Monday, November 18
We'll finish up the section on surface and upper
level winds in the Northern and Southern Hemisphere using page 129 and page 130. Then we'll
start the section on thunderstorms which includes page 151a, page 152a, page 152b, page 153, page 154, page 155, and page 156.
Summary & review
Here's a quick review of much of what we have
covered about upper level winds
Winds
spin counterclockwise around L pressure in the
northern hemisphere then switch direction and spin
clockwise around L pressure in the southern
hemisphere. I think by just remembering a
couple of things you can figure this out rather than
just trying to memorize it.
The pressure
gradient will start stationary air moving toward low pressure
(just like a rock placed on a slope will start to move
downhill)
The PGF can start
stationary air moving. The PGF always points
toward low pressure, so the direction of the
initial motion will always be toward low pressure.
The dots in the figure above show this initial motion
and its in toward the center of the picture.
These must both be centers of Low pressure.
Once the air starts moving the wind will turn to the
right or left depending on the hemisphere. This
is the effect of the Coriolis force, the CF
turns wind to the right in the northern hemisphere
and to the left in the southern hemisphere
(remember to always look down stream).
The
northern hemisphere winds are shown at left in the
figure above, the southern hemisphere winds are
shown at right. The inward pointing force is
always stronger than the outward force so that there
is a net inward pointing force.
This initial motion is outward away from the center in the
two figures below.
The outward moving air takes a right turn in the left
figure above, a left turn in the right figure (you may need to
rotate the picture so that you are looking downstream, in the
direction the wind is blowing to clearly see the left hand
turn).
Frictional force
Next we'll try to understand why friction causes surface winds
to blow across the contour lines (always toward low pressure).
With surface winds we need to take into account the PGF, the CF, and
the frictional force (F). That means we'll need some rules
for the direction and strength of the frictional force.
Friction arises with surface winds because the air is blowing
across (rubbing against) the earth's surface.
You're probably somewhat familiar with the effects of
friction. If you stop pedaling your bicycle on a flat road
you will slow down and eventually come to a stop due to air
friction and friction between the tires and road surface.
Friction always acts to slow a moving object it must point in a
direction opposite the motion.
The strength of the frictional force depends on wind speed.
The faster you try to go the harder it becomes because of
increased wind resistance. It's harder to ride on a rough
road than on a smooth road surface. In the case of air there
is less friction when wind blows over the ocean than when the air
blows over land. If the wind isn't blowing there isn't any
friction at all.
Step #9 The Frictional force causes surface
winds to blow across the contours (always toward Low pressure)
The top figure (page
129 in the ClassNotes) shows upper level winds
blowing parallel to straight contours. The PGF and CF
point in opposite directions and have the same strength (the
fact that there are only two forces present tells you these
are upper level winds). Note the CF is to the right of
the wind, this is a northern hemisphere case. The total
force, the net force, is zero. The winds would blow in a
straight line at constant speed.
We add friction in the second picture. It points in a
direction opposite the wind and acts to slow the wind
down.
Slowing the wind weakens the CF and it can no longer balance
the PGF (3rd figure). The stronger PGF causes the wind
to turn and start to blow across the contours toward
Low. This is shown in the 4th figure.
Step #10 - Surface winds blowing around H & L
pressure in the N. & S. hemispheres.
I think you'll be surprised at how easy it
is to determine whether each of the figures below (p. 129 in
the ClassNotes) is a surface center of H or L pressure,
found in the N or S hemisphere, and whether rising
or sinking air motions/clear or cloudy skies would be
associated with each figure.
Key point to remember:
surface winds blow across the contours always toward
low pressure.
It should be very easy to figure
out which two of the figures above are surface centers of low
and high pressure.
Next to determine whether each figure is in the northern
or southern hemisphere we will imagine approaching the upper left
figure in an automobile. We'll imagine it's a traffic circle
and the arrows represent cars instead of wind.
