Tue., Mar. 27 notes

Note that condensation onto certain kinds of condensation nuclei and growth of cloud droplets can begin even when the relative humidity is below 100%. These are called hygroscopic nuclei. Salt is an example; small particles of salt mostly come from evaporating drops of ocean water.

I might try to show a video tape, not a digital video but video recorded on a magnetic tape. It will depend first of all on there being a VCR in the classroom.

Here are some more of the details that we didn't cover in class.

To understand how condensation onto particles can begin even before the RH has reached 100% we first need to learn about the solute effect


solution droplet	pure water droplet

Water vapor condensing onto the particle in the left figure dissolves the particle. The resulting solution evaporates at a lower rate (2 arrows of evaporation). A droplet of pure water of about the same size would evaporate at a higher rate (4 arrows in the figure at right). Note the rates of condensation are equal in both figures above. This is determined by the amount of moisture in the air surrounding each droplet. We assume the same moist (the RH is 100%) air surrounds both droplets and the rates of condensation are equal.

The next figure compares solution droplets that form when the RH is 100% (left figure) and when the RH is less than 100%.


the droplet is able to grow	the droplet is in equilibrium with its surroundings even when the RH is less than 100%

The solution droplet will grow in the RH=100% environment at left. You can tell the RH is less than 100% in the figure at right because there are now only 2 arrows of evaporation. But because the solution droplet only has 2 arrows of evaporation it can form and be in equilibrium in this environment.

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.

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.

I'm hoping to at least get started on the next topic. I'm not sure how much of what follows we will be able to cover in class on Tuesday.

Identifying and naming clouds - 10 main cloud types
We'll spent the entire 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. In addition to learning the names, 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 a few key words.

*** 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). 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.

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


Thin fog (perhaps even wet haze) with pretty good visibility (source of the image)	Thick fog (visibility was less than 500 feet) (source of the image)

Pictures of fog like we sometimes get in Tucson (maybe once a year). The picture at left is looking east from my house and was taken early in the morning at the start of the spring semester in 2015. The picture at right is the view to the west. Visibility was perhaps 1/4 mile.