Mon., Mar. 28 notes

Stamps issued by the US Postal Service. We'll be learning how to identify and name clouds today.

And a couple of sketches of the day

Evaporative cooling will make you feel cold if you get out of a swimming pool on an 80 F day with dry air. You won't feel as cold if the air is humid. Sling psychrometers make use of this to measure relative humidity and dew point.

At left in the figure above a 40 F day with 30 MPH winds will feel colder (because of increased transport of energy from your body by convection) than a 40 F day with no wind. The wind chill temperature tells you how much colder it will feel. If your body isn't able to keep up with the heat loss, you can get hypothermia and die.

Your body tries to stay cool by perspiring. You would still feel hot on a hot dry day. The heat index measures how much hotter you'd feel on a hot humid day. The combination of heat and high humidity is a serious weather hazard because it can cause heatstroke (hyperthermia). Your sweat won't evaporate as quickly on a humid day and your body might not be able to keep itself cool.

Next up was a short demonstration - producing a cloud in a bottle. Cooling air, changing relative humidity, condensation nuclei, and scattering of light are all involved in this demonstration.

We used a strong, thick-walled, 4 liter flask (vaccum flasks like this are designed to not implode when all of the air is pumped out of them, they aren't designed to 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 with a bicycle pump. At some point the pressure blows the cork out of the top of the flask. 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% momentarily ) and water vapor condenses onto cloud condensation nuclei in the air. A very faint cloud became visible at this point.

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

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 about how clouds might change and how this might affect climate (remember too that clouds are good absorbers of IR radiation).

The remainder of today's class was devoted to learning 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. 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 5 key words tell you something about the cloud's altitude and appearance.

Drawing a chart like this on a blank sheet of paper is a good way to review cloud identification and classification.

There are 10 boxes in this chart, one for each of the 10 main cloud types. You should be able to put a cloud name, a sketch, and a short written description in each square.

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.

Clouds are grouped into one of three altitude categories: high, middle level, and low. 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 when we look at cloud pictures later in the class.

Cirrus or cirro identifies a high altitude cloud. There are three types of clouds found in the high altitude category..

Alto in a cloud name means the cloud is found at middle altitude. The arrow 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. Sometimes it might sneak into the low altitude category.

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. 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, wavy, or ripply) appearance. These are cumuliform clouds and will have cumulo or cumulus in their name. In an unstable atmosphere cumuliform clouds will grow vertically. Strong thunderstorms can produce dangerous severe weather.

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

The last key word, nimbo or nimbus, means precipitation (it is also the name of a local brewing company). Only two of the 10 cloud types are able to produce (significant amounts of) precipitation. It's not as easy as you might think to make 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 because you are putting together the right two key words.

Here's the cloud chart from earlier. We've added the three altitude categories along the vertical side of the figure and the two appearance categories along the top. By the end of the class we will add a picture to each of the boxes.

Next we looked at 35 mm slides of most of the 10 cloud types. 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

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 small pointed 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. They're so thin you can sometimes see blue sky through the cloud layer. Haloes are a pretty sure indication that a cirrostratus cloud is overhead. If you were painting Cs clouds you could dip a broad brush in white paint (diluted perhaps with water) and then paint back and forth across the canvas.

Haloes are produced by white light entering a 6 sided ice crystal is bent (refraction). The amount of bending depends on the color (wavelength) of the light (dispersion). The white light is split into colors just as light passing through a glass prism. Crystals like this (called columns) tend to be randomly oriented in the air. That is why a halo forms a complete ring around the sun or moon.

This is a flatter crystal and is called a plate. These crystals tend to all be horizontally oriented and produce sundogs which are only a couple of small sections of a complete halo. A sketch of a sundog is shown below.

Sundogs are pretty common and are just patches of light seen to the right and left of the rising or setting sun.

If you spend enough time outdoors looking up at the sky you will eventually see all 10 cloud types. Cirrus and cirrostratus clouds are fairly common. Cirrocumulus clouds are a little more unusual. The same is true with animals, some are more commonly seen than others.

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.

middle altitude clouds

Altocumulus clouds are pretty common. Note since it is hard to accurately judge altitude, you must rely on cloud element size (thumbnail size in the case of Ac) 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.

Altostratus clouds are thick enough that you probably won't see a shadow if you look down at your feet. The sun may or may not be visible through the cloud. When (if) an altostratus cloud begins to produce precipitation, its name is changed to nimbostratus.

low altitude clouds

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.

I didn't have any photos of stratus clouds :(

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.

Thunderstorms fit into all three altitude categories

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 (dust front might be a little more descriptive). 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.

This is as far as we were able to go in class on Monday, but I'll stick a couple more figures into the online notes.

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.

Here's the completed cloud chart.