Tue., Oct. 23 notes

Tuesday Oct. 23, 2007

Optional Assignment #4 (Controls of Temperature) and Optional Assignment #5 (Humidity) were collected today. Both assignments will be returned on Thursday. Assignment #4 answers are online, answers to Assignment #5 will be distributed in class on Thursday.

The 1S1P Assignment #2 reports are due on Thursday.

The Experiment #3 reports are due next Monday. You should collect your data this week (warm sunny weather is predicted starting Tuesday). Return the materials this week and pick up the supplementary information sheet.

The Quiz #3 Study Guide is now online.

We will be looking at the formation of haze, fog, and clouds in class today. Before getting into that material we began with a short light scattering demonstration. Condensation nuclei and the small water droplets that make up fog and clouds are too small to be seen, we can see them because they scatter light.

In the first part of the demonstration, a thin beam of bright red laser light was shined across the front of the classroom. No one in the class could see this beam of light. To see the beam you would need to stand over where the beam struck the wall and look back toward the laser. The laser light is very intense and could damage your eyes, so this wouldn't be a very good thing to do.

Students in the class could see a red spot on the wall because the light hitting the wall was scattered or splattered and sent off in a multitude of directions. A individual ray of laser light was sent to everyone in the class (and because the intense light is split up into so many rays, the individual rays are weaker and safe to look at).

Next we clapped a couple of chalkboard erasers together. When particles of chalk dust fell into the laser beam they intercepted some of the laser light and scattered it. Again everyone in the room got their own personal ray of light coming from each of the particles of chalk. We use chalk because it is white, it scatters rather than absorbs light. What would you have seen if black particles of soot had been dropped into the laser beam?

In the 3rd part of the demonstration we made a cloud by pouring some liquid nitrogen into a cup of water. The numerous little water droplets made very good scatterers. So much light was scattered that the spot on the wall fluctuated in intensity (the spot dimmed when lots of light was being scattered, and brightened when not as much light was scattered).

The blue color of the sky is caused by the scattering of light by air molecules. The air molecules preferentially scatter the shorter wavelengths in sunlight. We should at some point later in the semester have time to discuss the blue color of the sky as well as phenomena such as haloes and rainbows which involve the refraction (bending) of light.

This figure (redrawn after class for improved clarity) shows how cloud condensation nuclei and increasing relative humidity can affect the appearance of the sky and the visibility.

The air in the left most figure is relatively dry. Even though the condensation nuclei particles are too small to be seen with the human eye you can tell they are there because they scatter sunlight. When you look at the sky you see the deep blue color caused by scattering of sunlight by air molecules mixed together with some white light scattered by the condensation nuclei. This changes the color of the sky from a deep blue to a bluish white color. The more particles there are the whiter the sky becomes. This is called "dry haze."

The middle picture shows what happens when you drive from the dry southwestern part of the US into the humid southeastern US. One of the first things you would notice is the hazier appearance of the air and a decrease in visibility. Because the relative humidity is high, water vapor begins to condense onto some of the condensation nuclei particles (the hygroscopic nuclei) in the air and forms small water droplets. The water droplets scatter more sunlight than just small particles alone. The increase in the amount of scattered light is what gives the air its hazier appearance. This is called "wet haze."

Finally when the relative humidity increases to 100% fog forms. Fog can cause a severe drop in the visibility. The thickest fog forms in dirty air that contains lots of condensation nuclei. We were able to see this effect in the cloud-in-a-bottle demonstration.

Cooling air and causing relative humidity to increase, condensation nuclei, and scattering of light are all involved in this demonstration.

We used a strong thick-walled 4 liter flask (flasks like this are designed not to implode when all of the air is pumped out of them, they aren't designed not explode when pressurized). There was a little water in the bottom of the flask to moisten the air in the flask. Next we pressurized the air in the flask. At some point the pressure blows the cork out of the top of the flask (hopefully). The air in the flask expands outward and cools. This sudden cooling increases the relative humidity of the moist air in the flask to 100% (probably more than 100%) and water vapor condenses onto cloud condensation nuclei in the air. A cloud became visible at this point. The cloud droplets are too small to be seen with the human eye. You can see the cloud because the water droplets scatter light.

The demonstration was repeated an additional time with one small change. A burning match was dropped into the bottle. The smoke from the match added lots of very small particles, condensation nuclei, to the air in the flask. The cloud that formed this time was somewhat "thicker" and easier to see.

Clouds are one of the best ways of cleaning the atmosphere (cloud droplets form on particles, the droplets clump together to form a raindrop, and the raindrop carries the particles to the ground). A raindrop can contain 1 million cloud droplets so a single raindrop can remove a lot of particles from the air. You may have noticed how clear the air seems the day after a rainstorm. Gaseous pollutants can dissolve in the water droplets and be carried to the ground by rainfall also.

A cloud that forms in dirty air is composed of a large number of small droplets (right figure above). This cloud is more reflective than a cloud that forms in clean air, that is composed of a smaller number of larger droplets (left figure).

This is has implications for climate change. Combustion of fossil fuels adds carbon dioxide to the atmosphere. There is concern that increasing carbon dioxide concentrations will enhance the greenhouse effect and cause global warming. Combustion also adds condensation nuclei to the atmosphere (just like the burning match added smoke to the air in the flask). More condensation nuclei might make it easier for clouds to form, might make the clouds more reflective, and might cause cooling. There is still quite a bit of uncertainty how clouds might change and how this might affect climate (remember too that clouds are good absorbers of IR radiation).

