Tuesday Oct. 24, 2006

The Assignment #2b 1S1P reports were collected today.  That ends Assignment #2.  There will be atleast one more 1S1P assignment before the end of the semester.

Optional Assignments #4 and #5 are due at the beginning of class on Thursday

Remember there is a experiment report writing workshop on Wednesday (4:30 in Harvill 302).
We'll learn how clouds are identified and classified in today's class.  The ten main cloud types are listed below.


You will notice that the 10 cloud names are all built up using 5 "key words" (alto, cumulo or cumulus, strato or stratus, cirro or cirrus, and nimbo or nimbus). The best way to learn these cloud names and the characterstics of each cloud type is to learn the 5 key words. 
Clouds are classified according to the altitude at which they form and the appearance of the cloud.  The words cirro and alto tell you something about cloud altitude.



Cirrus or cirro identifies a high altitude cloud.  Alto in a cloud name means the cloud is found at middle altitude.  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.

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 often thicker and lower than an altostratus cloud.

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.



Nimbo or nimbus in a cloud name means the cloud is or is capable of producing significant precipitation.  Note that only 2 out of the 10 cloud types is capable of producing appreciable 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.  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.
Next we looked at photographs of most of the 10 main cloud types.  You'll find the written descriptions of the cloud types on pps 97-98 in the photocopied notes.  Some additional details and sketches have been added below that weren't covered in class.

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).  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 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.


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 most 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.  The figure below wasn't shown in class.




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. 
Here's a good way to learn the 10 basic cloud type names.  Draw a table like the one shown below.

Then fill in each square with an appropriate cloud name and a sketch of the cloud.  You should also be able to describe each cloud type in words.

Click here to see an example of a completed chart.
In the last part of the class we looked briefly at the three types of satellite photographs and what they tell you about clouds or wind motions in the atmosphere.  You'll find these discussed on pps 99-100 in the photocopied class notes.  You'll also find this topic discussed on pps 236-240 (pps 233-237 in the 3ed) in the text (Chap. 9).

An infrared satellite photograph detects the IR radiation actually emitted by clouds.  You don't depend on seeing reflected sunlight, so  clouds can be photographed during the day and at night.  Because the satellite detects 10 micrometer radiation, IR radiation emitted by the ground is also visible in regions where there aren't any clouds.

White on an IR photograph means the top radiating surface of the cloud is cold (found at high altitude).  It is sometimes hard to distinquish the tops of strong thunderstorms from high altitude cirrus or cirrostratus clouds even though the clouds are very different.  Warm, low level clouds appear grey.  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.

Thick clouds produce a white image on a visible satellite photograph.  Thin clouds appear grey.  Note a thunderstorm appears white on both IR and VIS satellite photographs.  By comparing images of clouds on both visible and IR images you can begin to distinquish between different kinds of clouds. 

The origin of the patchy cloud pattern seen behing cold fronts that are out over the ocean is shown below.  This figure wasn't shown in class.

Cold air warms and is moistened as it passes over warmer ocean water.  The air can eventually become bouyant and rise enough that a cloud forms.  Clouds develop best when there is a big temperature difference between the air and the water.  This is essentially the same as the Lake Effect you may have read about if you did a 1S1P report on air masses.

A water vapor satellite photograph is similar to an IR satellite photo (both types photograph IR radiation).  In this case both water vapor and clouds emit IR radiation (it is a slightly different wavelength than in a normal IR photograph) that is detected and displayed by the satellite.  Water vapor found at low altitude is warm and appears grey on the photograph (often hard to see on the satellite photograph).  High altitude water vapor is cold and appears white.  But remember the high altitude air is cold and there isn't much water vapor up there.  These photographs show air motions in regions where there aren't any clouds, motions that would otherwise be invisible.

Now that you have read through this information on satellite photographs, here is a sample satellite photograph question.