Thu., Oct. 25 notes

Thursday Oct. 25, 2007

1S1P Assignment #2 reports were collected today.

Quiz #3 is one week from today (Thu., Nov. 1, the day after Halloween). The complete Quiz #3 Study Guide is now online.

The Experiment #3 reports are due next Tuesday. Please try to return your materials by next Monday (you'll have to come to PAS 588), the experiment materials are needed for another class next week. Once you return the materials you can pick up a copy of the supplementary information sheet.

Computer generated grade summaries were distribted in class today. Please verify that the information recorded on your grade summary is correct.

We didn't finish the discussion of satellite photographs in class on Tuesday. Quite a bit of additional information was added to the Tue. Oct. 23 notes that was not covered in class.

Today we will first look at 35 mm slides of most of the 10 cloud types. Good photographs of the ten cloud types can also be found in Chapter 4 of the text and in a Cloud Chart at the end of the textbook. You'll find the written descriptions of the cloud types in the images below on pps 97-98 in the photocopied notes.

High altitude clouds are thin because the air at high altitudes is very cold and cold air can't contain much moisture (the saturation mixing ratio for cold air is very small). These clouds are also often blown around by fast high altitude winds. Filamentary means "stringy" or streaky. If you imagine trying to paint a Ci cloud you would dip a stiff brush in white paint brush it quickly and lightly across a blue colored canvas.

A cirrostratus cloud is a thin uniform white layer cloud (not purple as shown in the figure) covering part or all of the sky. Here you might first dilute your white paint with water and then brush back and forth across the canvas. The thin white paint might not be thick enough to hide the blue canvas but the white coating on the canvas would be uniform not streaky like with a cirrus cloud.

To paint a Cc cloud you would dip a sponge in white paint and press it gently against the canvas. You would leave a patchy, splotchy appearing cloud (sometimes you might see small ripples). It is the patchy (or wavy) appearance that makes it a cumuliform cloud.

Note since it is hard to accurately judge altitude, you must rely on cloud element size to determine whether a cloud belongs in the high or middle altitude category. The cloud elements in Ac clouds appear larger than in Cc because the cloud is closer to the ground.

Lenticular clouds (see Fig. 4.33 on p. 105 in the text) are a special type of altocumulus cloud.

When (if) an altostratus cloud begins to produce precipitation, its name is changed to nimbostratus.

This cloud name is a little unusual because the two key words for cloud appearance have been combined. Because they are closer to the ground, the separate patches of Sc are about fist size. The patches of Ac, remember, were about thumb nail size.

There weren't any examples of stratus clouds in the slide collection.

Cumulus clouds come with different degrees of vertical development. The fair weather cumulus clouds don't grow much vertically at all. A cumulus congestus cloud is an intermediate stage between fair weather cumulus and a thunderstorm.

There are lots of distinctive features on cumulonimbus clouds including the flat anvil top and the lumpy mammatus clouds sometimes found on the underside of the anvil. Cold dense downdraft winds hit the ground below a thunderstorm and spread out horizontally underneath the cloud. The leading edge of these winds produces a gust front. Winds at the ground below a thunderstorm can exceed 100 MPH, stronger than many tornadoes. The top of a thunderstorm is cold enough that it will be composed of just ice crystals. The bottom is composed of water droplets. In the middle of the cloud both water droplets and ice crystals exist together at temperatures below freezing (the water droplets have a hard time freezing). Water and ice can also be found together in nimbostratus clouds. We will see that this mixed phase region of the cloud is important for precipitation formation. It is also where the electricity that produces lightning is generated.

Here's one final feature to look for at the bottom of a thunderstorm.

Cold air spilling out of the base of a thunderstorm is just beginning to move outward from the bottom center of the storm in the picture at left. In the picture at right the cold air has moved further outward and has begun to get in the way of the updraft. The updraft is forced to rise earlier and a little ways away from the center of the thunderstorm. Note how this rising air has formed an extra lip of cloud. This is called a shelf cloud. You'll find a good photograph of a shelf cloud in Fig. 10.7 in the text.

We had a little bit of time 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. All of the figures below were redrawn for clarity (and to use in the MWF class online notes)

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?

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).

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.