Geography 140
Introduction to Physical Geography
Lecture: Storms as an Element of Weather
V. Storms are the last of the weather elements and, in a manner of speaking,
they are really nothing more than particular configurations of the other
three elements. Because of their importance, people have always been
trying to predict them and understand them. A big breakthrough toward
that goal was made in WWI by Jacob Bjerknes and his colleagues in Norway.
They developed something called "Air Mass Analysis."
A. Air masses
1. These are defined as bodies of air with uniform temperature
structures and humidity characteristics.
2. These properties derive from their "source regions," The source
regions yield a two way system of simple air mass analysis:
a. The first dimension of the classification system is the latitude
zone of source region, which governs the temperature
characterisitics of an air mass. There are four of these zones
in the Bjernkes system:
i. Equatorial (E)
ii. Tropical (T)
iii. Polar (P)
iv. Arctic or Antarctic (A or AA)
![[ Bjerknes' air mass map ]](https://home.csulb.edu/~rodrigue/geog140/airmassbjerknes.jpg)
b. The underlying surface of the source region also affects the
characteristics of the resulting air masses:
i. Maritime or oceanic surfaces (m), which create relatively
humid air masses
ii. Continental or land surfaces (c), which create relatively
dry air masses
c. So, in Bjerknes' system, there are six basic air mass types:
i. Arctic or Antarctic air (abbreviated A or AA):
a. Extremely cold
b. Very dry (because of the extreme cold)
ii. Polar continental air (abbreviated cP or Pc):
a. Very cold, having developed over northern Canada and
Russia
b. Very dry, partly because of the cold and partly from
having developed over land.
c. Yes, the terminology is a little confusing: "Polar" air
isn't the air mass that develops over the poles, because
THAT air is called Arctic or Antarctic....
d. Somehow I think of a Lincoln in Ottawa .....
iii. Polar maritime air (abbreviated mP or Pm):
a. Very cool because of the high latitude, but not as cold
as polar continental air because of the moderating
influence of the sea and the warm currents at these
latitudes (the North Pacific Drift that continues the
Japan or Kuroshio Current and the North Atlantic Drift
that continues the Gulf Stream)
b. Rather dry because of the coolness but not as dry as the
polar continental air because of evaporation from the
drifts
iv. Tropical continental (a Lincoln in Dallas? abbreviated cT
or Tc):
a. Warm because of the lower latitude
b. Dry because it formed over land
v. Tropical maritime (abbreviated mT or Tm):
a. Very warm because of the lower latitude
b. Very humid because of the tropical waters below it
vi. Equatorial (abbreviated E)
a. Hot!
b. Extremely humid
c. Continental is not differentiated from maritime here
because there's relatively little land along the equator
and much of that land is covered with tropical
rainforest, which evapotranspires as much water vapor
into the air as an equivalent area of ocean water.
3. Air masses usually have sharp boundaries between themselves. Air
masses of different characteristics, therefore, tend not to mix.
This boundary is called a "front" (the rather militaristic
vocabulary here may have had something to do with the pressure of
circumstances!).
B. Fronts
1. It is the fronts between air masses that create storms. This
happens when one mass pushes into another along their front and one
is forced upwards. This forced uplift produces adiabatic cooling
and so precipitation and storms.
2. There are three basic types of front in air mass analysis:
a. A cold front, where the invading air mass is colder and,
therefore, denser than the one it invades.
i. This produces fast uplift.
ii. Uplift produces a "squall line" of cumuliform clouds.
iii. Precipitation, then, is intense, due to the sharp uplift --
and it can fall on both sides of the front's contact with
the ground.
iv. Because cold air is denser than the warm air it is
invading, it can push the more buoyant warm air out of the
way very fast. Cold fronts are, therefore, fast moving.
b. A warm front, in which the invading air mass is warmer and,
therefore, lighter than the one it's invading.
i. This produces a slow, low angle, gradual uplift.
ii. This kind of gentle uplift creates stratiform clouds
blanketing huge areas. At the forward edge, there are
cirrus clouds of various types, which are replaced by alto-
stratus, stratus, and finally nimbo-stratus clouds. Now,
you can see why the appearance of certain kinds of clouds
is associated with the advent of stormy weather.
iii. These stratiform clouds yield less intense precipitation in
a large area ahead of the front's contact with the ground.
iv. Because warm air, particularly warm humid air, is lighter
than the air it's moving into, it meets strong resistance.
