Fri., Oct. 26 notes

Friday Oct. 26, 2007

The Experiment #4 materials were distributed today. A limited number of sets of materials will be available again next week.

The Experiment #3 reports and the revised Expt. #2 reports are due on Monday (Oct. 29).

A variety of question were asked during class. Answers to these questions constituted an inclass Optional Assignment. You will find the question (numbered 1-10) sprinkled throughout the class notes.

We didn't have quite enough time in class on Wednesday to get started on the next topic: formation of precipitation. It is not as easy to make precipitation as you might think. Only nimbostratus and cumulonimbus clouds are able to do it.

This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops (a human hair is about 50 um thick for comparison). As we saw in the cloud in a bottle demonstration it is relatively easy to make cloud droplets. You cool moist air to the dew point and raise the RH to 100%. Water vapor condenses pretty much instantaneously onto a cloud condensation nucleus to form a cloud droplet. It would take much longer (a day or more) for condensation to turn a cloud droplet into a raindrop. You know from personal experience that once a cloud forms you don't have to wait that long for precipitation to begin to fall.

Part of the problem is that it takes quite a few 20 um diameter cloud droplets to make one 2000 um diameter raindrop. How many exactly? The raindrop is 100 times bigger across. Volume is three dimensions. The raindrop is 100 times wider, 100 times deeper, and 100 times higher than the cloud droplet. The raindrop has a volume that is 100 x 100 x 100 times larger than the volume of the cloud droplets. Question 1a: What is 100 x 100 x 100? Question 1b: What is the volume of a sphere?

Fortunately there are two processes capable of quickly turning small cloud droplets into much larger precipitation particles in a cloud.

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

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. There is one part of this process that is a little harder to understand. Though this process can produce a variety of different kinds of precipitation (rain, snow, hail, etc).

Question 2: What are the names of the two cloud types capable of producing significant amounts of precipitation?

Here's what you might see if you looked inside a warm cloud with just water droplets:

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. A larger than average cloud droplet will overtake and collide with smaller slower moving ones.

This is an acclerating growth process. The falling droplet gets wider, falls faster, and sweeps out an increasingly larger volume inside the cloud.

The figure below shows the two precipitation producing clouds: nimbostratus (Ns) and cumulonimbus (Cb). Ns clouds are thinner and have weaker updrafts than Cb clouds. Cb clouds are thicker and have much stronger updrafts. The largest raindrops fall from Cb clouds because the droplets spend more time in the cloud growing.

Raindrops grow up to about 1/4 inch in diameter. When drops get larger than that, wind resistance flattens out the drop as it falls toward the ground. The drop begins to "flop" around and breaks apart into several smaller droplets. Solid precipitation particles such as hail can get much larger than 1/4 inch in diameter.

Question 3: How many 200 um diameter drizzle drops would you need to make one 2000 um diameter raindrop?

Before learning about the second precipitation producing process, the ice crystal process, we need to look at the structure of cold clouds.

The bottom of the thunderstorm, Point 1, is warm enough (warmer than freezing) to just contain water droplets. The top of the thunderstorm, Point 2, is colder than -40 C and just contains ice crystals. The interesting part of the thunderstorm and the nimbostratus cloud is the middle part, Point 3, 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 be able to 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. At Point 4 we see this is because it is much easier for small droplets of water to freeze onto an ice crystal nucleus (just like it is easier for water vapor to condense onto condensation nuclei rather than condensing and forming a small droplet of pure water). Not just any material will work as an ice nucleus however. The material must have a crystalline structure that is like that of ice.

Question 4a: What is the flat top of a thunderstorm called?
Queston 4b: What is the lumpy cloud feature on the bottom side of the flat top called?

Question 6: High altitude clouds are composed entirely of __________.

Question 5: Which process CC or IC produces rain in the summer in Tucson?

We'll see next how the ice crystal process works. There's a couple of tricky parts.

The first figure above (see p.101 in the photocopied Class Notes) shows a water droplet in equilibrium with its surroundings..The droplet is evaporating (the 3 green arrows in the figure). The rate of evaporation will depend on the temperature of the water droplet. The droplet is surrounded by air that is saturated with water vapor (the droplet is inside a cloud where the relative humidity is 100%). This means there is enough water vapor to be able to supply 3 arrows of condensation.

The next figure shows what is required for an ice crystal (at the same temperature) to be in equilibrium with its surroundings. First the ice crystal won't evaporate as rapidly as the water droplet (only one arrow is shown). Going from ice to water vapor is a bigger jump than going from water to water vapor. There won't be as many ice molecules with enough energy to make that jump. A sort of analogous situation is shown in the figure below (many people could jump up a 1 foot step, fewer people would be able to jump up a 3 foot step).

To be in equilibrium only one arrow of condensation is needed. There doesn't need to be as much water vapor in the air surrounding the ice crystal to supply this lower rate of condensation.

There are going to be fewer people able to make the big jump on the left just as there are fewer ice molecules able to sublimated. Going from water to water vapor is a "smaller jump" and more molecules are able to do just as more people would be able to make the shorter jump at right in the picture above.

Now what happens in the mixed phase region of a cold cloud is that ice crystals find themselves in the very moist surroundings needed for water droplet equilibrium. This is shown below.

The water droplet is in equilibrium (3 arrows of evaporation and 3 arrows of condensation) with the surroundings. The ice crystal is evaporating more slowly than the water droplet. Because the ice crystal is in the same surroundings as the water droplet water vapor will be condensing onto the ice crystal at the same rate as onto the water droplet. The ice crystal isn't in equilibrium, condensation exceeds evaporation and the ice crystal will grow.

Question 7: Are water droplets and ice crystals are found together in clouds at temperatures ABOVE (warmer than) or BELOW (colder than) freezing?

The equal rates of condensation are shown in the figure below using the earlier analogy.

We'll look at the variety of things that can happen to a growing ice crystal in a cold cloud in class on Monday.

Question 8: Do water droplets and ice crystals found together at the same temperature in a cloud evaporate at the SAME or at DIFFERENT rates?

Question 9: Ice crystals don't actually evaporate. What is the name of the solid to gas phase change?

Question 10: (changed slightly from the one given in class): Should 1, 2, or 3 arrows of condensation be drawn going to the ice crystal in the figure below?