Most everyone can manage to make the big or
the small jump down.
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 photocopied 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). A microscope
is needed because snow crystals are very small, usually 100
or a few 100s of micrometers in diameter (tenths of a
millimeter in diameter). I
might not have mentioned in class that the different
shapes are called "habits".
You'll find some much better photographs and a pile of
additional information about snow crystals at www.snowcrystals.com.
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.
Several snow crystals can collide and
stick together to form a snowflake. Snow crystals are
small, a few tenths of a millimeter across, almost too small
to be seen. 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).
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.
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.
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. As it appears in your ClassNotes, the picture
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. The
official
US record holder is a hailstone that fell in
Vivian, South Dakota on July 23, 2010.
It was 8 inches in diameter.
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. We will discuss these later in the
semester.
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 (and I got a little
carried away with the colored pencils).
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.
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 (the image shows a car) 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. It sometimes
takes several days for power to be restored.