Warm fronts are, accordingly, slow moving.
v. Warm fronts are found east of cold fronts.
![[ cold front and warm front, Jim Pejsa, 1999 ]](http://physics.uwstout.edu/WX/u8/U8_12c.gif)
c. An occluded front occurs when a fast moving cold front to the
west catches up with a slower moving warm front to the east.
i. The cold air behind the cold front links up with the cold
air ahead of the warm front.
ii. The warm air behind the warm front is caught in between
and, being more buoyant, is forced up off the ground
completely. You can't perceive it from the ground anymore:
It is hidden (hence the name, "occluded," for "hidden").
iii. This sudden uplift leads to intense rain.
iv. But then the warm air cools adiabatically and eventually
stabilizes, stops rising, and so the storm dies.
C. There are different types of storm. The variations are related to
latitude, as well as severity. At first, it was thought that the
mechanisms of air mass analysis could explain all of them, but it
turned out that there is too little difference between the air masses
that produce tropical and equatorial storms to account for their
nature in air mass analytic terms. So, I'll discuss storm types by
latitude and severity, starting with the mid-latitudes that most
conform to air mass analysis.
1. Mid-latitude traveling cyclones, sometimes called extratropical
cyclones.
a. Mid-latitude wave cyclones.
i. This is far and away the most common type of storm in the
middle latitudes.
ii. It develops along the polar front, which separates polar
air masses (Pc, Pm) from tropical air masses (Tc, Tm)
iii. As such, the polar front corresponds to the subpolar low in
the world pressure and wind system and is, therefore, a
powerful storm generator, because the air masses it
separates are very, very different in their temperature and
humidity characteristics.
iv. It also is associated with the polar jet stream, sometimes
called the polar front jet stream, a high speed westerly
wind in the upper troposphere, which forms where the
decline in thickness of the troposphere is the steepest
(remember? the troposphere is quite thick over the tropics
and thins towards the pole). The path of the jet stream is
unstable, sometimes being a fairly straight zonal west-east
flow and other times having a strong meridional north-south
component to it. This is important as it affects the paths
and the strengths of storms.
v. Mid-latitude traveling cyclones go through four different
stages of development and death.
a. Stage A: Situation normal. All along the polar front
the tropical air masses, propelled by the westerlies,
are confronting the polar air masses, themselves
propelled by the polar easterlies. For a while, this is
an even match along the front, and the polar front forms
a rather straight path from west to east.
b. Stage B: Sooner or later, this front develops bulges
north and south as first one air mass, then the other is
stronger at a given point.
1. This means a warm front sector is formed where warm
air pushes into polar air mass territory and a cold
front sector forms behind it, pushing into tropical
air territory. This, then, is the beginning of a
storm, with its distinctive fronts established.
2. On weather maps, it is conventional to mark the cold
front sector with a thick line with small triangles
on it pointing in the direction of the front's
advance; the warm front is marked by a heavy line
with small semicircles or bumps on it, again pointing
in the direction of the front's advance. The
occluded sector is shown by alternating points and
semicircles, all pointing in one direction. A section
of a front not moving, a stationary front, is also
shown with alterating triangles and semi-circles, but
they point in opposite directions. Precipitation
starts in with the uplift associated with the moving
fronts.
![[ sample weather map, Jim Pejsa, 1999 ]](http://physics.uwstout.edu/WX/u8/U8_11.gif)
3. At this point, the global circulation of Prevailing
Westerlies converging with Polar Easterlies at the
Subpolar Low gives way to the regional circulation of
a storm, with air spiraling into the deepest part of
the cyclone (where the cold front touches the warm
front).
A. In the Northern Hemisphere, the winds in front of
the cold front will be coming from the south or
southwest; after the cold front passes, there will
be a wind shift, with the wind now coming in from
the west or northwest.
B. You can see that in the weathervane symbols used
to show wind direction and strength in the map
above: The reporting station is shown as a circle
(black for cloudy and clear for, well, clear
weather) and the vane points in the direction from
which the wind is blowing. The number of angled
lines indicates wind speed.
C. Anyhow, look at the stations to the east of the
cold front and you'll see the winds are from the
southwest; for those west of the cold front, the
winds are coming from the northwest.
c. Stage C: The cold front sector moves faster, so it
catches up with the warm front sector, attacking it from
behind in a sort of rotational motion, lifting the warm
tropical air mass clear off the ground. This is the
beginning of the occluded stage.
