The air is initially at an altitude of 0km (sea level) and has a temperature of 30C and a dew point of 20C.  Make a table similar to what we did in class to show the temperature and the dew point of the air as it flows over the mountain and is forced to rise (and thus cool).   Does the air become saturated at anytime during the ascent (on the west side of the mountain)?  When the air reaches the top of the mountain it then flows down the east side of the mountain.  As it does so, it is compressed and thus its temperature is increased (just the opposite of the rising leg of the journey).  In this particular case we will assume that if there is any condensation, then the cloud that forms will move with the air parcel up and over the mountain and NO RAIN will fall from the cloud (no moisture is lost from the air).  So, in this case on the way down the mountain, the air parcel will be compressed and warm at the saturated rate (increase the temperature by 6C for every kilometer in altitude it descends) because the liquid water cloud droplets in the air parcel will evaporate.  The parcel will warm at this saturated rate until the dew point of the parcel on the east side is equal to the dew point of the parcel as it started out on the west side (20C).  After this point all the cloud liquid water has evaporated.  So the air will be unsaturated and the air will warm at the dry rate.  What is the air temperature and dew point of the air when it has descended the mountain on the east side to an altitude of 0km?
9. For the same case as above, but now we will assume that whatever water condenses will be lost from the air via the process of rain. The ultimate question is what is the air temperature and dew point of the parcel when it reaches the base of the mountain on the east side.  The book has a figure similar to this situation.  What actually happens in the atmosphere is somewhere in between the previous case and this case.

• Stability of the Atmosphere
1. Archimedes' principle states that an object immersed in a fluid (like water or air) is subject to an upward buoyant force.  What is this force equal to?
2. The net force is equal to the difference in ___ of the surrounding fluid and the object.
3. How does Charles's Law come into play in determining the direction of the net force of a parcel of air immersed in the atmosphere (this is just an object immersed in a fluid).
4. For a parcel of air, how does the release of latent heat during condensation affect the buoyant force, the weight of the parcel and the net force?
5. The atmosphere is stable if a parcel of air which is forced upwards, sinks back down to the original level when the lifting force stops.
6. Likewise, the atmosphere is unstable if a parcel of air which is forced upwards, continues to rise when the lifting force stops.
7. What is the level of free convection?
8. The bottom line for stability of the atmosphere is:
1. If the temperature of the atmosphere cools slower than a rising saturated air parcel (6C/km) or even warms as the altitude increases, then the atmosphere is ABSOLUTELY STABLE.
2. If the temperature of the atmosphere cools faster than a rising dry air parcel (10C/km) as the altitude increases, then the atmosphere is ABSOLUTELY UNSTABLE.
3. If the temperature of the atmosphere cools at a rate less than 10C/km, but greater than 6C/km, then is the atmosphere is CONDITIONALLY UNSTABLE. The condition is with regards to the whether or not the parcel is saturated.
9. For example;
1. The temperature sounding for Tucson on Nov. 10, 1999  shows that at 5 AM (the dotted curve) the temperature actually increases as the altitude increases (the atmosphere warms as the altitude increases) up to about 4000 ft.  Thus from the surface to 4000 ft the atmosphere is stable, because a parcel of air lifted from the surface would always be colder (more dense) than the surrounding air, regardless of whether the parcel is dry or saturated.  This layer of the atmosphere where the temperature increases with altitude is called an inversion.
2. The 5 PM temperature sounding, however, is unstable from the surface up to about 7000 ft since the atmosphere cools more rapidly than 5.5 F/1000 ft.  Thus a parcel of air starting at the surface with a temperature of 82 F that was nudged upward would continue to rise until about 7000 ft since it would cool at a rate of 5.5 F and always be slightly warmer than the surrounding air.  As soon as the temperature of the parcel is equal to that of the surrounding air, then the parcel will have the same density as the surrounding air and no longer be forced upward.  It would then remain at that level (altitude), which would be ~7000 ft in this specific case.
3. These two soundings illustrate how the stability of the atmosphere can be changed.  The stability of the atmosphere can be increased if the "bottom of the atmosphere" is cooled (from 5PM to 5AM).  Likewise, the instability of the atmosphere can be increased if the bottom of the atmosphere is warmed (from 5AM to 5PM).  This is discussed on page 108 of the text.
10. What type of clouds would you expect to form in a stable atmosphere?  What type of clouds would you expect to form in an unstable atmosphere.
11. Look at the tables that we made in class for the behavior of an air parcel for the three different cases. You should be able to fill-in the same sort of table and able to answer some questions using the information in the table.  Such as, you should be able to tell me if an air parcel becomes saturated and at what altitude (what is the LCL?), or what is the stability of the atmosphere and is there a level of free convection.

• Precipitation
1. Precipitation from "cold clouds" involves what 3 methods of "growth" in creating a precipitation particle.
2. If the air over a flat surface of pure liquid water is saturated (RH=100%), then the same air (has the same water vapor content and same temperature) over a flat surface of pure ice is below saturation, at saturation or above saturation?
3. The majority of precipitation that falls here on Earth in the middle and high latitude regions starts falling as what?