Monday Oct. 24, 2011
click here to download today's notes in a more printer friendly format

Only a single short music selection before class today.  You heard "Big Noise from Winnetka" performed by New Orleans' Own Dukes of Dixieland (this version on YouTube is a little longer than the one played in class).  I heard the song at the Pilobolus dance performance at Centennial Hall Sunday night.

The Humidity Optional Assignment was collected today.

Today's class was as best I can remember devoted entirely 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).


I'm hoping you'll 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.  Because they all sound alike 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.  Eventually, 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 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 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.  We'll start to look at precipitation producing processes in class on Wednesday.

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.


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.  Since I don't have digital copies of my slides and can't put them online, I've tried to find some good online photographs of all the cloud types we discussed in class.  The photographs that follow weren't shown in class.   

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.  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.

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.

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.



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.

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.





A very bright halo is shown at upper left with the sun partially blocked by a building.  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.

Now we'll turn our attention to
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".



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.(sources of the photographs: left photo, right photo ).  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 up) these clouds would probably be "fair weather" cumulus.  If the patches of cloud are touching then stratocumlus would be the correct designation. 




I didn't show any photos of stratus clouds in class.  Other than being closer to the ground it really isn'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 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 (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. 


Shelf clouds can sometimes be quite impressive (the picture above is from a Wikipedia article on arcus clouds)


Here's the much less impressive hand drawn cloud chart. 
And a link to a cloud chart on a National Weather Service webpage.  It has photographs of all the main cloud types we have discussed.