Tuesday Nov. 26, 2013
A mix of music this morning: it started with Mikis Theodorakis'
"Zorba the
Greek", that was followed by Dick Dale and "Misirlou",
Mark Howard, Stuart Duncan, & Sam Bush with "Orange Blossom
Special", and finally "Big Noise from
Winnetka" by the Dukes of Dixieland.
The last of the Optional Assignments and the revised Expt. #3
reports were collected today. I'll try to have all of this
graded by next week.
Lightning kills just under 100 people every year in the United
States (more than tornadoes or hurricanes but less than flooding,
summer heat and winter cold) and is the cause of about 30% of all
power outages. In the western United States, lightning
starts about half of all forest fires. Lightning caused
fires are a particular problem at the beginning of the
thunderstorm season in Arizona. At this time the air
underneath thunderstorms is still relatively dry. Rain
falling from a thunderstorm will often evaporate before reaching
the ground. Lightning then strikes dry ground, starts a
fire, and there isn't any rain to put out or at least slow the
spread of the fire. This is so called dry lightning.
Strong downdraft winds from the thunderstorm can help the fire
grow and spread.
We'll be concerned with the lightning produced by
thunderstorms but it has also be observed in dust storms and
volcanic eruptions such as in these other worldly pictures of the
2010 eruption of Eyjafjallajokull
in Iceland.
A typical summer thunderstorm
in Tucson is shown in the figure above (p. 165 in the
photocopied ClassNotes). Even on the hottest day in
Tucson in the summer a large part of the middle of the cloud
is found at below freezing temperatures and contains a mixture
of super cooled water droplets and ice crystals. This is
where precipitation forms and is also where electrical charge
is created. Doesn't it seem a little unusual that
electricity can be created in such cold and wet conditions?
Collisions between precipitation particles
produce the electrical charge needed for lightning. When
temperatures are colder than -15 C (above the dotted line in
the figure above), graupel becomes negatively charged after
colliding with a snow crystal. The snow crystal is
positively charged and is carried up toward the top of the
cloud by the updraft winds. At temperature warmer than
-15 (but still below freezing), the polarities are
reversed. A large volume of positive charge builds up in
the top of the thunderstorm. A layer of negative charge
accumulates in the middle of the cloud. Some smaller
volumes of positive charge are found below the layer of
negative charge. Positive charge also builds up in the
ground under the thunderstorm (it is drawn there by the large
layer of negative charge in the cloud).
Air is normally an insulator, but when the electrical
attractive forces between these charge centers gets high enough
lightning occurs. Most lightning (2/3 rds) stays inside the
cloud and travels between the main positive charge center near the
top of the cloud and a large layer of negative charge in the
middle of the cloud; this is intracloud lightning (Pt. 1).
About 1/3 rd of all lightning flashes strike the ground.
These are called cloud-to-ground discharges (actually negative
cloud-to-ground lightning). We'll spend most of the class
learning about this particular type of lightning (Pt. 2)
A couple of interesting things can
happen at the ground under a thunderstorm. Attraction
between positive charge in the ground and the layer of
negative charge in the cloud can become strong enough that a
person's hair will literally stand on end (see two photos
below). This is incidentally a very dangerous situation
to be in; I wouldn't wait around for my picture to be taken.
St.
Elmo's Fire (corona discharge) is a faint electrical
discharge that sometimes develops at the tops of elevated objects
during thunderstorms. The link will take you to a site that shows
corona discharge. Have a look at the first 3 pictures, they
probably resemble St. Elmo's fire. The remaining pictures
are probably different phenomena. St. Elmo's fire was first
observed coming from the tall masts of sailing ships at sea (St.
Elmo is the patron saint of sailors).
Positive polarity cloud to ground lightning (Pt. 3) accounts
for a few percent of lightning discharges. Upward lightning
is the rarest form of lightning (Pt. 4). We'll look at both
of these unusual types of lightning later in the lecture.
Most cloud to ground discharges begin with a negatively-charged
downward-moving stepped leader (the figure above is on p. 166 in
the ClassNotes). A developing channel makes its way down
toward the cloud in 50 m jumps that occur every 50 millionths of a
second or so. Every jump produces a short flash of light
(think of a strobe light dropped from an airplane that flashes on
and off as it falls toward the ground). The sketch below
shows what you'd see if you were able to photograph the stepped
leader on moving film. Every 50 microseconds or so you'd get
a new picture of a slightly longer channel displaced slightly on
the film (the highlighted segments would be photographed).