![[ cold front and warm front as map, Jim Pejsa,
1999 ]](http://physics.uwstout.edu/WX/u8/U8_12b.gif)
Here you see the cold and warm fronts, with two
transects, or trips we'll take in cross-section, from E
to A across the American South (the two kinds of front)
and from G to F across the upper Midwest to see the
situation above occlusion.
The cross-section above shows the distinct cold and warm
front sectors west of D and east of C.
![[ later occlusion in cross-section, Jim Pejsa,
1999 ]](http://physics.uwstout.edu/WX/u8/U8_12a.gif)
This cross section shows the area above occlusion, where
the warm air has been snapped up off the ground.
d. Stage D: The chilling of the warm air above the
occlusion, which puts a gradual end to the occluded
front and to the storm itself. The deceleration in
uplift eases up on the lower pressure of the cyclone,
which exerts less influence on regional air flow: This
allows the regional cyclonic circulation to give way to
the global pattern of converging westerlies and
easterlies. The end of occlusion, thus, re-establishes
Stage A, normalcy, until the next ripple comes along.
vi. The whole storm system, as it develops and dies, tracks
generally eastbound, under the path of the polar front jet
stream. The jet stream is affected by the Rossby wave
circulation around the poles, which is not perfectly
circular (west-east). The Rossby circulation is sometimes
nearly zonal (west-east) and other times markedly
meridional (north-south). So, the amplitude of the Rossby
waves shrinks and swells with time in a roughly three to
eight week cycle called the index cycle.
a. Low index means low zonal component
b. High index means high zonal component
c. During low index phases, the jet stream will curve
markedly (and the paths of the storms below it will,
too) and storm severity can be exaggerated as the curves
in the jet stream accentuate the clockwise spiraling of
surface highs and the counterclockwise spiraling of
surface lows in the Northern Hemisphere, as shown in
this graphic from USA Today:
![[ jet stream amplification of surface highs
and lows, Chad Palmer, 1999 ]](http://www.usatoday.com/weather/photos/cyccurv2.gif)
vii. These mid-latitude wave cyclones are the source of
California's winter storms. Such storms are spawned in
great numbers by the Aleutian Low as it and the polar front
are displaced south in winter with the migration of the sun
into the Southern Hemisphere. Remember, the Hawai'ian High
is also displaced south and weakened, taking our protection
from these frontal cyclones away. We have experienced
winters with a markedly low index, which has brought the
jet stream right over California, clobbering us with volley
after volley of mid-latitude traveling cyclones. Other
times, we experience high index, with the jet stream and
its Aleutian storms tracking well north of us and giving us
a dry winter.
b. Tornadoes -- much more "entertaining"
![[ tornado in Dimmitt, Texas, USA Today ]](http://www.usatoday.com/weather/tornado/photos/dimmitt.jpg)
i. Also mid-latitude phenomena, these are the most violent
storms on our planet.
ii. They're an occasional by-product of very fast moving cold
fronts in the mid-latitudes. When these fronts invade very
moist maritime tropical air, the warm, wet air is forced
suddenly aloft. This low is deepened further by the latent
heat suddenly released by the intense condensation of the
wet air racing aloft.
iii. These fronts are moving so fast that they tend to run ahead
of themselves: The winds on the ground are impeded by
friction with the ground, while the air aloft pulls ahead
of the front's contact with the ground. The front in
cross-section would be even blunter than shown in the
cross-section above.
iv. This creates a pipeline of rolling air on the ground, kind
of like that pipeline that surfers love so much just ahead
of a tumbling wave crest, which is sort of what this is.
v. At some point, the horizontally-rolling vortex of air can
detach and tilt upward on one side and permit extremely
fast spiraling airflow up into the cumulo-nimbus cloud
above, which creates extremely low pressure.
