Monday, Apr. 4, 2016
Maserati "Oaxaca"
(8:11)
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 p. 101 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.
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. Because
the ice crystal is in the same surroundings as the water droplet
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 what
makes the ice crystal process work.
The equal rates of condensation are shown in the figure
below using the earlier analogy.
Snow
Crystals
Now we will see what can happen once the ice crystal has had a
chance to grow a little bit.
Once an ice crystal has grown a little bit it becomes a snow
crystal (this figure is on p. 102 in the ClassNotes). 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 (also several slides explaining the
difference between hail, graupel, and sleet).
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 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.
Hail that falls to the ground in Tucson usually just has a
graupel core and a single 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 p. 103 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.
