Monday, Mar. 28, 2016

La Santa Cecilia: "I Won't Cry For You" (3:40), "Vámonos" (3:40), "Sacudo la Pereza" (4:15), "Calaverita" (3:43)


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 about these kinds of 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.

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.  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 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 wasn't).  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 today and more so on Thursday.

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
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 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 on Thursday.  You'll need to organize this material is a clear compact way.  Here's something that may 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 categories run along the top (note 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. 

Something that I often forget to mention in class, the colors used on the clouds at right.  Green is indicating clouds that are warm (above freezing) and made up of only water droplets.  Purple or violet indicates very cold high altitude clouds composed of only ice crystals.  The middle level clouds shaded blue are colder than freezing and contain both unfrozen water droplets and ice crystals.  That's a little surprising but turns out to be a very important part of precipitation producing (and lightning producing) clouds.

We'll be looking at pictures of most of the 10 main cloud types today.  I'm hoping you'll go outside and have a look at clouds when and if they're in the sky.  But also a warning, real world examples are often much complex than what we'll be looking at here.  ATMO 170 is not going to make you cloud identification experts.

Try to organize this class notes material as you read through it.  For each of the cloud types sketch the cloud, write down its name and add a word or two of description on a small index card size piece of paper.  Put that piece of paper in its proper position on a larger cloud chart.  I.e. does that cloud belong at high, middle or low altitude.

Names, pictures and short descriptions of most of the 10 main cloud types
(many of the descriptions below are found on pps 97 & 98 in the ClassNotes)

Something I don't usually mention in class.  If you get a particularly good photograph of a cloud or if you are an artist (as I know some of you must be) and are able to draw some really nice cloud pictures, I'd like to see them (and include them in the class notes).  So send them in.
 
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.  Look down at your feet and see if you cast a 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 (dispersion).  The white light is split into colors 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 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.  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 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.  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 (actually I've seen most 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


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 Urbana-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.

Without being there, it is hard to tell whether this is an altrostratus, a nimbostratus, or 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



Pretty common.  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.



No photographs of stratus clouds, sorry.  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.

You should end up with something like this at the end of class.  Your cloud chart will also include some words of description or clues that help you identify and name a cloud.  I've used abbreviations for the cloud names (Cc = cirrocumulus, As = altostratus etc).


Here's a link to a cloud chart on a National Weather Service webpage with actual photographs.  27 clouds are shown.  This is because slightly different versions of the 10 main cloud types are shown.

Below is the actual cloud chart we ended up with at the end of class