Voilà! -- a funnel cloud. In a manner of
speaking, a tornado is sort of like a cumulo-nimbus cloud
on steroids.
vi. As an extremely low pressure cyclone, this rogue cloud
sucks air into it up to 400 km/h!
vii. It's a very narrow storm: 100-500 m.
viii. The tall, narrow vacuum with the twisting funnel that
marks it writhes back and forth, touching ground
frequently. Its dark color is that of sudden condensation
plus all the stuff it yanks up off the ground.
ix. A tornado is immensely destructive. It's so low a cyclone
that it causes houses and buildings in its path to explode
when the vacuum and the extremely high speed winds hit
them. Also, its vortex tends to yank things up: cars,
animals, people, trees, parts of houses -- in addition to
its explosive effect. These flying objects are lethal to
anything they hit at these speeds: Even a piece of straw
becomes a bullet.
x. At heart, a tornado is a cumulo-nimbus cloud gone very,
VERY bad. It exhibits all the damage of a regular cumulo-
nimbus cloud, only much worse: Unbelievably torrential
rain, hail, and lightning (as well as really weird aurora-
like electrical effects: funny glowing colors, some pink,
but mainly puke chartreuse or yellow). I was in Worcester,
Massachusetts, when a tornado came through in 1979. I was
unaware of what it was, because I did not see the funnel
cloud: All I was aware of was a staggering amount of rain
pouring out of a black sky edged with barf-colored green-
yellow clear sky. The noise from lightning and wind was
astonishing, too. It was all over about 15 minutes later.
I didn't realize what it was until I read about some
children in a campground nearby who'd been killed -- by a
tornado. In Massachusetts, of all places.
xi. About their geography: They tend to be largely New World
phenomena (look at the topography of the Western
Hemisphere: The mountains tend north-south, affording no
block between polar and tropical air masses. The Old World
has the Himalayas and Caucasus and Zagros and Taurus and
other east-west ranges to keep Siberian air out of the
tropics).
![[ NOAA map of tornadoes, 1989-1998 ]](http://www.spc.noaa.gov/archive/tornadoes/10yr_avg.gif)
x. Uhhh, check out California. We get more than most
Californians think! We are in denial, folks: People came
here from Kansas to get away from them. When we get them,
the newspapers will call them "freak windstorms,"
"waterspouts" (even up in Pasadena), and the current
favorite, "microbursts." According to Warren Blier of the
Weather Service, our tornado incidence is much higher than
locals perceive, the difference being that we get tornadoes
of smaller intensity than the Midwest does. We get F0s,
F1s, and the rare F2s on the Fujita Scale, while Texas and
Oklahoma have gotten them as high as F5s.
xi. The Fujita Scale (sometimes called the Fujita-Pearson
Scale) is a way of representing the intensity of a tornado,
judging from the specific patterns of damage it does.
a. Weak Scale Class
1. F0 -- Gale tornado -- winds 64-115 km/h (40-72 mph)
-- Some damage to chimneys; breaks branches off
trees; pushes over shallow-rooted trees; damages sign
boards.
2. F1 -- Moderate tornado -- winds 116-179 km/h (73-112
mph) -- Moderate Damage: Surface of rooves peeled
off, mobile homes pushed off foundations or
overturned, outbuildings demolished, moving autos
pushed off the roads, trees snapped or broken;
beginning of hurricane-speed winds.
b. Strong Scale Class
1. F2 -- Significant tornado -- winds 180-251 km/h
(113-157 mph) -- Considerable Damage: Roofs torn off
frame houses, mobile homes demolished, frame houses
with weak foundations lifted and moved, large trees
snapped or uprooted, light-object missiles generated.
2. F3 -- Severe tornado -- winds 252-330 km/h (158-206
mph) -- Severe Damage: Roofs and some walls torn off
well-constructed houses; trains overturned; most
trees in forest uprooted, heavy cars lifted off the
ground and thrown, weak pavement blown off the roads.
c. Violent Scale Class
1. F4 -- Devastating tornado -- winds 331-416 km/h (207-
260 mph) -- Devastating Damage: Well-constructed
houses leveled, structures with weak foundations
blown off the distance, cars thrown and
disintegrated, trees in forest uprooted and carried
some distance away.
2. F5 -- Incredible tornado -- winds 417-509 km/h (261-
318 mph) -- Incredible Damage: Strong frame houses
lifted off foundations and carried considerable
distance to disintegrate, automobile-sized missiles
fly through the air in excess of 300 feet, trees
debarked, incredible phenomena will occur.
3. F6 -- Inconceivable tornado -- 510-606 km/h (319-379
mph) -- These winds are very unlikely. The small area
of damage they might produce would probably not be
recognizable along with the mess produced by F4 and
F5 wind that would surround the F6 winds. Missiles,
such as cars and refrigerators would do serious
secondary damage that could not be directly
identified as F6 damage. If this level is ever
achieved, evidence for it might only be found in some
manner of ground swirl pattern, for it may never be
identifiable through engineering studies.