This is a good point to learn more about how to identify and name clouds. The ten main cloud types are listed below (you'll find this list on p. 95 in the photocopied class notes).

You should try to learn these 10 cloud names. Not just because they might be on a quiz (they will) but because you will be able to impress your friends with your knowledge. There is a smart and a not-so-smart way of learning these names. The not-so-smart way is to just memorize them. You will inevitably get them mixed up. A better way is to recognize that all the cloud names are made up of key words. The key words, we will find, tell you something about the cloud altitude and appearance.

Clouds are classified according to the altitude at which they form and the appearance of the cloud. There are two key words for altitude and two key words for appearance.

Cirrus or cirro identifies a high altitude cloud. There are three high altitude cloud names.

Alto in a cloud name means the cloud is found at middle altitude. The dotted line connecting altostratus and nimbostratus indicates that they are very similar. When an altostratus cloud begins to produce rain or snow its name is changed to nimbostratus. A nimbostratus cloud is also often somewhat thicker and lower than an altostratus cloud.

It is very hard to just look up in the sky and determine a cloud's altitude. You will need to look for other clues to distinquish between high and middle altitude clouds. We'll learn about some of the clues you can look for in class on Wednesday.

There is no key word for low altitude clouds. Low altitude clouds have bases that form 2 km or less above the ground. The summit of Mt. Lemmon in the Santa Catalina mountains north of Tucson is about 2 km above the valley floor. So low altitude clouds will have bases that form at or below the summit of Mt. Lemmon.

Clouds can have a patchy of puffy (or lumpy or wavy) appearance. These are cumuliform clouds and will have cumulo or cumulus in their name. In an unstable atmosphere cumuliform clouds will grow vertically.

Stratiform clouds grow horizontally and form layers. They form when the atmosphere is stable.

The last key word, nimbo or nimbus, means precipitation. Two of the 10 cloud types are able to produce (significant amounts of) precipitation.

Nimbostratus clouds tend to produce fairly light precipitation over a large area. Cumulonimbus clouds produce heavy showers over localized areas. Thunderstorm clouds can also produce hail, lightning, and tornadoes. Hail would never fall from a Ns cloud.

While you are still learning the cloud names you might put the correct key words together in the wrong order (stratonimbus instead of nimbostratus or nimbocumulus instead of cumulonimbus). You won't be penalized for those kinds of errors in this class.

We'll look at pictures of most of the 10 cloud types in class on Wednesday. Drawings and written descriptions have already been placed on the Thurs., Oct. 25 notes pages.

We had just a little time at the end of the period to look at satellite photographs. Here is a much more detailed discussion of three types of satellite photographs. You'll find these discussed on pps 99-100 in the photocopied class notes (also on pps 240-243 (Chap. 9) in the 5th eds of the text, pps 236-240 in the 4th edition of the text)

The most common type of satellite photograph is an infrared satellite photograph. We had time to discuss this briefly in class.

1. An infrared satellite photograph detects the 10 um IR radiation actually emitted by the ground or by clouds. You don't depend on seeing reflected sunlight, so clouds can be photographed during the day and at night. You may recall that 10 um radiation is in the middle of the atmospheric window, so this radiation is able to pass through air without being absorbed.

2.   Clouds absorb 10 um radiation and then reemit radiation. The top surface of a low altitude cloud will be relatively warm. Warmer objects emit IR radiation at a greater rate or at higher intensity (the Stefan Boltzmann law from Chap. 2). This is shown as grey on an IR satellite photograph. A grey unimpressive looking cloud on an IR satellite photograph may actually be a thick nimbostratus cloud that is producing a lot of rain or snow.

3.   Cloud tops found at high altitude are cold and emit IR radiation at a lower rate or at lower intensity. This shows up white on an IR photograph.

4.   Two very different clouds (a thunderstorm and a cirrostratus cloud) would both appear white on the satellite photograph and would be difficult to distinquish. Meteorologists are interested in locating tall thunderstorms as they can produce severe weather.

5.   The ground changes temperature during the course of the day. On an infrared satellite animation you can watch the ground change from black (afternoon when the ground is warmest) to grey (early morning when the ground is cold) during the course of a day. The ocean right alongside doesn't change temperature much during the day and remains grey throughout the day.

The remaining information wasn't covered in class on Tuesday. We'll review it quickly at the start of class on Thursday.

A visible satellite photograph photographs sunlight that is reflected by clouds. You won't see much on a visible satellite photograph at night. Thick clouds are good reflectors and appear white. Thinner clouds don't reflect as much light and appear grey. The low altitude layer cloud and the thunderstorm would both appear white on this photograph and would be difficult to distinquish.

Here's a summary of what we have learned so far.

The figure below shows how if you combine both visible and IR photographs you can begin to distinquish between different types of clouds.

There is one more type of satellite image worth mentioned, a water vapor image.

This is also a type of IR photograph. It detects a different wavelength of IR radiation. 6.7 um radiation is absorbed and emitted by water vapor in the atmosphere. Warm low altitude water vapor appears grey and unimpressive. Higher altitude water vapor appears white on the satellite photograph. But remember the high altitude air is cold and there isn't much water vapor up there. The utility of these photographs is not to show you whether a lot of moisture is moving into an area but rather they reveal wind motions in regions where there aren't clouds.