Here's an actual slow motion
movie of a stepped leader. The video camera
collected 7207 images per second ( a normal video camera takes
30 images per second). The images were then replayed at
a slower rate. A phenomenon that takes a fews tens of
milliseconds to occur is spread it out over a longer period of
time so that you can see it.
As the leader channel
approaches the ground strong electrical attraction develops
between negative charge in the leader channel and positive
charge on the surface of the ground. Several
positively charged sparks develop and move upward toward the
stepped leader. One of these will intercept the stepped
leader and close the connection between negative charge in the
cloud and positive charge on the ground.
Here's a sketch of one of the best photographs ever
taken of an upward connecting discharge.
You can see the actual photograph on the photographers
homepage. There were at least 3 upward
discharges initiated by the approach of the stepped leader
(1, 2, and 3 in the sketch). Streamer 1 connected to
the bottom of the stepped leader. It isn't clear where
the exact junction point was. The downward branching
at Point 4 indicates that was part of the descending stepped
leader. A very faint upward discharge can be seen at
Point 3. Here's another more recent
photograph (click on Galleries on the bar near the top
of the page, then click on Lightning Gallery 1).
Lightning rods (invented by Benjamin Franklin)
make use of the upward connecting discharge.
Houses with and without lightning
rods are shown above. When lightning strikes
the house without a lightning rod at left the
powerful return stroke travels into the house
destroying the TV and possibly starting the house on
fire. With a
lightning rod, an upward discharge will intercept
the stepped leader and safely carry the lightning
current through a thick wire around the house and
into the ground. Lightning rods do work and
they have changed little since their initial
development in the 1700s. Most of the newer
buildings on campus are protected with lightning
rods.
The connection between the stepped leader and
the upward discharge creates a "short circuit" between
the charge in the cloud and the charge in the ground.
A powerful
current travels back up the channel from the ground
toward the cloud. This is the 1st return
stroke. Large currents (typically 30,000
amps in this 1st return stroke) heat the air to
around 30,000K (5 times hotter than the surface of
the sun) which causes the air to explode. When
you hear thunder, you are hearing the sound produced
by this explosion.
The figure below shows what we've
covered so far in simplified form
A downward negatively charged
stepped leader followed by a positively charged upward moving
return stroke.
Many cloud-to-ground flashes end at this point. In
about 50% of cloud to ground discharges, the stepped
leader-upward discharge-return stroke sequence repeats itself
(multiple times) with a few subtle differences.
A downward dart leader travels
from the cloud to the ground. The dart leader doesn't step but
travels smoothly and follows the channel created by the
stepped leader (avoiding the branches). It is followed
by a slightly less powerful subsequent return stroke that
travels back up the channel to the cloud. This second
stroke might be followed by a third, a fourth, and so on.
Here's a stepped leader-upward connecting
discharge-return stroke animation
(you'll see the stepped leader, upward discharges, and the
first return stroke. Two additional subsequent
strokes are shown without the dart leader).
The sketch above and the photo below show a multiple stroke
flash consisting of 4 separate return strokes. There is enough
time between separate return strokes (around 1/20th to 1/10th of
a second) that your eye can separate the individual flashes of
light.
Here are some unusual types of lightning.
We've been looking at strikes
that originate in the negative charge center is a thunderstorm
(discharge at left in figure above). Occasionally
a lightning stroke will travel from the positive charge region
in the top of the thunderstorm cloud to ground (shown at right
in the figure above). These types of strikes are more
common at the ends of storms and in winter storms. This is
probably because the top part of the cloud gets pushed sideways
away from the middle and bottom portions of the cloud.
Positive strokes are very powerful. They sometimes produce
an unusually loud and long lasting clap of thunder.
Here's an even rarer form of lightning.
Lightning sometimes starts at the ground and travels
upward. Upward lightning is generally only initiated by
mountains and tall objects such as a skyscraper or a tower of
some kind (the Empire State Building is struck many times every
year but lightning and usually it's lightning that the building
itself caused).
Note the discharge is different in another way also.
These discharges are initiated by an upward leader. This
is not followed by a return stroke, like you might expect, but
by a more normal downward leader. Once the 2nd leader
reaches the ground, an upward return stroke travels back up the
channel to the cloud.
The fact that lightning could
begin with an upward discharge that begins at the ground led
(French) scientists to develop a technique to trigger lightning
by firing a small rocket up toward a thunderstorm. The
rocket is connected by a thin wire to the ground. When the
rocket gets 50 to 100 m above the ground an upward streamer will
develop off of the top of the wire. Once the streamer
reaches the cloud it can initiate a "normal" series of downward
dart leaders and upward subsequent return strokes.