2. Tropical weather systems aren't too similar to mid latitude storms:
The contrast between air masses (e.g., mT and E) is much less.
They do have tremendous capacities to cause precipitation through
convection, however. Remember, tropical air is very moist. Once
frontal or convectional uplift gets going, the latent heat energy
released by all that condensation accelerates uplift, leading to
intense precipitation.
a. Easterly waves are the most common source of stormy weather in
the tropics.
i. They are slow moving (300-500 km/day) low pressure troughs
in the Trades, about 5-30° N or S.
ii. A trough is an area of low pressure, stretched out along a
line or arc, in this case pointing poleward from the
equatorial area. If you examined the pressure gradient in
this area, you would see that, for the most part, the
isobars trend east-west. If the area of an easterly wave,
however, they bend poleward to create a trough:
![[ easterly wave isobar map, University of East
Anglia ]](http://web.archive.org/web/20030324221854/http://www.uea.ac.uk/~e930/e174/fig/eastwv.gif)
iii. You can understand the "trough" expression by drawing a
straight line east-west right across the middle of the
trough. Mark the places your line crosses isobars and note
the reading there. Copy the line and its tick marks onto
another sheet of paper. Now, construct a Y axis going up
from this line and mark it with isobar readings from, in
this case, 1010 to 1020 mb (or hPa). Above each tick mark
on the X axis, put a dot across from its isobar reading.
Now, connect the dots with a smooth curve and you'll see it
dips in the middle: That dip in your curve is the trough,
or area of low pressure.
iv. Air at the surface will cross the isobars from high
pressure to low pressure, slightly deflected to the right
in the Northern Hemisphere (to the left in the Southern
Hemisphere). This means the wind is bent so that it
converges along the trough and just east of it, which means
it tends to rise there, and it diverges (and sinks) on the
west side and far to the east.
v. So, as the easterly wave approaches, you would first enjoy
clear weather. Then, as the trough passes over you, you'd
experience the scattered showers and thunderstorms
associated with the convergence and uplift of hot, humid
tropical air for a day or so. Then, it would clear with
the return of slightly divergent and subsiding air behind
the easterly wave.
b. Tropical cyclones (aka hurricanes in the Atlantic, typhoons in
the Northwestern Pacific, and cyclones in the Indian Ocean, and,
sometimes, chubascos along the west coast of Mexico)
i. These often form when the low pressure of an easterly wave
deepens (the isobars would become "pointier" in their bend
poleward -- as in the dashed lines on the sketch map
above), as uplift is accelerated by unusual heat release
during condensation.
ii. This becomes a self expanding process, because of the great
amount of water vapor held in mT air over oceans. The
uplift/condensation of that huge amount of water is a
powerful source of heat energy to speed up the uplift and
deepen the low pressure.
iii. A hurricane is seen on weather maps when the isobars keep
bending until they cut themselves off into concentric
circular patterns (instead of wavy zonal patterns) and when
they attain hurricane force winds (118 km/h) -- they're
called "tropical depressions" or "tropical storms" when
they develop the circular isobar pattern but before their
winds hit the requisite speed.
iv. Description:
a. A hurricane is a circular center of very low pressure
(a few have been known to plunge below 920 mb)
b. This profound low draws in high speed winds: 120-200
km/h
c. The uplift of these swirling winds creates heavy rain
(through convergent and convectional uplift).
d. The storm is relatively small, about 150-1000 km in
diameter.
e. Hurricanes usually move about 25-30 km/h, though they
can stall and just sit there, creating devastating
amounts of rain and storm surge. On the other hand,
they have been known to race along at up to 100 km/h!
