Thursday Oct. 29, 2009
click here to download today's notes in a more printer friendly format

Some music from Robert Plant and Alison Krauss from their Raising Sand CD before class today.

The humidity Optional Assignment was collected today.  Answers are now available online.  That should be it for optional assignments until after next week's quiz.

The Experiment #3 reports are due next Tuesday.  You should be thinking about bringing your materials by my office later this week or early next week so that you can pick up the supplementary information handout.  There's a box just inside the door (PAS 588) where you can drop off the materials.

The preliminary version of the Quiz #3 Study Guide is now online.

We'll spend most of today's class learning how to identify and name clouds.  There'll be a short section on satellite photographs of clouds at the end of the period.

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 5 key words, we will find, mostly tell you something about the cloud altitude and appearance.

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


Each of the clouds above has a box reserved for it in the figure.

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.

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.

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.

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.

Cirrus clouds are sometimes considered to be a third type of cloud appearance.


The last key word, nimbo or nimbus, means precipitation.  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.  This particular crystal is called a column and is fairly long.

This is a flatter crystal and is called a plate.  These crystals tend to all be horizontally oriented and produce sundogs.  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.
Cirrus and cirrostratus clouds are fairly common.  Cirrocumulus clouds are a little more unusual.



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.

No pictures of stratus clouds were shown in class.



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 (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.

Here's one final feature to look for at the bottom of a thunderstorm.






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.  One day you'll be able to buy a NATS 101 Intro. to Weather & Climate T-shirt.  This is what will be on it.

This is a logical time to learn a little bit about the 2 most common types of satellite photographs.  You'll find this discussed on pps 99-100 in the photocopied ClassNotes.  Throw those old notes away, the figures below are better and were on a handout distributed in class.


When you see satellite photographs of clouds on the TV weather you are probably seeing an infrared satellite photograph.

1. An infrared satellite photograph detects the 10 um IR radiation actually emitted by the ground, the ocean and 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.  If clouds don't get in the way, you can see the ground on an IR photograph.

2.   Clouds absorb 10 um radiation and then reemit IR 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).  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 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 because 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 dark grey or black (afternoon when the ground is warmest) to lighter grey (early morning when the ground is cold) during the course of a day.  Because of water's high specific heat, the ocean right alongside doesn't change temperature much during the day and remains grey throughout the day.  Here's a link to an IR satellite photograph loop on the UA Atmospheric Sciences Dept. webpage.
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 above wasn't shown in class.

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



Note You'll probably find some introductory material on precipitation producing processes online before class next Tuesday.  Have a quick look at it, if you have time, so that you can familiarize yourself with some of the material before coming to class.