Scientists are able to take closeup photographs and make
measurements of lightning currents using triggered
lightning. Triggered lightning can also be used to test the
operation of lightning protection devices.
One of the class TAs, Daile ZHANG, took over at this point.
She is a PhD student in the Atmospheric Sciences Department and
lightning is her specialty.
Here's a link
to the video that she showed in class.
The abbreviation NLDN that you'll see at the start of the video
stands for National
Lightning Detection Network. The headquarters of this
company are hear in Tucson.
In the first 1:30 of the video you'll see natural lightning
occurring in the Tucson area during the summer (both intracloud
and cloud to ground discharges).
Between 1:30 and about 2:00 you'll see lightning activity
photographed at the Grand Canyon.
Next, between about 2:00 and 2:40 photographs of lightning
striking large wind turbines in Kansas. A lightning strike
to one of the turbine blades can cause damage that is very
expensive to repair. At 2:16 and again at about 2:24 you'll
see very bright lightning flashes that momentarily overexpose the
video. These are probably positive cloud to ground
discharges. And look carefully at the discharge that occurs
between about 2:28 and 2:31 on the video. Notice the upward
pointing branching. This was an upward discharge initiated
by one of the wind turbines.
The remainder of the video shows rocket triggered
lightning. These experiments were done at the International
Center for Lightning Research and Testing (ICLRT) run by the
University of Florida near Gainesville, FL.
Ms Zhang asked a question, during the video, about what causes the
green color that is sometimes seen in photographs of triggered
lightning. The answer was vaporization of the copper wire
that is carried upward by the rocket. If you're someone that
enjoys watching lightning storms you may have seen a green glow
when lightning strikes the ground. This is often produced by
an exploding transformer on an electric power pole. The
copper wire in the transformer is vaporized by the lightning.
The vaporization of different chemical compounds is what gives
fireworks there distinctive colors. This
link lists some of the chemical compounds and the colors
they produce.
We didn't have time to cover the
following material in class but I'm including it
just so that everything will be together in one place.
When lightning strikes the
ground it will often melt the soil (especially sandy soil) and
leave behind a rootlike structure called a fulgurite. A
fulgurite is just a narrow (1/2 to 1 inch across) segment of
melted sand (glass).
Lightning is a serious weather hazard. Here are some
lightning safety rules that you should keep in mind during
thundery weather.
Stay
away
from
tall
isolated
objects
during
a
lightning
storm.
You
can
be
hurt
or
killed
just
by
being
close
to
a
lightning
strike
even
if
you're
not
struck
directly.
Lightning currents often travel outward along
the surface of the ground (or in water) rather
than going straight down into the ground.
Just being close to something struck by
lightning puts you at risk.
An
automobile
with
a
metal
roof
and
body
provides
good
protection
from
lightning.
Many
people think this is because the tires insulate the
car from the ground. But the real reason cars
are safe is that the
lightning
current
will
travel
through
the
metal
and
around
the
passengers
inside. The rubber tires really don't play any
role at all. The people in Florida in the
video that were triggering lightning with rockets
were inside a metal trailer and were perfectly
safe. All of the connections made to equipment
outside the trailer were done using fiber optics,
there were no metal wires entering or leaving the
trailer.
You shouldn't use a corded phone or
electrical appliances during a lightning storm because
lightning currents can follow wires into your
home. Cordless phones and cell phones are
safe. It is also a good idea to stay away from
plumbing as much as possible (don't take a shower during
a lightning storm, for example). Vent pipes that
are connected to the plumbing go up to the roof of the
house which puts them in a perfect location to be
struck.
To estimate the distance to a lightning strike
count the number of seconds between the flash of light and
when you first hear the thunder. Divide this by 5 to get
the distance in miles.
For example, a delay of 15 seconds between the flash of light
and the sound of thunder would mean the discharge was 3 miles
away. Research studies have shown that about 95% of cloud to
ground discharges strike the ground within 5 miles of a point
directly below the center of the storm. That's a 10 mile
diameter circle and covers the area of a medium size city.
The latest lightning safety recommendation is
the 30/30 Rule.
The 30/30 rule
People should seek shelter if the delay between a lightning
flash and its thunder is 30
seconds or less.
People should remain under cover until 30 minutes after the final
clap of thunder.