f. Perhaps the greatest peculiarity is the eye. This is a
small area of absolutely clear weather right in the
center of the storm. The eye is usually about 30-60 km
across and can last an hour or so. The eyes are clearly
visible in the images of the 1992 Hurricane Andrew on
the left (the most expensive hurricane disaster ever to
hit the United States, doing at least $20 billion of
damage) and the 1998 Hurricane Mitch on the right (which
devastated Central America, killing over 10,000 people):
![[ Hurricane Andrew, NOAA, NASA, 1992 ]](http://rsd.gsfc.nasa.gov/rsd/images/andrewOverhead_lg.jpg)
![[ Hurricane Mitch, NOAA, NASA, 1998 ]](http://rsd.gsfc.nasa.gov/rsd/images/Mitch/mitch2_md.jpg)
The eye is produced by subsidence of air at the core of
the spiraling storm. Air races towards the center of a
hurricane, spiraling to form a large vortex in the
center, made up of rapidly uplifted air (and
condensation). This forms the eyewall, the wall of
immense cumulo-nimbus clouds around the eye. Here is an
image of the eyewall seen from just inside the eye:
![[ Katrina Eyewall, NOAA, Dewie Floyd, Flight
Engineer ]](http://www.katrina.noaa.gov/eyewall/images/katrina-eyewall-2-08-28-2005b.jpg)
The wind at the top of the storm spirals outward for the
most part, but some, trying to diverge, manages to head
inward where it converges and most of it sinks. The
subsiding air, of course, brings dry clear weather at
the heart of the storm.
![[ hurricane circulation, NOAA ]](http://web.archive.org/web/20060502074920/http://hurricanes.noaa.gov/prepare/images/structure1.gif)
g. Another distinctive feature is the spiral rainbands that
swirl outward from the eyewall, rather like the arms of
a spiral galaxy. These are made of cumulo-nimbus clouds
formed from eddies or individual updrafts in the general
inward rush of winds toward the center of the storm.
The bands themselves spiral (counterclockwise in the
Northern Hemisphere) slowly about the eyewall. They can
stretch out anywhere from 75 to 750 km from the eye.
h. Another feature of a hurricane is wind reversal after
the eye passes. If you are in the direct path of a
hurricane in the Northern Hemisphere, which is moving
from east to west, the wind will first come at you from
the north and you'll really get pounded, the winds
accelerating as the eyewall approaches. Then, the eye
passes over you, and things are sunny and tranquil for
half an hour or an hour or so. Just when you start to
relax, you'll be smashed by really fast winds, this time
coming from the south. If the storm is coming up from
the south to the north in the Northern Hemisphere, then
the wind reversal would be easterly winds before the eye
and westerly after. The opposite is true in the
Southern Hemisphere.
i. Another feature of direct relevance to disaster planning
is the notion of the (more) dangerous side. The winds
spiral into the storm at some high rate of speed but you
have to remember the storm itself is moving. You have to
add and subtract the storm's speed to get the effective
speed of winds in any part of the hurricane. In the
Northern Hemisphere, the dangerous side is the right
side; in the Southern, it's the left side. So, if the
storm were moving, say, 25 km/hr westbound in the
Northern Hemisphere and the winds were coming in at,
say, 150 km/h, the winds to the right would effectively
be moving at 175 km/hr, while those on the left side
would effectively be moving at 125 km/hr. Here is a
spiffy animation making the same point with a hurricane
moving north into Louisiana and Mississippi at 30 mph
with winds of 100 mph.
![[ hurricane dangerous side, NOAA ]](http://web.archive.org/web/20060426063318/http://hurricanes.noaa.gov/prepare/images/winds_H2.gif)
j. Direction of travel: generally westward at first, driven
by the easterly Trades and the circulation around the
oceanic subtropical high cell. That said, hurricanes
pretty much go where they "want" to, driven by quirks in
their own internal circulation and interactions with the
regional circulation, which is itself affected by the
position of the world pressure and wind belts, seasonal
movements in these, and the land and sea pressure
differential that is so prominent in the summer.
Hurricanes will often initially start out westbound and
then curve poleward along the isobars defining the
Bermudas or Hawai'ian High. If there is a marked low on
land, hurricanes will head for land. Here we see the
North Atlantic hurricane tracks for 1999:
![[ Atlantic hurricane tracks, NOAA, 1999 ]](http://www.nhc.noaa.gov/tracks/1999atl.gif)
1. If they manage their poleward turn out at sea, notice
how long their tracks can be. By staying out over
water, they stay in contact with their power source:
warm tropical water. By power source, I mean they
pick up enormous amounts of evaporated water and the
latent heat it carries, the air rises, cooling
adiabatically enough to create condensation (clouds
and rain) and this releases the latent heat, which
accelerates the original uplift. Hurricanes can ride
the Gulf Stream or similar warm currents pretty far
poleward and can eventually head east along the
current, moving along those summer isobar patterns
into the Westerlies belt. Sooner or later, they
degrade into lower power tropical or extratropical
storms because they are slowly moving into ever
cooler waters, which cuts them slowly off from their
power source.
