Tuesday Mar. 31, 2015

Today's notes were assembled in a hurry late in the afternoon and might have some typos or confusing sections.  I'll read through them more carefully on Wednesday.

Music from Rupa and the April Fishes: "Cochabamba" (3:57), "Build" (4:37), "Este Mundo" (3:18) with some nice Day of the Dead costumes, "Culpa de la Luna" (3:08), "Trouble" (4:10) , "Une Americaine a Paris" (3:56), "Neruda" (3:13), "Electric Gumbo Radio" (4:22)

The 1S1P reports on Ultraviolet Light were collected today.  The 1S1P Equinoxes and Manhattanhenge reports have been graded and were returned.  Two new 1S1P topics are now available for your consideration.

The Experiment #3 reports were collected today together with the Expt. #2 report revisions.  Don't expect to get the Expt. #3 reports back before Thursday next week.

Finally the first part of the Quiz #3 Study Guide is now online.  The remaining part will be online by Thursday this week.

Condensation and the formation of dew, frost, haze, fog, and clouds
Here's part of what we'll be covering today.


A variety of things can happen when you cool air to the dew point and the relative humidity increases to 100%.  Point 1 shows that when moist air next to the ground is cooled to and below the dew point, water vapor condenses onto (or is deposited onto) the ground or objects on the ground.  This forms dew, frozen dew, and frost. 

When air above the ground cools to the dew point, it is much easier for water vapor to condense onto small particles in the air called condensation nuclei.  Both the condensation nuclei and the small water droplets that form on them are usually too small to be seen with the naked eye.  We can tell they are present (Point 3) because they scatter sunlight and make the sky hazy.  As humidity increases dry haze turns to wet haze and eventually to fog (Point 4).  We'll try to make a cloud in a bottle and you'll be able to better appreciate the role that condensation nuclei play. 

The main event on Thursday will be learning how to look up in the sky and identify and name clouds, but we'll get started on that today.

Here's a quick demonstration, it's something I wanted to do in class last week.

The drinking bird
Evaporative cooling and saturation are involved in the "drinking bird".  Here's a video.


The drinking bird is filled with a volatile liquid of some kind (ether?).  Initially the bird's head and butt are the same temperature.  The liquid evaporates and saturates the air inside with vapor.

Next you get the bird's head wet. 

The amount of vapor in the cool saturated air in the head is less than in the warmer saturated air at the bottom.


The differences in amounts of vapor produce pressure differences.  The higher pressure at the bottom pushes liquid up the stem of the bird.  The bird becomes top heavy and starts to tip.

At some point the bottom end of the stem comes out of the pool of liquid at the base.  Liquid drains from the neck.

and the bird straightens back up.
 
You can arrange the bird so that when it tips its beak dips into a small cup of water.  This keeps the head moist and cool and the dipping motion could go on indefinitely.


Condensation nuclei and the role they play in cloud droplet formation

You might have thought that once the relative humidity in the air (RH) reaches 100% that water vapor would simply condense and form little droplets.  This is not the case; we will find that small particles in the air called condensation play an essential role in cloud formation.

it is much easier for water vapor
to condense onto small particles
called condensation nuclei
 it is much harder for water vapor
to just condense and form
small droplets of pure water

There are always lots of CCN (cloud condensation nuclei in the air) so this isn't an impediment to cloud formation.  The following information is from p. 91 in the ClassNotes.



 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.

The following figure is at the bottom of p. 91 in the ClassNotes and illustrates how cloud condensation nuclei and increasing relative humidity can affect the appearance of the sky and the visibility.



The air in the left most figure is relatively dry.  Even though the condensation nuclei particles are too small to be seen with the human eye you can tell they are there because they scatter sunlight.  When you look at the sky you see the deep blue color caused by scattering of sunlight by air molecules mixed together with some white sunlight scattered by the condensation nuclei.  This changes the color of the sky from a deep blue to a bluish white color.  The more particles there are the whiter the sky becomes.  This is called "dry haze."  Visibility under these conditions might be anywhere from a few miles up to a few tens of miles.



(source of the image above)

Photographs of fairly recent pollution problems in Paris illustrate an extreme case of dry haze (this is more common and more severe in China).   In Paris cars with even numbered license plates weren't allowed into the city on Monday, odd numbers were banned on Tuesday.  Public transportation was free for a short time to try to reduce automobile use.

The middle picture shows what happens when you drive from the dry southwestern part of the US into the humid southeastern US or the Gulf Coast.  One of the first things you would notice is the hazier appearance of the air and a decrease in visibility.  Because the relative humidity is high, water vapor begins to condense onto some of the condensation nuclei particles (the hygroscopic nuclei) in the air and forms small water droplets.  The water droplets scatter more sunlight than just small particles alone.  The increase in the amount of scattered light is what gives the air its hazier appearance. This is called "wet haze."  Visibility now might now only be a few miles.




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)

Finally when the relative humidity increases to 100% fog forms and water vapor condenses onto all the condensation nuclei.  Fog can cause a severe drop in the visibility.  The thickest fog forms in dirty air that contains lots of condensation nuclei.  That is part of the reason the Great London Smog of 1952 was so impressive.  Visibility was at times just a few feet!



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.

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

Identifying and naming clouds

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.