The top
of a thunderstorm is cold enough that it will
be
composed of just ice crystals. The bottom is composed of water
droplets. In the middle of the cloud both water droplets and ice
crystals
exist together at temperatures below freezing (the water droplets have
a hard
time freezing). Water and ice can also be found together in
nimbostratus
clouds. We will see that this mixed phase region of the cloud is
important
for precipitation formation. It is also where the electricity
that
produces lightning is generated.
There are lots of distinctive features on cumulonimbus clouds such as the flat anvil top and the lumpy mammatus clouds that are sometimes found on the underside of the anvil.
Mixed in wind with heavy precipitation are cold dense downdraft winds that hit the ground below a thunderstorm and spread out horizontally underneath the cloud. The leading edge of these winds produces a gust front (dust front might be a little more descriptive term). These winds at the ground below a thunderstorm can exceed 100 MPH, stronger than many tornadoes.
Here's one final feature to look for at the bottom of a thunderstorm.
There are lots of distinctive features on cumulonimbus clouds such as the flat anvil top and the lumpy mammatus clouds that are sometimes found on the underside of the anvil.
Mixed in wind with heavy precipitation are cold dense downdraft winds that hit the ground below a thunderstorm and spread out horizontally underneath the cloud. The leading edge of these winds produces a gust front (dust front might be a little more descriptive term). These winds at the ground below a thunderstorm can exceed 100 MPH, stronger than many tornadoes.
Here's one final feature to look for at the bottom of a thunderstorm.
Cold air spilling
out
of the base
of a thunderstorm is just beginning to move outward from the bottom
center of the
storm in the picture at left. In the picture at right the
cold air
has moved further outward and has begun to get in the way of the
updraft.
The updraft is forced to rise earlier and a little ways away from the
center of
the thunderstorm. Note how this rising air has formed an extra
lip of
cloud. This is called a shelf cloud.
Now that
we've finished with the section on identifying clouds this is a logical
time to learn a little bit about the 2
most common types of satellite photographs. You'll find this
discussed on pps 99-100 in the photocopied ClassNotes.
IR satellite photographs
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 um 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. You may recall that 10 um 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 on an IR photograph.
2. Clouds do absorb 10 um radiation and then reemit 10 um IR radiation upwards toward the satellite and down toward the ground. 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 weaker radiation (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 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 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. An infrared satellite photograph detects the 10 um 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. You may recall that 10 um 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 on an IR photograph.
2. Clouds do absorb 10 um radiation and then reemit 10 um IR radiation upwards toward the satellite and down toward the ground. 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 weaker radiation (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 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 grey throughout the day.
Visible satellite photographs
A visible satellite photograph photographs sunlight that is reflected by clouds. You won't see clouds on a visible satellite photograph at night. Thick clouds are good reflectors and appear white. Thinner clouds don't reflect as much light and appear grey. The low altitude layer cloud and the thunderstorm would both appear white on this photograph and would be difficult to distinquish.
Here's a summary of what we have
learned so far. This
figure 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, that we didn't discuss in class.
There is a 3rd type of satellite photograph, a water vapor image, that we didn't discuss in class.
It is also an IR satellite
photograph but detects and photographs 6.7 um 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).
This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops (a human hair is about 50 um 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.
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?
This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops (a human hair is about 50 um 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 um
diameter cloud
droplets to make one 2000 um diameter raindrop. How many
exactly? Before answering that question we will look at a cube
(rather than a sphere).
It would take 64 individual sugar
cubes to make a 4 cube x 4 cube x 4 cube cube. That is because
the bigger cube is 4 times wider, 4 times deeper, and 4 times
taller. Volume is 3 dimensions. (27 sugar cubes would be
needed to make the 3 cube x 3 cube x 3 cube box discussed in class)
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.
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 or something resembling hail called graupel often falls 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).
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 or something resembling hail called graupel often falls 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
what you might see if you looked inside a warm cloud with just water
droplets:
The collision coalescence process
works best in a cloud
filled with cloud droplets of different sizes. A short video
showed that 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 acclerating 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)
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" 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).
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" 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).