Wednesday Oct. 25, 2006

Optional Assignments #4 and #5 were collected today.
Answers to the Assignment #5 questions will be distributed in class on Friday.  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.

Today (and Friday) 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 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.  The biggest raindrops are about 1/4 inch across.

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. 


Before learning about the ice-crystal process we need to know something about the structure of cold clouds (clouds that contain both water droplets and ice crystals).


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.