You're approaching the traffic
circle, what direction would you need to turn in order to
merge with the other cars. In this case it's
left. That left turn is the Coriolis force at work
and tells you this is a southern hemisphere map.
The remaining examples are shown below
Converging winds cause air to rise. Rising air expands and
cools and can cause clouds to form. Clouds and stormy
weather are associated with surface low pressure in both
hemispheres. Diverging winds created sinking wind motions
and result in clear skies.
Somethings change when you move form the northern to the
southern hemisphere (direction of the spinning winds).
Somethings stay the same (winds spiral inward around centers of low
pressure in both hemispheres, rising air motions are found with
centers of low pressure in both hemispheres).
Thunderstorms pt. 1 - Introduction
Thunderstorms come in different sizes and levels of
severity. We will mostly be concerned with ordinary
single-cell thunderstorms (also referred to as air mass
thunderstorms). They form in the middle of warm moist air,
away from fronts. Most summer thunderstorms in Tucson are
this type. An air mass thunderstorm has a vertical
updraft. A cell is just a term that means a single
thunderstorm "unit" (a storm with an updraft and a downdraft).
In class I'll show a particularly nice image of a single cell
thunderstorm from a gallery of storm images (http://www.mikeolbinski.com/storms/)
that were taken by Mike Olbinski (it's 11 rows down in the
collection of images at the bottom of his home page).
Tilted updrafts are found in severe and supercell
thunderstorms. As we shall see this allows those storms to
get bigger, stronger, and last longer. The
tilted updraft will sometimes begin to rotate. We'll see
this produces an interesting cloud feature called a wall cloud and
sometimes tornadoes. The fact that lightning is
X'd out does not mean that severe thunderstorms don't produce
lightning, they do. It's just that lightning by itself does
not indicate a severe thunderstorm.
Supercell thunderstorms have a complex internal structure;
I'll try to show a short video at some point that shows a computer
simulation of the complex air motions inside a supercell
thunderstorm. The 1st and 5th images
in Olbinski's gallery show the base of a supercell thunderstorms
photographed in Texas with wall clouds (in the 5th image, air from
the downdraft is being sucked back up by the updraft).
There are additional images further down in the gallery.
We won't spend anytime discussing mesoscale convective systems
except to say that they are a much larger and longer lasting storm
system. They can cover a large portion of a state.
They move slowly and often thunderstorm activity can persist for
much of a day. Occasionally in the summer in Tucson we'll
have activity that lasts throughout the night. This is often
caused by an MCS.
The buildup to an air mass thunderstorm
The following somewhat tedious material is intended to
prepare you to better appreciate a time lapse video movie of a
thunderstorm developing over the Catalina mountains. The
newest 1S1P/Optional Assignment will make use of the same 10
C/km and 6 C/km rates of cooling for rising parcels of
unsaturated and saturated air.
\
Refer back and forth between the lettered points in the
figure above and the commentary below.
The numbers in Column
A show the temperature of the air in the atmosphere
at various altitudes above the ground (note the altitude scale on
the right edge of the figure). On this particular day the
air temperature was decreasing at a steady rate of 8 C per
kilometer. This rate of decrease is referred to as the
environmental lapse rate (lapse rate just means rate of decrease
with altitude). Temperature could decrease more quickly than
shown here (thunderstorms would be more likely in that case) or
less rapidly (thunderstorms would be less likely).
Temperature in the atmosphere can even increase with increasing
altitude (a temperature inversion).
At Point B,
some of the air at the ground is put into an imaginary container,
a parcel so that we can keep track of it. Then a
meteorological process of some kind lifts the air to 1 km altitude
(in Arizona in the summer, sunlight heats the ground and air in
contact with the ground, the warm air becomes buoyant - that's
called free convection). The rising air will expand and cool
as it is rising. Unsaturated air (RH
is less than 100%) cools at a rate of
10 C per kilometer. So the 15 C surface air will have
a temperature of 5 C once it arrives at 1 km altitude.
