Wed., Apr. 2 notes

Here are the 10 cloud names that I am hoping you will learn. My recommendation is that you concentrate on the "key words" shown above that tell you something about a cloud's altitude and appearance. Then you'll not only be able to name the cloud, but will be able to describe it in words.

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. Eventually, you should be able to put a cloud name, a sketch, and a short written description in each square.

Start by writing down the key words for altitude along the vertical side of the chart and key words for appearance across the top. You'll find a completed version of the chart at the end of today's notes.

Clouds are classified according to the altitude at which they form and the appearance of the cloud. We'll discuss altitude again briefly.

Cloud Altitude

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 distinguish 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 basically the same kind of cloud. When an altostratus cloud begins to produce rain or snow its name is changed to nimbostratus. A nimbostratus cloud may become 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.

High clouds are found up near the top of the troposphere. Moving from left to right across the top of the picture are cirrocumulus, cirrostratus, and cirrus. It's cold up at high altitude and violet in this picture means the cloud is composed entirely of ice crystals.

Green at the bottom of the figure indicates clouds that form in warmer air and are composed of just water droplets, As you move left to right at the bottom of the picture are cumulus, stratus, and stratocumulus. Note how the base of these low clouds is at or below the summit of the mountain in the sketch.

Blue, at middle levels indicates a mixture of ice crystals and supercooled water droplets (water that has cooled to below freezing but hasn't frozen). The figure shows altocumulus, altostratus, and nimbostratus producing rain and snow.

The tall cloud at right is a thunderstorm, also named cumulonimbus. The cloud starts in the low altitude category and grows all the way up to the top of the troposphere, the top of the high altitude category.

A key word for cloud appearance is the second part of most cloud names.

Cloud Appearance

We'll start with cumuliform. In some ways cumulus clouds resemble a head of cauliflower. C for cumuliform and C for cauliflower. Cumulus clouds might also have a lumpy, patchy or puffy appearance and might form in waves or ripples. You'll find cumulo or cumulus in a cloud that has texture like this

Cumuliform cloud source	Head of cauliflower source
lumpy cloud source	patchy appearing cloud source
ripples or waves source	higher altitude cloud with ripples & a contrail source

Clouds can have a patchy of puffy (or lumpy, wavy, splotchy 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. You'll find strato or stratus in the cloud name.

stratiform - as in rock strata, stratosphere

rock strata at the Grand Canyon source
A side view of a layer cloud. How much sunlight is able to shine through the cloud depends on how thick the cloud is. A person on the ground may or may not cast a shadow.	A view from the ground look up at the sun through a middle level layer cloud. The sun is visible but blurred. (source)

There's a 5th key word that I have been forgetting to mention.

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. We'll start to look at precipitation producing processes in the next class.

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, for example). You won't be penalized for those kinds of errors in this class because you are putting together the right two key words.

Next we looked at photographs 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 ClassNotes. You won't find the pictures, you should draw those in yourself.

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 raw material needed to make clouds (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 might dip a small pointed brush in white paint brush it quickly and lightly across a blue colored canvas. Here are some pretty good photographs of cirrus clouds (they are all from a Wikipedia article on Cirrus Clouds)

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 watered down white paint and then paint back and forth across the canvas.

Now a detour to briefly discuss haloes and sundogs.

Haloes are produced when white light (sunlight or moonlight) enters a 6 sided ice crystal. The light is refracted (bent). 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. You don't usually see all the colors, usually just a hint of red or orange on the inner edge of the halo.

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. Keep an eye out for them whenever you see high thin clouds in the sky at sunrise or sunset.

A very bright halo is shown at upper left with the sun partially blocked by a building (the cloud is very thin and most of the sunlight is able to shine straight through). A halo like this would draw a crowd. Note the sky inside the halo is darker than the sky outside the halo. The halo at upper right is more typical of what you might see in Tucson. Thin cirrus clouds may appear thicker at sunrise or sunset because the sun is shining through the cloud at a steeper angle. Very bright sundogs (also known as parhelia) are shown in the photograph at bottom left. The sun in the photograph at right is behind the person. You can see both a halo and a sundog (the the left of the sun) in this photograph. Sources of these photographs: upper left, upper right, bottom row.

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 in the desert around Tucson (and even in town) than others.

To paint a Cc cloud you could 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.

The table below compares cirrostratus (the cloud on the left without texture) with a good example of a cirrocumulus cloud (the "splotchy" appearing cloud on the right). Both photographs are from the Wikipedia article mentioned earlier.

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. A couple of photographs are shown below (source: Ron Holle for WW2010 Department of Atmospheric Sciences, the University of Illinois at Urband-Champaign)

There's a much larger collection in this gallery of images. The fact that there are so many examples is an indication of how common this particular type of cloud is.

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. Three examples are shown below (the first is from a Wikipedia article, the middle and right photograph are from an Environment Canada web page)

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

Unless you were there and could see if it was raining or snowing you might call this an altostratus or even a stratus cloud. The smaller darker cloud fragments that are below the main layer cloud are "scud" (stratus fractus) clouds (source of this image).

LOW ALTITUDE CLOUDS

This cloud name is a little unusual because the two key words for cloud appearance have been combined, but that's a good description of this cloud type - a "lumpy layer cloud". Remember there isn't a key word for low altitude clouds.

Because they are closer to the ground, the separate patches of Sc are bigger, about fist size (sources of these images:left photo, right photo ). The patches of Ac, remember, were about thumb nail size.. If the cloud fragments in the photo at right are clearly separate from each other (and you would need to be underneath the clouds so that you could look to make this determination) these clouds would probably be "fair weather" cumulus. If the patches of cloud are touching each other (clearly the case in the left photo) then stratocumulus would be the correct designation.

I didn't show any photos of stratus clouds in class. Other than being closer to the ground they really aren't much different from altostratus or nimbostratus.

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.

A photograph of "fair weather" cumulus on the left (source) and cumulus congestus or towering cumulus on the right (source)

THUNDERSTORMS FIT INTO ALL 3 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 (in Arizona 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 (violet in the sketch) is cold enough that it will be composed of just ice crystals. The bottom (green) is composed of water droplets. In the middle of the cloud (blue) 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.

The top left photo shows a thunderstorm viewed from space (source: NASA Earth Observatory). The flat anvil top is the dominant feature. The remaining three photographs are from the UCAR Digital Image Library. The bottom left photograph shows heavy by localized rain falling from a thunderstorm. At bottom right is a photograph of mammatus clouds found on the underside of the flat anvil cloud.

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 a photograph of the dust stirred up by the thunderstorm downdraft winds (blowing into Ahwatukee, Pheonix on Aug. 22, 2003). The thunderstorm would be off the left somewhere and the dust front would be moving toward the right. Dust storms like this are often called "haboobs" (source of this image). We'll learn more about the hazards associated with strong downdraft winds later in the semester when we cover thunderstorms.

Shelf clouds can sometimes be quite impressive (the picture above is from a Wikipedia article on arcus clouds). The main part of the thunderstorm would be to the left. Cold air is moving from left to right in this picture. The shelf cloud forms along the advancing edge of the gust front.

Here's the completed cloud chart and here's a link to a cloud chart on a National Weather Service webpage with actual photographs. See if you can fill in the cloud names using just the abbreviations and pictures of the clouds as clues.