Thursday Mar. 29, 2018

Adele "If It Hadn't Been For Love" (3:08), Laura Marling "Blackberry Stone" (3:31), Norah Jones "Black" (3:22), Gotye "Somebody That I Used to Know" (4:07), Koop "Koop Island Blues" (4:33), "Come to Me" (2:50), Elle King "Under the Influence" (3:18)
 
 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 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 warmer than 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.  You'll often find many different cloud type mixed together.  One day in 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 usually don't 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 your body is casting 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.   The photograph above (source) is like 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 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 (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


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.

We often work on a cloud chart during class.  Here's the example from today's class





Formation of precipitation in clouds
The last two topics we will cover before next week's quiz are precipitation formation and types of precipitation. 

Only two of the 10 main cloud types (nimbostratus and cumulonimbus) are able to produce significant amounts of precipitation and produce precipitation that can survive the fall from cloud to ground without evaporating.  Why is that? 

Before we get into the details you will notice that significant amounts is underlined in the sentence above.  That is because you will sometimes see streamers of precipitation falling from some of the other cloud types, clouds that you would not have thought capable of producing precipitation.   A couple of examples are shown below


Streamers of snow falling from either mid or high altitude clouds at sunset.  (source of this image)
 Snow falling from high altitude cirrus uncinus clouds, photographed in Catalina, Arizona, I believe.  (source of this image)

Precipitation like the examples above will almost always evaporate (or sublime) before reaching the ground.  If the clouds are closer to the ground some of the drops of rain or drizzle or flakes of snow might survive the fall to the ground, but it would be very light - probably not even enough to dampen the ground.

Why is it so hard for clouds to make precipitation?

This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops (a human hair is about 50 μm thick for comparison).  As we saw in the cloud in a bottle demonstration it is relatively easy to make cloud droplets.  You cool moist air to the dew point and raise the RH to 100%.  Water vapor condenses pretty much instantaneously onto cloud condensation nuclei to form cloud droplets.  It would take much longer (a day or more) for condensation to turn a cloud droplet into a raindrop.  You know from personal experience that once a cloud forms you don't have to wait that long for precipitation to begin to fall.

 Part of the problem is that it takes quite a bit more water to make a 2000 μm diameter raindrop than it does to make 20 μm diameter cloud droplets .  A raindrop is about 100 times bigger across than a cloud droplet.  How many droplets are needed to make a raindrop?  Before answering that question we will look at a cube (rather than a sphere).





How many sugar cubes would you need to make a box that is 4 sugar cubes on a side?


It would take 16 sugar cubes to make each layer and there are 4 layers.  So you'd need 64 sugar cubes.  The key point is that we are dealing with volumes,  in the case of a cube, volume is length x width x height.

The raindrop is 100 times wider, 100 times bigger from front to back, and 100 times taller than the cloud droplet.  The raindrop has a volume that is 100 x 100 x 100 = 1,000,000 (one million) times larger than the volume of the cloud droplets.  It takes about a million cloud droplets to make one average size raindrop.

Precipitation-producing processes
Fortunately there are two processes capable of quickly turning small cloud droplets into much larger precipitation particles in a cloud.




The collision coalescence process works in clouds that are composed of water droplets only.  This is often called the "warm rain" process.  Clouds like this are found in the tropics (and very occasionally in Tucson).  We'll see that this is a pretty easy process to understand. 



This process will only produce rain, drizzle, and something called virga (rain that evaporates before reaching the ground).  Because the clouds are warm and warm air can potentially contain more water vapor than cooler air, the collision-coalescence process can produce very large amounts of rain.

The ice crystal process produces precipitation everywhere else.  This is the process that normally makes rain in Tucson, even on the hottest day in the summer (summer thunderstorm clouds are tall and reach into cold parts of the atmosphere, well below freezing).  Hail and graupel often fall from these summer storms; proof that the precipitation started out as an ice particle).  Thunderstorms also produce lightning and later in the semester we will find that ice is needed to make the electrical charge that leads to lightning. 



There is one part of this process that is a little harder to understand, but look at the variety of different kinds of precipitation particles (rain, snow, hail, sleet, graupel, etc) that can result.