Thu., Oct. 30 notes

Thu., Oct. 30, 2014

I was having trouble again finding some music for this morning's class and finally settled on "Jezebel" from Iron & Wine, "In Control" Greensky Bluegrass, and someone I just heard about Shakey Graves with "Roll the Bones".

The 1S1P reports on two topics: "The Causes of the Seasons" and "The Equinoxes and Manhattanhenge" have both been graded and were returned in class. Reports on a couple of additional topics are due next Tuesday (Nov. 4). I have also just added some report writing guidelines for "The Earth's Changing Temperature" topic.

The Experiment #3 reports are due next Tuesday. The revised Experiment #2 reports are due next Thursday.

Quiz #3 is also next Thursday and the 2nd part of the Quiz #3 Study Guide is now online.

I will also try to get answers to the questions on the handout distributed in class on Tuesday online sometime today or tomorrow.

We started class with a quick look at condensation nuclei and the role they play in the formation of haze, fog, and clouds. You'll find this all at the end of the Tuesday Oct. 28 online notes. There's a good chance I'll also put some optional supplementary reading online on this topic.

Making a cloud in a bottle
Next up was the cloud-in-a-bottle demonstration. Cooling air & increasing relative humidity, condensation nuclei, and scattering of light are all involved in this demonstration.

We used a strong, thick-walled, 4 liter vacuum flask (designed to not implode when all of the air is pumped out of them, they really aren't designed to be 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. Believe it or not that's the way I like the demonstration to work.

The demonstration was repeated an additional time with one small change. A burning match was dropped into the bottle. The smoke from the matches added lots of very small particles, condensation nuclei, to the air in the flask. The same amount of water vapor was available for cloud formation but the cloud that formed this time was quite a bit "thicker" and much easier to see. To be honest the burning match probably also added a little water vapor (water vapor together with carbon dioxide is one of the by products of combustion).

Clouds and climate change
This effect has some implications for climate change.

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).

Combustion of fossil fuels adds carbon dioxide to the atmosphere. There is concern that increasing carbon dioxide concentrations (and other greenhouse gases) 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 that clouds are good absorbers of IR radiation and also emit IR radiation.

Something I didn't mention today (but probably did earlier in the semester when covering air pollution): clouds are one of the best ways of cleaning the atmosphere

A cloud is composed of small water droplets (diameters of 10 or 20 micrometers) that form on particles ( diameters of perhaps 0.1 or 0.2 micrometers). The droplets "clump" together to form a raindrop (diameters of 1000 or 2000 micrometers which is 1 or 2 millimeters), and the raindrop carries the particles to the ground. A typical raindrop can contain 1 million cloud droplets so a single raindrop can remove a lot of particles from the air. You may have noticed how clear the air seems the day after a rainstorm; distant mountains are crystal clear and the sky has a deep blue color. Gaseous pollutants can dissolve in the water droplets and be carried to the ground by rainfall also. We'll be looking at the formation of precipitation later this week.

And here's Mother Nature's version of the cloud in a bottle demonstration.

A brush fire in this picture is heating up air and causing it to rise. Combustion also adds some moisture and lots of smoke particles to the air. You can see that initially the rising air doesn't form a cloud. A little higher and once the rising air has cooled enough (to the dew point) a cloud does form. And notice the cloud's appearance - puffy and not a layer cloud. Cumulo or cumulus should be in the cloud name. These kinds of fire caused clouds are called pyrocumulus clouds. The example above is from a Wikipedia article about these kinds of clouds. The fire in this case was the "Station Fire" burning near Los Angeles in August 2009.

10 main cloud types
We spent the remainder of the class learning to identify and name clouds.

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. And I'm hoping you'll be able to sketch each of the clouds and describe them in words. That gets to be a lot of material to try to just memorize.

A better way is to recognize that all the cloud names are made up of key words. Clouds are classified using just two criteria: altitude and appearance. There are 4 key words that tell you something about the cloud's altitude and appearance (and a 5th key word for clouds that are producing precipitation wasn't mentioned in class). My recommendation is to learn the key words and what they mean. Then you can usually construct a cloud name by taking key words from both the altitude and appearance groups and combining them.

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.

Examples of puffy patchy (cumuliform) clouds found at different altitudes

	high altitude cloud the patches of cloud are small because they are far away a cirrocumulus cloud cirro means high altitude, cumulus means "patchy".
	middle altitude cloud the patches of cloud are bigger because they closer to the ground an altocumulus cloud
	low altitude cloud cumulus clouds (there is no key word for low altitude)

Cloud Appearance

Cumulus clouds are often described as resembling a head of cauliflower. 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.

Cumuliform cloud source	Head of cauliflower source
lumpy cloud source	patchy appearing cloud source
ripples or waves note the size, this is probably a middle or low level cloud source	This is probably a middle or high level cloud because the ripples are smaller (higher) source

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 looking up at the sun through a middle level layer cloud. The sun is visible but blurred. (source)

Cloud appearance comparison


featureless Stratiform cloud (layer cloud) an altostratus cloud	in between case, a "lumpy layer cloud" this is named stratocumulus	patchy, puffy Cumuliform cloud cumulus clouds

cirriform is sometimes used
as an appearance key word
source of this image

Trying to draw the different clouds will help you to visualize the differences in appearance

To draw the cirriform cloud you could use the sharp end of a pencil. Using the side of a pencil as you would if you were shading in or coloring in a picture was used in the center picture. To make the right picture I put a bunch of ink on the side of a sponge and pressed it against the paper.

There's a 5th key word that I have been neglecting 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.

No penalty for putting the key words in the wrong order

We've covered a fair amount of information and we're going to be looking at a lot of cloud pictures. You'll need to organize this material is a clear compact way. Here's something that my help.

Take out a blank sheet of paper and draw a chart like shown above at left. There are 10 boxes, one for each of the cloud types. The three altitude categories run along the vertical side of the chart and the two appearance categores run along the top (not the exceptions column). This will force you to remember the key words. Then you should be able to put a name in each box, sketch each of the clouds (as done above at right), and a short written description of each cloud.

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 (as I tried to do earlier). 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.

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

THUNDERSTORMS

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.