Thursday Apr. 2, 2015

Just in time for Easter, some gospel music from The Blind Boys of Alabama "Soldier", "Amazing Grace", "Nobody's Fault But Mine", "Run On", "Down By the Riverside", "Way Down in the Hole"

Quiz #3 is one week from today and the complete Quiz #3 Study Guide is now available online.

The goal today is to spend the first part of the period looking at examples of the 10 main clouds. Most of the written descriptions below come from pps 97 & 98 in the ClassNotes.

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.









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.



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.




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.


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



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.



Here's the completed cloud chart and here's a link to a cloud chart on a National Weather Service webpage with actual photographs.  See if you can fill in the cloud names using just the abbreviations and pictures of the clouds as clues.


Precipitation producing processes
The last topic we will cover before next week's quiz is 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?  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 a cloud condensation nucleus to form a cloud droplet.  It would take much longer (a day or more) for condensation to turn a cloud droplet into a raindrop.  You must 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 few 20 μm diameter cloud droplets to make one 2000 μm diameter raindrop.  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.  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.

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.  Clouds like this are only found in the tropics.  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 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 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.

Here's how the collision coalescence process works.  The picture below shows what you might see if you looked inside a warm cloud with just water droplets:


The collision coalescence process works in a cloud filled with cloud droplets of different sizes, that's critical.  The larger droplets fall faster than the small droplets.  A larger-than-average cloud droplet will overtake and collide with smaller slower moving ones.





This is an accelerating growth process.  The falling droplet gets wider, falls faster, and sweeps out an increasingly larger volume inside the cloud.  The bigger the droplet gets the faster it starts to grow (think of a growing ball of snow as it rolls down a snow-covered hill and picks up snow, grows, and starts to roll faster and faster;  or think of an avalanche that gets bigger and moves faster as it travels downslope)






A larger than average cloud droplet can very quickly grow to raindrop size.



The figure shows the two precipitation producing clouds: nimbostratus (Ns) and cumulonimbus (Cb).  Ns clouds are thinner and have weaker updrafts than Cb clouds.  The largest raindrops fall from Cb clouds because the droplets spend more time in the cloud growing. In a Cb cloud raindrops can grow while being carried upward by the updraft and also when falling in the downdraft.

Raindrops grow up to about 1/4 inch in diameter.  When drops get larger than that, wind resistance flattens out the drop as it falls toward the ground.  The drop begins to "flop" or "wobble" around and breaks apart into several smaller droplets.  Solid precipitation particles such as hail can get much larger (an inch or two or three in diameter).

And actually my sketch at lower left above isn't quite accurate as this video of the breakup of a 5 mm diameter drop of water shows.