Fri., Oct. 27 notes

Friday Oct. 27, 2006

The Experiment #3 reports are due next Monday. 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 Monday.

Last Wednesday we saw how the collision coalescence process turns cloud droplets into precipitation size particles. The collision-coalescence process works in warm clouds that just contain water droplets.

Rain, drizzle, and virga are really the only types of precipitation that you would expect to fall from a warm cloud.

The Ice Crystal process works in cold clouds which are found in many more locations than warm clouds. The mixed phase portion of a cold cloud is shown in the figure below. In the mixed phase region you find both supercooled water droplets and also ice crystals.

We are going to see that a variety of different types of ice particles can form in a cold cloud. An that is just the beginning of the story; look at all the different types of precipitation that can fall from this cloud.

Now just 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 3 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 have the 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.

The ice crystal process can produce a variety of types of precipitation particles inside the cloud. Once the precipitation particle falls from the cloud it change in a variety of ways before it reaches and hits the ground.

In the example above at left the particle first melts and then evaporates before reaching the ground. Rain that evaporates before reaching the ground is called virga.

A similar thing can happen with snow crystals or snow flakes. They sublimate away; the streamers of falling precipitation are called fall streaks (I said I would accept the name virga for this process also since it is the same overall idea). You'll see white streamers falling from cirrus clouds sometimes.

The frozen precipitation particles produced by the ice crystal process (graupel or snow) can melt before reaching the ground. This would be rain (or drizzle if the drops are small). Rain in most locations at most times of the year starts out as frozen precipitation (even in Tucson in the summer).

If you are on a mountain top you might see some of the frozen precipitation before it melts. You might see graupel falling from a summer thunderstorm, for example, while the people in the valley only observe rain.

Sometimes the frozen precipitation will melt and then fall into a thick layer of cold air and refreeze. The resulting particle is called sleet (or ice pellets). The clear ice in sleet is noticeably different from the frosty, milky white, rime ice in graupel.

Rain that falls into a shallow cold air layer and freezes after reaching the ground is called freezing rain. It is nearly impossible to drive during one of these "ice storms." Sometimes the coating of ice is heavy enough that power lines are brought down and branches on trees are broken. It sometimes takes several days for power to be restored.