Monday Apr. 6, 2009
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Music today was from the Andrea Bocelli Vivere Live in Tuscany
DVD. The name of the song was La Voce del Silenzio.
Andrea Bocelli was joined by an Italian singer named Elisa.
The Expt. #3 reports have been graded. Revised reports are due in
two weeks, on or before Monday Apr. 20. Please return your
original report with your revised report.
The Expt. #4 reports are due today.
The ice
crystal
process can
produce a variety of precipitation particles inside the cloud.
Further
changes can occur once the particle falls from the cloud. The
pictures
were redrawn because I decided the figures on p. 103 were kind of ugly.
In
the example above at left the ice particles (graupel or snow) first
melt and
then evaporate before reaching the ground. Rain that evaporates
before
reaching the ground is called virga.
A similar thing can happen with snow crystals or snow flakes.
They
sublimate away; the streamers of falling precipitation are called fall
streaks
(as far as I'm concerned you can use the name virga for this process
since it
is the same overall idea). You'll see white streamers of snow
falling
from high altitude cirrus clouds fairly often.
The frozen
precipitation particles
produced by the ice crystal process (graupel or snow) can melt before
reaching
the ground. This would be rain (or drizzle if the drops are
small).
This is where rain in Tucson comes from even in the summer. Rain in
most
locations at most times of the year starts out as frozen precipitation.
If you are on a mountain top you might see some of the frozen
precipitation
before it melts. You might see graupel falling from a summer
thunderstorm, for example, while the people in the valley only observe
rain. Sometimes the graupel particles due reach the ground in
Tucson.
Sometimes the
frozen precipitation
will melt and then fall into a thick layer of cold air and
refreeze. The
resulting particle is called sleet (or ice pellets). Sleet is
rain that
freezes before hitting the ground. The clear ice in sleet is
noticeably
different from the frosty, milky white, rime ice in graupel.
Rain that falls into a shallow cold air layer and only freezes after
reaching
the ground is called freezing rain. 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.
Satellite
photographs are a good way of observing clouds
(especially out over the ocean). Using both
visible and
infrared light satellite photographs, you can get a good idea of cloud
type. However satellite
photographs don't really tell you whether a cloud is producing
precipitation or not. For that you need radar. The following information on radar was
not covered in class.
An ordinary radar periodically transmits a short burst of
microwave
radiation. This radiation penetrates a cloud but is reflected by
precipitation particles. The radar keeps track of what direction
the antenna is pointing and determines how long it takes for a signal
to go out and return. The radar also measures the strength of the
return signal (a cloud with lots of relatively large precipitation
particles will produce a stronger reflection than a cloud with less and
smaller particles). Conventional radar can thus determine the
direction and distance to a precipitating cloud and make an estimate of
the rainfall rate or intensity.
The radar antenna slowly spins as it is transmitting so it
scans a full
360 degrees in a minute or two.
Information from a single radar or a combination of data from many
radars are drawn on weather maps (the PPI display above shows the data
from a single radar, the radar would be at the center of the
picture). This would show where precipitation is occurring.
The radar data is often combined with satellite photographs.
Colors are used to indicate the intensity of the precipitation.
Yellows, oranges and reds generally indicate the heaviest precipitation
(often coming from thunderstorms).
In research the radar can be used to scan vertically through a storm,
this produces an RHI display.
By detecting changes in the frequency of the reflected
signal, a
doppler radar can measure the speed at which precipitation particles
are moving toward or away from a radar antenna. By combining data
from 2 or more radars (and some complicated computer processing),
three-dimensional wind motions inside a cloud can be mapped out.
Doppler radars can detect a rotating thunderstorm updraft (a
mesocyclone) that could indicate a thunderstorm capable of producing
tornadoes. Small mobile doppler radars are being used to try to
measure wind speeds in tornadoes. Police use doppler radar to
measure the speeds of automobiles on the highway (moving toward or away
from the police car).
Dual polarization radar is a research tool that can be used
to learn something about the kinds of
precipitation particles inside a cloud.
The
material on Why the Wind Blows Like it Does will be moved to the Friday
Apr. 10 notes.
