Tue., Oct. 29 notes

Making a cloud in a bottle
Cooling air (caused by sudden expansion) & 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 more than 100% momentarily and water vapor condenses onto cloud condensation nuclei in the air.

I like it best when a faint, hard to see, cloud becomes visible. That's because there is something we can add to the demonstration that will make the cloud much "thicker" and easier to see.

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 (you could see the swirls of smoke, the small particles scattered light). 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).

I have found a couple of online versions of the demonstration. The first is performed by Bill Nye "The Science Guy" and is pretty similar to the one done in class. The second differs only in the way that is used to caused the sudden expansion and cooling of the air (I didn't care much for the music (probably your opinion of the music I play before class) and I would recommend turning down the sound while watching the video).

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 (the RH is still less than 100%). 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 is the word used to describe a cloud with this appearance. These kinds of fire caused clouds are called pyrocumulus clouds. The example above is from a Wikipedia article fire-caused clouds. The fire in this case was the "Station Fire" burning near Los Angeles in August 2009. We sometimes see clouds like this in the summer when lightning starts a fire burning in one of the nearby forests. The pyrocumulus cloud caused by the fire is sometimes the only cloud in the sky.

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

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 far IR radiation and also emit IR radiation. Clouds often raise nighttime low temperatures.

Clouds are one of the best ways of cleaning the atmosphere. This is something we mentioned earlier in the semester and you're now in a position to understand it better.

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 in more detail later this week.

We should be able to get well into this next topic in class today. You'll find the following on page 95 in the ClassNotes.

Identifying and naming clouds - 10 main cloud types

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. The names all sound alike and you will inevitably get them mixed up.

A better way is to recognize that all the cloud names are made up of a few key words. Try to learn the key words and what they tell you about clouds. In addition to learning the names, I'm hoping you'll be able to sketch each of the clouds and describe them in words. The key words will help with that also.

*** Clouds are classified using just two criteria: altitude and appearance ***

There are 2 key words that tell you something about the cloud's altitude and 2 more for cloud appearance (there's a 5th key word for clouds that are producing precipitation). In many cases you'll be able to 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 (that's why the 2 to 5km and 5 to 10 km altitude ranges are X'd out). 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.

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. This is a cirrocumulus cloud, cirro means high altitude, cumulus means "patchy". The air is cold at high altitude and doesn't contain much water vapor. High altitude clouds are thin, there's not much raw material available to make the cloud.
	middle altitude cloud the patches of cloud are bigger because they closer to the ground. This is 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. These clouds are as tall or taller than they are across, in an unstable atmosphere cumuliform clouds will grow vertically and turn into thunderstorms. Strong thunderstorms can produce dangerous weather.

Here are some examples of the different textures or features that characterize cumuliform clouds:

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 and further away) 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, or 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 (and a clue about the cloud's altitude). 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
You'll become more familiar with different types of cloud appearances by looking at clouds and cloud photographs.


featureless Stratiform cloud (layer cloud) an altostratus cloud	an 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
a cirrus cloud
source of this image

Note the stratocumulus cloud name is formed by combining the two key words for appearance which is a little unusual.

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 more 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 together in the wrong order

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. Cirrostratus clouds are so thin that you should be able to look down at your feet and see your shadow.

Haloes and sundogs

Haloes are produced when white light (sunlight or moonlight) enters a 6 sided ice crystal. The light is bent (refraction). The amount of bending depends on the color (wavelength) of the light. The white light is split into colors (dispersion) just as when light passes 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 ice 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. The photograph above (source) is like what you might see in Tucson (except perhaps for the lake in the foreground). The sun is in the center of the photograph and the sundog is send at right. The photograph also illustrates how thin cirriform clouds will often appear thicker at sunrise or sunset because the rays of sunlight shine through them at an angle.

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 might cause a crowd to gather. 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. 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 (to 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. If you click on the link you'll see pictures of some of the wild animals that live in and around Tucson. With the exception of a skunk I've seen all of them in my neighborhood in central Tucson (often in my backyard).

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.

Cirrostratus - Cirrocumulus comparison

Middle altitude clouds

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.