Thu., Oct. 26 notes

Thursday Oct. 26, 2006

Optional Assignments #4 and #5 were collected today.
Answers to the Assignment #5 questions were distributed in class. Answers to the Controls of Temperature assignment will appear online (that material won't be covered on the next quiz).

The Atmospheric Stability worksheet has been graded and was returned in class. There is a link on the class homepage where you can check to on how the 1S1P Assignment #2 report grading is progressing.

The Quiz #3 Study Guide has been updated and is now nearly finished (there may not be any additional changes made)

Some reading has been assigned from Chapter 5.

The Experiment #3 reports are due next Tuesday. If you haven't returned your materials yet, you will now have to come by my office (PAS 588) to leave your materials and to pick up the supplementary information sheet. The Expt. #2 revised reports and the Scientific Paper report are also due next Tuesday.

Today we will be learning how clouds make precipitation. It's not as easy as you might think.

This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops. As we saw in the cloud in a bottle demonstration it is relatively easy to make cloud droplets. You raise the RH to 100% and water vapor condenses pretty much instantaneously onto a cloud condensation nucleus to form a cloud droplet. It would take much longer (days) for condensation to turn a cloud droplet into a raindrop. Part of the problem is that it takes about 1 million cloud droplets of water to make a raindrop.

A raindrop is about 100 times bigger across than a cloud droplet. You must remember that volume depends on length x width x height. So a 2000 micrometer diameter raindrop contains 1 million times as much volume as a 20 micrometer diameter droplet.

There are two processes capable of quickly producing precipitation sized particles in a cloud.

The collision coalescence process works in clouds that are composed on water droplets only. Clouds like this are found in the tropics. We'll see that this is a pretty easy process to understand.

The ice crystal process produces precipitation everywhere else. This is the process that makes rain in Tucson, even in the hottest part of the summer.

The collision coalescence process works best in a cloud filled with cloud droplets of different sizes. As we saw in a short video the larger droplets fall faster than the small droplets. The large droplets overtake and collide with the smaller ones. The droplets then stick together and form any even larger droplet that will fall faster than before and sweep out a larger volume. In this accelerating growth process an above averaged sized droplet can quickly turn into a raindrop.

The raindrops that fall from nimbostratus clouds tend to be smaller than the raindrops that fall from cumulonimbus clouds. The growing raindrops don't spend as much time in the Ns cloud because the cloud is thin and the updrafts are weak.

So how big can the raindrops that fall from cumulonimbus clouds get? The answer is about 1/4 inch in diameter. The wind resistance that a large drop encounters as it falls through a cloud causes it to flatten out, start to flop around or wiggle, and eventually break into smaller pieces.

You may have noticed a few what seem to be very large raindrops hitting the ground with an impressive splot at the beginning of a summer thunderstorm. The figure below is one explanation of this phenomenon.

A bunch of raindrops of different sizes fall from the cloud. The big ones fall fastest and reach the ground first.

Now we're ready to tackle the ice crystal process. First though we must learn something about the structure of cold clouds.

The top of the thunderstorm is so cold that there are just ice crystals there. The bottom is warm enough to just contain water droplets. The interesting part of the thunderstorm and the nimbostratus cloud is the part that contains both supercooled water droplets (water that has been cooled to below freezing but hasn't frozen) and ice crystals. This is called the mixed phase region. This is where the ice crystal process will produce precipitation. This is also where the electrical charge that results in lightning is generated.

The supercooled water droplets aren't able to freeze even though they have been cooled below freezing. This is because it is much easier for small droplets of water to freeze onto an ice crystal nucleus. Not just any material will work, the material must have a crystalline structure that is like that of ice.

Now what happens inside the mixed phase region in a cold cloud. You will find most of the following on pps 101 and 101a in the photocopied class notes.

In a cold cloud the supercooled water droplets are in equilibrium with their surroundings. This means the air must be moist enough to provide enough condensation to balance evaporation from the droplets (3 arrows of evaporation from each drop above is balanced by three arrows of condensation).

In Step 2, an ice crystal at the same temperature won't sublimate as quickly as a supercooled water droplet. It is a bigger step to go from solid to gas than from liquid to gas (see figure below). There doesn't need to be as much moisture in the air to keep an ice crystal in equilibrium with it surroundings.

Most of the students in a class would be able to make a 1 foot vertical jump. The rate of evaporation from a supercooled water droplet is relatively high because many of the water molecules having energy needed to evaporate.

Only a few students would be able to make a 3 foot vertical jump, just as fewer ice molecules will have the energy needed to sublimate.

In a cold cloud ice crystals are found in the very moist air needed to keep supercooled water droplets in equilibrium. Water will condense onto the water droplets and be deposited onto the ice crystals at equal rates (see figure below). The ice crystal will grow under these circumstances. This is what gets the ice crystal process started.

Equal numbers of students could jump down the 1 and 3 foot drops, just as water vapor condenses onto the water droplet and ice crystal at equal rates.

Once it gets started, ice crystal process can produce a variety of types of particles inside the cloud.
We'll look at some of the possibilities next.

Once an ice crystal has grown a little bit it becomes a snow crystal. Snow crystals can have a variety of shapes (called crystal habits, sketched above) depending on the conditions (temperature and moisture) in the cloud. Dendrites are the most common because they form where there is the most moisture available for growth. With more raw material available it makes sense there would be more of this particular snow crystal shape.

Here are some actual photographs of snow crystals (taken with a microscope). You'll find even better photographs at
www.snowcrystals.com

A variety of things can happen once a snow crystal forms. First it can break into pieces, then each of the pieces can grow into a new snow crystal. Because snow crystals are otherwise in rather short supply, ice crystal multiplication is a way of increasing the amount of precipitation that ultimately falls from the cloud.

This is incidentally the idea behind cloud seeding, to increase the number of ice crystals and hopefully the amount of precipitation. A substance called silver iodide is often used. Silver iodide is one of the relatively rare materials that can act as an ice crystal nucleus. However it is possible to "overseed" a cloud and end up with too many ice crystals. Then they all fight for a limited amount of water vapor and, as a result, do not get very big. Overseeding a cloud could decrease the precipitation from a cloud.

Several snow crystals can collide and stick together to form a snowflake. Snow crystals are small, a few tenths of a millimeter across. Snowflakes can be much larger and are made up of many snow crystals stuck together. The sticking together or clumping together of snow crystals is called aggregation.

Snow crystals can collide with supercooled water droplets. The water droplets may stick and freeze to the snow crystal. This process is called riming or accretion (note it is really the same idea as collision and coalescence). If a snow crystal collides with enough water droplets it can be completely covered with ice. The resulting particle is called graupel (or snow pellets). Graupel is sometimes mistaken for hail and is called soft hail. Rime ice has a frosty milky white appearance. A graupel particle resembles a miniature snow ball. Graupel particles often serve as the nucleus for a hailstone.

Hail forms in thunderstorms with very strong updrafts. In the figure above the hailstone starts with a graupel particle (colored green to represent rime ice). The graupel falls or gets carried into a part of the cloud where it collides with a large number of supercooled water droplets which stick to the graupel but don't immediately freeze. The graupel gets coated with a layer of water (blue). The particle then moves into a colder part of the cloud and the water layer freeze producing a layer of clear ice (the clear ice, colored violet, has a distinctly different appearance from the milky white rime ice). The particle then can pick up a new layer of rime ice, followed by another layer of water which subsequently freezes to produce a layer of clear ice.

Large hailstones can be composed of many alternating layers of rime and clear ice. An unusually large hailstone (2.5 to 3 inches in diameter) has been cut in half to shown the different layers of ice.