Wednesday, October 30


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 have no idea why bumping the bottle of water initiates freezing.

Superheated water
It is also possible to superheat water, i.e. heat water to above 212 F without it boiling.  When the superheated water is disturbed it suddenly and explosively begins to boil.  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 super heat 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 super heating water, here's a far safer experiment to try.

Carbonated drinks 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).  The juices don'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.

Next- the "tricky" part of the Ice Crystal Process: what actually gets precipitation formation started

We'll see next why or how the ice crystal process works, this is the "tricky" part.  It's a 3-step process (summarized on page 102b in the ClassNotes)


The first figure above shows a water droplet in equilibrium with its surroundings.  The droplet is evaporating (the 3 blue arrows in the figure).  The rate of evaporation will depend on the temperature of the water droplet.  There will be some evaporation even from a droplet that is very cold.  The rate of condensation will depend on the amount of water vapor in the air.  The processes of evaporation and condensation operate independently of each other.  The droplet is surrounded by air that is saturated with water vapor (the droplet is inside a cloud where the relative humidity is 100%).  This means there is enough water vapor to be able to supply 3 arrows of condensation.  Because the droplet loses and gains water vapor at equal rates it doesn't grow or shrink.


This figure shows what is required for an ice crystal (at the same temperature) to be in equilibrium with its surroundings.  First, the ice crystal won't evaporate as rapidly as the water droplet (only 1 arrow is shown).  Going from ice to water vapor is a bigger "jump" than going from water to water vapor.  There won't be as many ice molecules with enough energy to make that jump.  A sort of analogous situation is shown in the figure below.  The class instructor could and most of the people in the room could jump from the floor to the top of a 10 or 12 inch tall box.  It would be much tougher to jump to the top of the table (maybe 30 inches off the ground) or the cabinet (maybe 36 inches) at the front of the room.  There wouldn't be as many people able to do that. 

Back to the figure above.  To be in equilibrium with its surroundings, the ice crystal only needs 1 arrow of condensation.  There doesn't need to be as much water vapor in the air surrounding the ice crystal to supply this lower rate of condensation. 


Sort of an analogous situation: sublimation (solid to gas) is a bigger jump than evaporation (liquid to gas).  Not as many ice molecules are able to make the big jump as there are making the smaller jump

Now what happens in the mixed phase region of a cold cloud is that ice crystals find themselves in the very moist surroundings needed for water droplet equilibrium. This is shown below.

The water droplet is in equilibrium (3 arrows of evaporation and 3 arrows of condensation) with the surroundings.  The ice crystal is evaporating more slowly than the water droplet.  But the ice crystal is in the same surroundings as the water droplet and water vapor will be condensing onto the ice crystal at the same rate as onto the water droplet.  The ice crystal isn't in equilibrium, condensation (3 arrows) exceeds evaporation (1 arrow) and the ice crystal will grow.  That's part of what makes the ice crystal process work.



Snow Crystals
Now we will see what can happen once the ice crystal has had a chance to grow a little bit (the next few figures are on page 103a in the ClassNotes)

Once an ice crystal has grown a little bit it becomes a snow crystal.  Snow crystals can have a variety of shapes (plates, dendrites, columns, needles, etc.; these are called crystal habits) depending on the conditions (temperature and moisture) in the cloud.  Dendrites are the most common because they form where there is the most moisture available for growth.  With more raw material available it makes sense there would be more of this particular snow crystal shape.



Here are some actual photographs of snow crystals (taken with a microscope).    Snow crystals are usually 100 or a few 100s of micrometers in diameter (tenths of a millimeter in diameter).  That's visible but you'd need a microscope to see the detail shown above. 

You'll find some much better photographs and a pile of additional information about snow crystals at www.snowcrystals.com. Here's another source of some pretty amazing photographs.

This Snow Crystal Morphology Diagram (from SnowCrystals.com) illustrates how the shapes of snow crystals depends on temperature and the amount of moisture in the air.

Inside a cold cloud, once the ice crystal process is underway

A variety of things can happen once a snow crystal forms.

First it can break into pieces, then each of the pieces can grow into a new snow crystal.  Because snow crystals are otherwise in rather short supply, ice crystal multiplication is a way of increasing the amount of precipitation that ultimately falls from the cloud.

Snowflakes

 

Several snow crystals can collide and stick together to form a snowflake.  Snow crystals are small, a few tenths of a millimeter across.  Snowflakes can be much larger and are made up of many snow crystals stuck together.  The sticking together or clumping together of snow crystals is called aggregation.  I drew an X through the name in the figure because I frequently forget the name of this process and don't expect you to remember it either.

Riming (accretion) and graupel (aka snow pellets & soft hail)
The next process and particle are something that I hope you will remember.

