Thu., Oct. 31, 2013

Music from Rupa and the April Fishes "L'Elephant", "Este Mundo" with some nice Day of the Dead costumes, "Gone", "Culpa de la Luna", "Trouble" to get the blood circulating on a cold morning.

The Experiment #1 revised reports have been graded and were returned in class today.  Click on this link to see if you have a graded report waiting to be picked up.  The Experiment #3 reports are due next Tuesday and the Experiment #2 report revisions are due next Thursday.

The Controls of Temperature and the Humidity Example Problems Optional Assignments were collected today.  You'll get at least they humidity assignment back next Tuesday to review before the quiz.  Here are answers to the questions on the Humidity Assignment.

The 3rd of the 1S1P Assignment #2 topics is due next Tuesday.

If you're not doing anything Sunday evening you should wander downtown and watch (or participate in) the 2013 edition of the All Souls Procession.

We had a little bit of material left over from Tuesday's class to finish before getting into the main topic of the day: Identifying and Naming Clouds.  Most of the information below was on a class handout.


Clouds clean particulates and pollutants from the air
 Particles (so called cloud condensation nuclei) play an essential role in cloud formation
 clouds and climate change

Earlier in the semester we learned that clouds are the best way of cleaning particulates and gaseous pollutants from the atmosphere. 

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.


We'll first illustrate that when the air is saturated with water vapor (the relative humidity is 100%) the rates of evaporation and condensation above a flat surface of water will be equal. 

It's hard for water vapor to condense and form a small droplet of water because small droplets evaporate at a very high rate.  This is known as the curvature effect and is illustrated below.



The surface of the smallest droplet above at left has the most curvature and the highest rate of evaporation (6 arrows).  The middle droplet is larger and does not evaporate as quickly.  The largest drop has an evaporation rate (3 arrows) that is the same as would be found over a flat surface of water.  If a small droplet of pure water were to form it wouldn't survive; condensation from the moist surroundings would not be enough to overcome the high rate of evaporation.  A droplet must somehow reach a critical size before it will be in equilibrium with its surroundings.

Particles in the air, cloud condensation nuclei (CCN), make it much easier for cloud droplets to form.  The figure below explains why.

By condensing onto a particle, the water droplet starts out large enough and with an evaporation rate low enough that it is in equilibrium with the moist surroundings (equal rates of condensation and evaporation). 

There's something else that can happen, something that I didn't mention in class.



Water vapor condensing onto the particle will sometimes cause the particle to dissolve.  This will lower the rate of evaporation even more.  Now there may even be more condensation than evaporation and the droplet can grow.

I wanted to mention the role that condensation nuclei (particles) play in cloud formation because we will be able to see it at work in the cloud in a bottle demonstration.  Cooling air, changing relative humidity, condensation nuclei, and scattering of light are all involved in this demonstration.



To make a cloud we need to raise the relative of the air in the bottle to 100%.                 RH = 100% x r/rS        We can do this by both moistening the air in the bottle (this increases r) and by cooling  the air (that decreases rS ).  There was a little water in the bottom of the flask to moisten the air.  Next we pressurized the air in the flask with a bicycle pump.  At some point the pressure blew the cork out of the top of the flask.  The air in the flask expanded outward and cooled.  This sudden cooling increased the relative humidity of the moist air in the flask to 100% ( probably more than 100% momentarily ) and water vapor condensed onto cloud condensation nuclei in the air.  A very faint cloud became visible at this point.   The demonstration wasn't terribly impressive at this point but actually it was working just as it should.

There's something else we could add to the air in the flask.  So far today's class has been about particles and the role they play in cloud formation.  We could also add some particles to the air in the flask.


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).

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.

The rest of today's class was devoted to learning how to identify and name clouds.  The ten main cloud types are listed below (you'll find this list on p. 95 in the ClassNotes).


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.  A better way is to recognize that all the cloud names are made up of key words.  The 5 key words tell you something about the cloud's altitude and appearance.  My recommendation is to learn the key words.


Drawing a chart like this on a blank sheet of paper is a good way to review cloud identification and classification. There are 10 boxes in this chart, one for each of the 10 main cloud types.  Eventually, you should be able to put a cloud name, a sketch, and a short written description in each square.

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.  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.


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.

Stratiform clouds grow horizontally and form layers.  They form when the atmosphere is stable.  You'll find strato or stratus in the cloud name.


The last key word, 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 class next Tuesday.

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.






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.

A short time ago we and created a cloud in a bottle.  We were able to make the cloud more visible by adding smoke from a burning match to the demonstration.  The smoke particles acted as condensation nuclei. 




Here's Mother Nature's version of the cloud in a bottle demonstration.  A forest 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.

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).  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.  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.

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

THUNDERSTORMS FIT INTO ALL 3 ALTITUDE CATEGORIES

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.

That was enough for today.  I'm surprised we were able to get through it all.  Have a nice weekend.  I'll see you perhaps at the All Souls Procession 2013.