Friday, Oct. 26, 2012
I'll be working on the uiz for next week this weekend and needed
some inspiration. "Stairway to Heaven"
by Led Zeppelin was just what was needed.
The Humidity Problems Optional Assignment turned in on Wednesday
has been graded and was returned in class today. You'll find
answers to the questions and some explanation here.
After learning about clouds on Wednesday this seemed like a
logical
place to briefly discuss the 2
most common types of satellite photographs.
You'll find the following figures discussed on pps 99-100 in the
photocopied ClassNotes.
IR
satellite photographs
When you see satellite photographs
of clouds on the TV weather you are probably seeing an infrared
satellite photograph.
1.
An
infrared
satellite
photograph
detects
the
10
μm IR
radiation
actually
emitted by the ground, the ocean and by clouds. You don't depend
on seeing
reflected
sunlight, so clouds
can be photographed during the day and at
night, 24 hours per day. You may recall that 10 μm
radiation is in the
middle of
the atmospheric window, so this radiation is able to pass through air
without being absorbed. If clouds don't get in the way, you can
see the ground and the ocean on an IR photograph.
2. Clouds do absorb 10 μm
radiation and then reemit
10 μm IR radiation upwards toward the satellite
and down toward the
ground. It is the radiation emitted by the top surface of the
cloud that will travel through the atmosphere and up to the
satellite. The top surface of a low altitude cloud will be
relatively warm. Warmer objects emit stronger IR radiation than a
cool object (the Stefan Boltzmann law).
This is shown as grey on an IR satellite photograph. A
grey
unimpressive
looking
cloud
on
an
IR
satellite
photograph
may
actually
be
a
thick
nimbostratus
cloud
that
is
producing
rain
or
snow.
3. Cloud tops found at high altitude are cold and emit
IR radiation at a lower rate or lower intensity. This shows up
white on an IR photograph.
4. Two very different clouds (a thunderstorm and a
cirrostratus cloud) would both appear white on the satellite photograph
and would be difficult to distinquish. Meteorologists are
interested in locating thunderstorms because they can produce
hazardous severe
weather. This can't be done using just IR photographs.
5. The ground changes temperature during the course of
the
day. On an infrared satellite animation you can watch the ground
change from dark grey or black (afternoon when
the ground is warmest) to lighter grey (early morning when the ground
is cold)
during the course of a day. Because of water's high specific
heat, the ocean right alongside doesn't
change temperature much during the day and remains the same shade of
grey throughout the
day.
Here's a link
to an IR satellite photograph loop on the UA Atmospheric Sciences Dept.
webpage.
Visible satellite photographs
1. A visible satellite photograph
photographs sunlight that is
reflected
by clouds. It shows what you would see if you were out in space
looking down at the earth. You won't see clouds on a visible
satellite photograph
at night.
2. Thick clouds are good reflectors and appear
white. The low altitude layer cloud and the
thunderstorm would
both appear white on this photograph and would be difficult to
distinquish.
3. Thinner clouds don't reflect as much light and appear
grey.
Here's a summary (that wasn't shown
in class)
The figure below shows
how
if
you
combine
both
visible
and
IR
photographs
you
can
begin
to
distinquish
between
different
types of
clouds.
You can use this figure to answer
the satellite
photograph
question that is on the Quiz #3 Study Guide.
There is a 3rd type of satellite photograph, a water vapor
image. This is just for your information purposes and wasn't discussed in
class.
This is also an IR satellite
photograph, but the satellite detects and photographs 6.7 μm
radiation.
This type of image can show
air motions in regions where there aren't any clouds because the
6.7 um radiation (Point 1) is absorbed by water vapor. The water
vapor then emits IR radiation upward toward the satellite where it can
be photographed. Water vapor from lower in the
atmosphere emits more strongly and appears grey (Point 2), water
vapor
from
high
in
the
atmosphere
emits
weak
radiation
and
appears
white
(Point
3).
The last
big topic we will cover
before next week's quiz is precipitation formation and types of
precipitation. Only two of the 10 main cloud types (nimbostratus
and cumulonimbus) are able to produce
significant amounts of
precipitation. Why is that? Why is it so hard for clouds to
make precipitation?
This figure shows typical sizes of
cloud
condensation nuclei (CCN), cloud droplets, and raindrops (a human hair
is about 50 μm thick for comparison). As
we
saw in the cloud in a bottle demonstration it is relatively easy to
make cloud droplets. You cool moist air to the dew point and
raise the RH to 100%. Water vapor
condenses pretty much instantaneously onto a cloud condensation nucleus
to form a cloud droplet. It
would take much longer (a day or more) for condensation to turn a cloud
droplet
into a
raindrop. You know from personal experience that once a cloud
forms you don't have to wait that long for precipitation to begin to
fall.
Part of the problem is that it
takes quite a few 20 μm
diameter cloud
droplets to make one 2000 μm diameter
raindrop. How many
exactly? Before answering that question we will look at a cube
(rather than a sphere).
How many sugar cubes would you need
to make a box that is 4 sugar cubes on a side?
It would take 16 sugar cubes to make each layer and there are 4
layers. So you'd need 64 sugar cubes. Volume is length x
width x height.
The raindrop is 100 times wider,
100 times
deeper, and 100 times taller than the cloud droplet. The raindrop
has a volume that is 100 x 100 x 100 = 1,000,000 (one million) times
larger than the volume of
the cloud droplets. It takes about a million cloud
droplets to make one average size raindrop.
Fortunately
there
are
two
processes
capable
of
quickly
turning
small
cloud droplets
into much larger precipitation particles in a cloud.
The collision coalescence process
works in clouds that
are
composed of water droplets only. Clouds like this are only found
in
the tropics. We'll see that this is a pretty easy process to
understand.
This process will only produce rain, drizzle, and
something called virga (rain that evaporates before reaching the
ground).
The ice crystal process produces precipitation everywhere
else.
This is the process that makes rain in
Tucson, even on the hottest day in the summer (summer thunderstorm
clouds are tall and reach into cold parts of the atmosphere, well below
freezing. Hail and graupel often
fall from these storms; proof that the precipitation started out as an
ice particle). There is one part
of this process that is a little harder to understand.
This
process can produce a variety of different kinds of precipitation
particles (rain, snow, hail, sleet, graupel, etc).
Here's how the collision coalescence process works. The
picture 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. The larger
droplets fall
faster than the small droplets. A larger-than-average cloud
droplet will overtake and collide with smaller slower moving
ones.
This is an accelerating growth
process.
The
falling droplet
gets
wider, falls faster, and sweeps out an increasingly larger volume
inside the cloud. The bigger the droplet gets the faster it
starts to grow (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)
A larger than average cloud droplet can very quickly 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 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 begins to "flop" or "wobble" around
and breaks
apart
into several smaller droplets. Solid precipitation particles such
as hail can get much larger (an inch or two or three in diameter).