Early in the day, Mother Nature is only able to lift the air from
the ground to 1 km altitude (she'll lift it higher and higher
later in the day). At Point C note that the air inside the
parcel is slightly colder than the air outside (5 C inside versus
7 C outside). The air inside the parcel will be denser than
the air outside and the parcel will sink back to the ground.
You wouldn't be able to see this because the air is clear,
invisible.
By 10:30 am the parcel is being lifted to 2 km as shown at Point D.
It is still cooling 10 C for every kilometer of altitude
gain. At 2 km, at Point E the
air has cooled to its dew point temperature, the relative humidity
is now 100%, and a cloud has formed. This level is
referred to as the condensation level. A dew point
temperature of -5 C was used in this example. It could be
warmer or colder than that.
Notice at Point F,
the air in the parcel or in the cloud (-5 C) is still colder and
denser than the surrounding air (-1 C), so the air will sink back
to the ground and the cloud will disappear. Still no
thunderstorm at this point.
At noon, the air is lifted to 3 km. Because the air
became saturated at 2 km, it will cool at a different rate between
2 and 3 kilometers altitude. Saturated air cools at a
rate of 6 C/km instead of 10 C/km. The saturated air
cools more slowly because release of latent heat during
condensation offsets some of the cooling due to expansion.
The air that arrives at 3km, Point H, is again still colder than the
surrounding air and will sink back down to the surface.
By 1:30 pm the air is getting high enough that it has become
neutrally buoyant, it has the same temperature and density as the
air around it (-17 C inside and -17 C outside). This is
called the level of free convection, Point J in
the figure.
If you can, somehow or another, lift air above the level
of free convection it will find itself warmer and less dense than
the surrounding air as shown at Point K and will float
upward to the top of the troposphere on its own, it doesn't need
Mother Nature's help anymore. This is really the beginning
of a thunderstorm. The thunderstorm will grow
upward until it reaches very stable air at the bottom of the
stratosphere the rising air will quickly become colder and denser
than the surrounding air if it travels into the stratosphere).
Here's a time
lapse video showing a day's worth of work leading eventually
to the development of a thunderstorm over the Catalina mountains
north of Tucson. Firefox seems to have trouble playing the
file, it usually works fine with Chrome (though that wasn't the
case in class today).
Air mass thunderstorm life cycle
The events leading up to the initiation of a summer air mass
thunderstorm are summarized in the figure below (page 151 in the
ClassNotes). It
takes some effort and often a good part of the day before a
thunderstorm forms. The air must be lifted to just above
the level of free convection (the dotted line at middle left in
the picture). Once air is lifted above the level of free
convection it finds itself warmer and less dense that the air
around it and floats upward on its own. I've tried to show this with colors below.
Cool colors below the level of free convection because the air
in the lifted parcel is colder and denser than its
surroundings. Warm colors above the dotted line indicate
parcel air that is warmer and less dense than the
surroundings. Once the parcel is lifted above the level
of free convection it becomes buoyant; this is the
moment at which the air mass thunderstorm begins.
Once an air mass
thunderstorm gets above the level of free convection it goes
through a 3-stage life cycle
In the first stage you would only find updrafts inside the
cloud (that's all you need to know about this stage, you don't
even need to remember the name of the stage).
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 denser).
The
thunderstorm is strongest in the mature stage. This
is when the heaviest rain, hail, strongest surface winds, and
most of the lightning occur.
Eventually the downdraft spreads
horizontally throughout the inside of the cloud and begins to
interfere with and get in the way of the updraft. This
marks the beginning of the end for this thunderstorm.
The downdraft eventually fills the
interior of the cloud. In this dissipating stage you would
only find weak downdrafts
throughout 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. Convergence
between the preexisting winds and the thunderstorm downdraft
winds creates rising air that can initiate a new thunderstorm.