2. If they do their poleward swing onshore, they die
suddenly. Notice how generally short the tracks are
that come ashore and how quickly they degrade into
the yellow tracks of an ordinary tropical storm or
the green tracks of a tropical depression. This
sudden flameout (if that's the right metaphor) is a
result of the hurricane suddenly being cut off from
access to its power source: warm tropical waters.
k. Global distribution. From the foregoing, we can
generally predict where and when hurricanes are going to
occur globally.
1. They are largely summer phenomena, given the
influence of the oceanic cells into which the
Subtropical High break: Hurricanes flow more or less
along the curving isobars that encircle those oceanic
peaks of high pressure. Hurricane season in the
North Atlantic and North Pacific, then, is largely
July to October; in the North Indian Ocean, there are
two peaks, one in May and another in November. In
the South Pacific and the South Indian oceans, it's
more like late October to May.
2. Hurricanes are tropical but not equatorial. Why
don't they form right along the equator, where,
presumably, the ocean water is warmest? Because
Coriolis Effect is non-existent. There has to be
some Coriolis Effect to induce the inspiraling so
characteristic of these storms. They generally
originate somewhere between 8 and 15 degrees north or
south of the equator.
3. They mainly afflict east coasts of continents because
of their dependence on a warm current, such as the
Gulf Stream. We in California are protected from
full-blown hurricanes by the cold California current.
Occasionally, a hurricane will actually survive the
short trip across Central America and wind up off the
west coast, where they quickly degrade over the cold
water. These systems are sometimes called
"chubascos." We sometimes get the tropical moisture
from a chubasco, usually in August or September,
receiving thunderstorms in the summer which,
otherwise, is pretty rare for us. An interesting gap
is present on this map of typical hurricane storm
tracks: the South Atlantic is free of them. Current
debates focus on ocean temperatures in the South
Atlantic just south of the equator (they may not
break the 26° C threshhold that seems necessary
to trigger hurricanes) and on vertical wind shear in
the troposphere there (too strong to support the
vertical uplift of the eyewall).
![[ global hurricane geography ]](http://rst.gsfc.nasa.gov/Sect14/mapXX.jpg)
l. There are several separate sources of hazard in a
hurricane:
1. Extreme winds.
2. Torrential rain and the resulting freshwater flooding
and mudslides (which is what killed most of the more
than 10,000 people who died in Hurricane Mitch in
1998).
3. Saltwater flooding from the storm surge and high
winds.
A. The extreme low pressure of a hurricane actually
pulls up the ocean surface in the area under it!
When this surge strikes the coast, waves can
penetrate farther inland than they would
otherwise, sort of the mother of all high tides in
effect.
B. Waves in water are a function of wind speed,
duration, and fetch (the distance of open water in
the direction the wind is blowing): Hurricanes
generate really high wind speeds for a sustained
period of time and the waves that come at a coast
may have had a long stretch of open water.
C. So, you have really big waves and an elevated
ocean surface and that translates into serious
saltwater flooding along coastal lowlands.
D. Human life is endangered both by the possibility
of drowning and by the debris carried by these
waves.
4. The dangerous side of hurricanes also often spawns
tornadoes to make the misery complete.
5. For a hurricane to become a disaster, it needs people
and property in the way. The Red Spot on Jupiter,
apparently some kind of hurricane, is not a natural
hazard, because there are no people and assets at
risk to it. Over the course of this century,
hurricanes have become costlier and costlier as the
human population grows and as it concentrates on
hurricane coasts. A hurricane (called a "cyclone"
locally) hit Bangladesh in 1970 and destroyed over
500,000 human beings. Hurricane Andrew did over $20
billion dollars of damage in 1992, because there are
now so many people and economic activities and assets
in South Florida.
m. Like the Fujita Scale with tornadoes, there is a
hurricane intensity scale, which is called the Saffir-
Simpson Scale. It has five hurricane levels, 1-5,
covering damage levels from minimal to catastrophic:
Saffir-Simpson Scale
Type Damage Press: mb Wind: km/h Surge: m
Depression (easterly wave develops circular isobars)
Tropical storm (many hurricane traits,
but not strong enough yet)
Hurr. 1 minimal >980 <118 1.25-1.75
Hurr. 2 moderate 965-980 118-154 1.75-2.75
Hurr. 3 extensive 945-965 154-178 2.75-4.00
Hurr. 4 extreme 920-965 178-210 4.00-5.50
Hurr. 5 catastrophic <920 >210 >5.50
c. Polar outbreaks
i. Cold air from polar regions breaks through to very low
latitudes in the Western Hemisphere sometimes (remember in
the discussion of tornadoes, I commented on how the New
World's mountains generally trend north-south, allowing
Arctic air to contact warm tropical air? Well, the
topography is relevant to this type of storm, too).