Snow crystals can collide with supercooled water droplets.  The water droplets may stick and freeze to the snow crystal.  This process is called riming or accretion (note this isn't called collision coalescence even though it is the same idea).  If a snow crystal collides with enough water droplets it can be completely covered with ice.  The resulting particle is called graupel.  Graupel is sometimes mistaken for hail and is called soft hail or snow pellets.  Rime ice has a frosty milky white appearance.  A graupel particle resembles a miniature snow ball.  Or smaller finer grained version of the shaved ice in a "snow cone."   Graupel particles often serve as the nucleus for a hailstone.  You'll find lots of pictures on the internet .

Graupel is made of milky white frosty rime ice.  Sleet, we will find, is made of clear ice.  Here are some pictures to help you better appreciate the differences in appearance. 





Here's a snowball.  It's white and you can't see through it.  It's made up of lots of smaller crystals of ice.  Graupel is just a small snowball.  source
The ice in a snow cone is basically the same.  Lots of smaller chunks of ice.  The ice is frosty white (before you added the flavored syrup).   source

Graupel vs sleet, rime ice vs clear ice
Graupel is sometimes referred as snow pellets.  Sleet is sometimes called ice pellets.

clear transparent sugar crystals
source of this photograph



a frosty white sugar cube made up
of many much smaller grains of sugar

Appreciating the differences in the appearance of clear ice and rime ice.

Formation of hail
This figure (page 103b in the ClassNotes) gives you an idea of how hail forms.


In the figure above a hailstone starts with a graupel particle (Pt. 1, colored green to represent rime ice).  The graupel falls or gets carried into a part of the cloud where it collides with a large number of supercooled water droplets which stick to the graupel but don't immediately freeze.  The graupel gets coated with a layer of water (blue) at Pt. 2.  The particle then moves into a colder part of the cloud and the water layer freeze producing a layer of clear ice (the clear ice, colored violet, has a distinctly different appearance from the milky white rime ice), Pt. 3.  In Tucson this is often the only example of hail that you will see: a graupel particle core with a single layer of clear ice (you can look through the clear ice layer and see the graupel inside, here is a link to some photographs from a Arizona Daily Star June, 2015 article).  You'll also find photographs if you search "hail crossection photograph"



Here's a photograph of a hail stone that fell from a thunderstorm cloud in midtown Tucson during the Fall 2018 semester (photo credit: Jim Dugan).  It clearly shows the frosty white graupel particle surrounded by a layer of clear ice.  It's pretty unusual to get hailstones this large in Tucson.
In the severe thunderstorms in the Central Plains, the hailstone can pick up additional layers of rime ice and clear ice and hailstones can be composed of many alternating layers of rime and clear ice.  An  unusually large hailstone (around 3 inches in diameter) has been cut in half to show (below) the different layers of ice.  The picture below is close to actual size.  If something like this were to hit you in the head it would split your skull open.  Here's some pretty good video of a hailstorm in Phoenix.

Hail is produced in strong thunderstorms with tilted updrafts.  You would never see hail (or graupel) falling from a nimbostratus cloud. Here is a photo of a record setting 8" diameter hailstone collected in South Dakota (it weighed nearly 2 pounds).  I believe it is still the national record holder.     Click here to see a gallery of images showing hail damage to automobiles.


The growing hailstone can fall back into the updraft (rather than falling out of the cloud) and be carried back up toward the top of the cloud.  In this way the hailstone can complete several cycles through the interior of the cloud.  The article above mentions a supercell thunderstorm.  We will discuss these later in the semester.

Types of precipitation
Finally on page 104a in the ClassNotes are illustrations of some of the things that can happen once a precipitation particle falls from a cloud.  I've split this into two groups for clarity.



Essentially all the rain that falls in Tucson is produced by the ice crystal process.  The left figure above shows how this happens.  A falling graupel particle or a snow flake moves into warmer air and melts.  The resulting drops of water fall the rest of the way to the ground and would be called RAIN. 

In the middle picture graupel particles can survive the trip to the ground without melting even in the summer.  Many people on the ground would call this hail but that wouldn't be quite right.  Graupel is less common in the winter because it comes from thunderstorms and they don't form very often in the winter.  Snow can survive the trip to the ground in the winter but not the summer.  Snow does occasionally make it to the valley floor in Tucson.

Sometimes the falling raindrops will evaporate before reaching the ground.  This is called VIRGA and is pretty common early in the summer thunderstorm season in Arizona when the air is still pretty dry.  Lightning that comes from thunderstorms that aren't producing much precipitation is called "dry lightning" and often starts brush fires.





Rain will sometimes freeze before reaching the ground.  The resulting particle of clear ice is called SLEET.  FREEZING RAIN by contrast only freezes once it reaches the ground. Everything on the ground can get coated with a thick layer of ice.  It is nearly impossible to drive during one of these "ice storms."  Sometimes the coating of ice is heavy enough that branches on trees are broken and power lines are brought down (either by the weight of ice or falling tree limbs).   It sometimes takes several days for power to be restored.  Here's a gallery of images taken after ice storms.