Friday Nov. 17, 2006
Today was the first of two 1S1P Assignment #3 due dates. The last
due date in next Monday. Some students will have reached the 45
pt maximum number of points allowed for 1S1P reports. You can
check this link to see if you have
reached 45 pts.
I forgot to mention in class that the Quiz #4
Study Guide is now available online.
Last
Wednesday we began a detailed look at what it required to initiate a
thunderstorm. That was finished at the beginning of class today.
You will find the complete discussion in the Wednesday
class notes. To review, parcels of air are lifted from the
ground beginning early in the day. As the day goes on the parcels
are lifted higher and higher. The rising parcels cool at either a
10 C/km rate (if the air is unsaturated) or a 6 C/km rate (saturated
air). Early in the day the lifted parcels end up colder and
denser than the surrounding air and sink back to the ground.
Eventually the parcels might be lifted above the level of free
convection where they become warmer and less dense than the surrounding
air. This is the point at which a thunderstorm begins.
The top portion of this figure repeats what we just
discussed: it takes some effort and often a good part of
the
day before a thunderstorm forms.
An ordinary single cell
thunderstorm goes through a 3-stage life cycle. In
the first stage, the cumulus stage, you would find updrafts throughout
the inside of the cloud.
Once precipitation has formed and grown to a certain size,
it will
begin to fall and drag air downward with it. This is the
beginning of the
mature stage where you find both an updraft and a downdraft inside the
cloud. The falling precipitation will also pull in dry air from
outside the thunderstorm (this is called entrainment).
Precipitation will mix with this drier air and evaporate. The
evaporation will strengthen the downdraft (the
evaporation cools the air and makes it more dense). The
thunderstorm is strongest in the mature stage.
Eventually the downdraft spreads throughout the inside of the cloud and
interferes with or cuts off the updraft. This marks the beginning
of the end for this thunderstorm. In the dissipating stage you
would find just weak downodrafts throughout the interior of the cloud.
Note how the winds from one thunderstorm can cause a region of
convergence on one side of the original storm and can lead to the
development of new storms. Preexisting winds refers to winds that
were blowing before the thunderstorm formed.
We have talked about most of the features in the top picture
before. The dust storms that thunderstorms stir up can cause a
sudden drop in visibility and are a serious risk to automobile traffic
on the interstate highway.
A narrow intense downdraft is called a microburst. At the ground
microburst winds will sometimes reach 100 MPH (over a limited area);
most tornadoes have winds of 100
MPH or less. Microburst winds can damage homes, uproot trees, and
will blow over a line of electric power poles at some point during our
summer thunderstorm season. Microbursts are
a serious threat to aircraft especially when they are close to the
ground during landing or takeoff (see Fig. 10.10 in the text).
Falling rain could warn of a (wet)
microburst.
In other cases, dangerous (dry) microburst winds might be invisible
(the virga, evaporating rain, will cool the air, make the air more
dense, and the downdraft winds will strengthen).
A simple demonstration gives you an idea of what a microburst
might
look like. A large plastic tank was filled with water, the water
represents air in the atmosphere. Then some colored liquid that
is a little denser than water is poured into the tank. This
represents the cold dense air in a thunderstorm downdraft. The
colored liquid sinks to the bottom of the tank and then spreads out
horizontally. In the atmosphere the cold downdraft air hits the
ground and spreads out horizontally. These are the strong
microburst winds that can reach 100 MPH.
The demonstration was followed with a short time lapse video showing a
microburst that occured over the Santa Catalina mountains. Cold
air and rain suddenly fell out of a thunderstorm sank to the ground and
then spread out sideways. The surface winds could well have been
strong enough to blow down a tree or two.
Note how the winds are increasing in speed with increasing
altitude. This is vertical wind shear (changing wind direction
with altitude is also wind shear).
The thunderstorm is moving more quickly to the right than the air
next to the ground. The thunderstorm will move to the right more
rapidly than the air in the thunderstorm updraft which originates at
the ground. Rising air that is situated at the front bottom edge
of the thunderstorm will find itself at the back edge of the storm when
it reaches the top of the cloud. This produces a tilted
updraft.
Remember that an ordinary air mass thunderstorm will begin to dissipate
when the downdraft grows horizontally and cuts off the updraft.
In a severe storm the updraft is continually
moving to the right and staying out of the downdraft's way.
Severe thunderstorms can get bigger, stronger, and last longer than
ordinary air mass thunderstorms. The strong updraft winds can
keep hailstones in the cloud longer which will allow them to grow
larger.
We will find that sometimes the tilted updraft will begin to
rotate. A thunderstorm with a rotating updraft is capable of
producing tornadoes.
This gives some general information about tornadoes.
The United States has more tornadoes than any other country in the
world. This is shown in the figure below.
The central US has just the right mix of
meteorological
conditions. In the spring cold air collides with warm moist air
from the Gulf of Mexico to make strong thunderstorms.
Most
tornadoes last only a few minutes and leave a path a few miles long on
the ground. There are of course exceptions. One is
discussed below.
The path of the 1925 "Tri-State Tornado" is shown
above. The
tornado path was 219 miles long, the tornado last about 3.5 hours and
killed 695 people. It is the deadliest single tornado ever.
Most tornadoes move from the SW toward the NE (the motion of
thunderstorms forming along a SW to NE oriented cold front). This
shown clearly in the figure below.
Tornadoes often occur in "outbreaks." Paths of 148
tornadoes
during the April 3-4, 1974 "Jumbo Tornado
Outbreak" are shown above. Note the first tornadoes were located
in the upper left corner of the map. The tornadoes were produced
by thunderstorms forming along a cold front. During this two day
period the front moved from the NW part toward the SE part of the
figure. Note that all the tornado paths have a SE toward NE
orientation.
We'll spend the entire period next Monday on tornadoes.