Friday, Apr. 1, 2016

Copenhagen Philharmonic Symphony "Ravel's Bolero" (4:52), Grieg's "Peer Gynt" (2:16), Naturally 7 performing Phil Collin's "In the Air Tonight" (5:12)

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? 

Before we get into the details you will notice I underlined significant amounts 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.  There were a few raindrops falling from stratocumulus clouds after the 11 am class last Wednesday.  I've included some other examples 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 this that evaporates (or sublimes) before reaching the ground is called virga.  The rain that was coming from low altitude clouds on Wednesday this week survived the fall to the ground, but it was very light.  There wasn't enough to even 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 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.

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 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.





The bigger droplets fall faster than the slower ones.  They collide and stick together (coalesce).  The big drops gets even bigger, fall faster, and collide more often with the smaller droplets.  This is an accelerating growth process - 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.

image source

source of this image









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.

The ice crystal process works in most locations most of the time.  Before we can look at how the ice crystal process actually works we need to learn a little bit about clouds that contain ice crystals - cold clouds.

Cold clouds
The figure below shows the interior of a cold cloud.


The bottom of the thunderstorm, Point 1, is warm enough (warmer than freezing) to just contain water droplets.  The top of the thunderstorm, Point 2, is colder than -40 F (which, coincidentally, is equal to -40 C) and just contains ice crystals.  The interesting part of the thunderstorm and the nimbostratus cloud is the middle part, Point 3, that contains both supercooled water droplets (water that has been cooled to below freezing but hasn't frozen) and ice crystals.  This is called the mixed phase region.  This is where the ice crystal process will be able to produce precipitation.  This is also where the electrical charge that results in lightning is created.

Ice crystal nuclei

The supercooled water droplets aren't able to freeze even though they have been cooled below freezing.  This is because it is much easier for small droplets of water to freeze onto an ice crystal nucleus (just like it is easier for water vapor to condense onto condensation nuclei rather than condensing and forming a small droplet of pure water).  Not just any material will work as an ice nucleus however.  The material must have a crystalline structure that is like that of ice.  There just aren't very many materials with this property and as a result ice crystal nuclei are rather scarce.  In most of the mixed phase region there are more supercooled water droplets than ice crystals.

Supercooled water
Here are a couple of demonstrations involving supercooled water that I showed in class.  In the first demonstration, some supercooled water (cooled to -6 F (-21 C)) is poured into a glass bowl sitting at room temperature.  Just pouring the water into the bowl is enough of a "disturbance" to cause the supercooled water to freeze.  Just bumping a bottle of supercooled water in the second video is enough to cause the water to freeze.  I don't know why that happens.

Superheated water
It is also possible to superheat water.    When the superheated water is disturbed it suddenly and boils explosively.   This is a potentially dangerous demonstration to attempt, better to watch a video online.

Here are a some precautions just in case you're ever tempted to try an experiment like this. 

It is probably easier to superheat distilled water than ordinary tap water.  So you might put two cups of water into a microwave, one with tap water the other filled with distilled water.  The cup of tap water will probably start boiling when it is supposed to, i.e. before it can become superheated.  You can watch the tap water and get an idea of how long you need to heat the distilled water to superheat it.  I suspect impurities in the tap water might act as nuclei to initiate the boiling.

Then once you think you have superheated the cup of distilled water be very careful taking it out of the microwave (better yet leave it in the microwave).  Just the slightest disturbance might start the water boiling.  You want your hands, arm, body and faced covered and protected just in case this happens.  Tape a spoon onto the end of a long stick and put a little sugar or salt into the spoon.  Then drop the salt or sugar into the cup of superheated water.

Chemists will often use "boiling chips" to make sure water will start to boil when it is supposed to (at 212 F) rather than becoming superheated.
Bubbles in beer or soda

Rather than superheating water, here's a far safer experiment to try.

Carbonated drinks all contain dissolved carbon dioxide.  The drink containers are pressurized.  When you open the can or take the cap off the pressure inside is released and dissolved carbon dioxide gas starts to come out of solution and forms small bubbles.  Often you will see the bubbles originate at a point on the side or bottom of the glass.  These are nucleation sites and are often small scratches or pits on the surface of the glass that are filled with a small bubble of air.  You can think of these bubbles of air as being "bubble nuclei."  When the carbon dioxide comes out of solution rather than forming a small bubble of its own, it makes use of and builds on these existing bubbles of air.  The bubble, now a mixture of air and carbon dioxide, grows until it is able to break free and float to the surface (a little gas is left behind in the scratch so the process can start over again).

This is actually a michelada, I think; a mixture of beer, lime, and tomato juice(image source) but that doesn't affect the bubble formation

The next time you are drinking one of these carbonated beverages sprinkle in a few grains of sugar or salt.  These will serve as additional bubble nucleation sites and additional bubbles will form.  This is exactly what happened in the superheated water demonstration above.

This is as far as I dared go in class today.  I've moved the rest of the information about the formation of precipitation and types of precipitation to the Monday, Apr. 4 notes.