Tuesday., Nov. 6, 2018
Andrea Bocelli "Melodramma"
(1:15 - 5:15 = 4:00), "La
Voce del Silenzio" feat. Eliza (5:22), "Dancing"
Eliza (5:17), "Dare
to Live" feat. Laura Pausini (4:20)
We've got a lot to cover, a lot of sketching and
writing. Fortunately much of what we need is on page 101b, page 102a, page 102b, page 103a, page 103b, and page 104a in the
ClassNotes.
Precipitation-producing processes
There are two processes capable of quickly turning small cloud
droplets into much larger precipitation particles in a cloud.
Here's a recap
The collision coalescence process works in clouds that are
composed of water droplets only. Here's how it
works. The picture (found on page 101b in the
ClassNotes) 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.
Very quickly a larger than average cloud droplet can 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 (in this case the
smaller droplets are catching and colliding with the larger
droplets, but the end result is the same) 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 quicly 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 (see page
102a in the ClassNotes)
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 have
no idea why bumping the bottle of water initiates freezing.
Superheated water
It is also possible to superheat water.
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 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). 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
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.
To be in equilibrium 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.
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 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 (here is a nice side
by side comparison of graupel and hail from South New Jersey Today ).
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
|
frosty white sugar cubes
are 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 a couple of weeks ago
(photo credit: Jim Dugan). It clearly shows the frosty
white graupel particle surrounded by a layer of clear ice.
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 is currently the national
record holder. Here's
another hailstone that is almost as big. It holds
the record for Oklahoma. 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.