The photograph below (taken near Swifts Creek, Victoria
(Australia) by fir0002 | flagstaffotos.com.au
) shows a mature thunderstorm with an anvil in the
background. New thunderstorms are trying to develop in
the foreground (this photo was in a Wikipedia
article about thunderstorms)
Here's a sketch of 4 thunderstorm clouds, what information
could you add to each picture.
Click here to
see the answer
Gust fronts and the dust storms (haboobs) they can
produce
The picture below shows some of the features at
the base of a thunderstorm.
The cold downdraft air spilling out of a thunderstorm hits the
ground and begins to move outward from underneath
the thunderstorm. The leading edge of this outward moving
air is called a gust front. You can think of it as
a dust front because the gust front winds often stir up
a lot of dust here in the desert southwest (see below).
There are several very nice examples in Mike Olbinski's Storm
Gallery (http://www.mikeolbinski.com/storms/).
The gust front in this picture
(taken near Winslow,
Az) is moving from the right to the left.
Visibility in the dust cloud can drop to near zero which
makes this a serious
hazard (link was corrected after
class) to automobile traffic. Dust storms
like this are sometimes called "haboobs".
There's lots of dust storm video on YouTube. Here's an example
from Gilbert Arizona (July 5, 2011). You
can
see day literally turn to night when the dust cloud is
overhead (start at about 1:20). Here's another video of the
same storm
from a different location (South Mountain). A
July 2018 dust storm was captured on video by storm chaser
Reed Timmer. Time lapse video (00:30 to about 00:50 near the
start of the video) shows the approaching dust cloud. Notice
the cloud forming near the top of the gust front. The storm
chasers drive into the dust cloud at about 2:20 and you can see
how much the visibility can drop. There are also several
time lapse videos on Mike Olbinski's Time Lapse Gallery page (http://www.mikeolbinski.com/timelapse/#top).
Here's a video
from a summer 2012 dust storm captured from the front window
of a vehicle that drove through the storm. Check the last
minute or two of the video where visibility drops to near zero
(about 9:00 minutes into the video). Officials recommend
that you drive off the highway under conditions like this, turn
off your lights, and take your foot off the brake so that your
brake lights are not on (otherwise someone might follow your
lights thinking you're still on the highway and run into you from
behind).
Microbursts
Thunderstorm downdraft winds can be a serious hazard whether they
stir up dust or not.
A narrow intense
thunderstorm downdraft is called a microburst. At the
ground microburst winds will sometimes reach 100 MPH or more
(over a limited area). As we will see most tornadoes have
winds of 100 MPH or less. Microburst winds can damage
homes (especially mobile homes that aren't tied to the ground),
uproot trees, and seem to blow over a line of electric power
poles at some point every summer in Tucson (with the downed power lines
remaining energized). Wind damage from a microburst is
often incorrectly attributed to a tornado.
Microbursts
are a serious threat to aircraft especially when they are close
to the ground during landing or takeoff. Microburst
associated wind shear was largely responsible for the crash of Delta
Airlines
Flight 191 while landing at the Dallas Fort Worth
airport on Aug. 2, 1985 (caution some of the links at the end of
the article contain audio of actual cockpit communications).
Falling rain could warn of a
wet microburst (see photo below). In other cases, dangerous dry microburst winds might be
invisible (the virga, evaporating
rain, will cool the air, make the air more dense, and strengthen
the downdraft winds).
Here are a couple of microburst
videos. The first video
was taken in the heavy rain and strong winds under a
thunderstorm in the microburst. You'll see a power pole
snapped in half by the microburst winds at about 2:26 into the
video. We'll look at portions of a 2nd longer
video in class. It was taken in or near San Tan,
Arizona. The microburst doesn't look too impressive at the
start of the footage but the storm winds soon get pretty violent
(at about the 3:15 point in the video) and winds were strong
enough to break the trunks of at least one tree (6:45 into the
video). It is probably more common to see recently planted
trees uprooted.