ii. A regular "squall line" of cumulo-nimbus clouds heralds the
cold front.
iii. Strong, steady winds follow the squall line.
iv. This brings unusually cool, clear weather to the tropics --
this can chill out many tropical crops grown on tropical
highlands, such as coffee ("mountain-grown," as the ads
have it!), wiping out much of the crop and raising the
price of your morning cuppa joe.
v. In the Caribbean and Central America, these outbreaks are
called "nortes," because they come from the north; in South
America, they're called "pamperos," because they blow in
from Antarctica over the Pampas (the famous grassland in
Argentina).
vi. In the low-lying areas, this actually brings some of the
most pleasant weather there: cooler and drier.
3. Equatorial storms are really different from both mid-latitude and
from tropical storms. They are often referred to as "weak
equatorial lows."
a. There is almost no difference among air masses, which we saw
earlier in discussing tropical storms (e.g., very little to
distinguish mT from E air masses), so, like the tropical storms,
air mass analysis isn't helpful.
b. Unlike the tropical storms, though, there is too little or no
Coriolis Effect to induce spiraling.
c. These storms consist of individuals or groups of cumulo-nimbus
clouds scattered in an area.
d. They are associated with the Inter-Tropical Cconvergence Zone
(ITCZ): Basically, the converging Trades produce uplift, which
generates many individual convectional and convergent storms.
e. A typical day along the ITCZ consists of day breaking clear and
sunny. By afternoon, thunderheads are piling up, especially
wherever there's some relief in the landscape: islands in the
open sea or mountains. By late afternoon, it's "raining cats
and dogs" (or is it "pitchforks and hammer handles"?). This
goes on for a short while, maybe 20 minutes to a couple of
hours, and then, as the sun sets, the clouds die down and
dissipate and the stars come out in a largely clear sky. This
goes on for weeks at a time. Pretty exciting, huh?
4. High latitude storms. Not much is known about the storms
experienced in the polar and circumpolar latitudes. The climate is
pretty dry up there, given the low temperatures, so not much
precipitation happens most of the time (these are sometimes called
"the polar deserts"). But, when it does, it comes with great
suddenness and violence.
a. Polar lows: Some of these storms are similar to mid-latitude
wave cyclones, with front-like features and the asymmetry of the
precipitation bands we associate with classic mid-latitude
cyclones. They may result from A or AA or cP air interacting
with mP air blowing in from over a warm current, such as the
Gulf Stream/North Atlantic Drift or the Japan Current/North
Pacific Drift.
b. Others actually look suspiciously like hurricanes, of all
things, complete with an eye. Some people call them even call
them "(Ant-) Arctic hurricanes" or "polar hurricanes."
Sometimes they're called "bomb cyclones," because the air
pressure in them drops catastrophically (like a bomb), at least
24 mb in 24 hours (there have been a few that dropped 60 mb or
hPa in 24 hours!!!).
i. So, even though they may look like a hurricane, they are
still pretty different:
a. They have cold air and cold fronts, which would destroy
a conventional tropical cyclone or hurricane.
b. They form under strong upper-level winds (remember one
of the explanations for the South Atlantic being free of
hurricanes is unusually strong vertical wind shear?).
c. They form in the winter, and tropical cyclones are
summer affairs.
d. They form in the Northwest Pacfic, Northwest Atlantic
("the Perfect Storm"), and, rarely, even in the
Mediterranean, while true hurricanes need warm tropical
oceans to form and these bomb cyclones may have
something to do with the famous, sudden-onset
Nor'easters on the American East Coast.
ii. One possible explanation for these is that they are
hybrids, formed when an unusually strong mid-latitude wave
cyclone somehow connects up with a tropical cyclone moving
into higher latitudes, allowing the mid-latitude cyclone to
exploit the massive amounts of water vapor carried by the
tropical cyclone.
c. In short, not a lot is known about high latitude weather and
it's an area of active research and controversy, with many
opportunities for students who acquire the physical geography,
physics, chemistry, meteorology, or remote sensing background to
help out (and who actually like freezing in the field).
D. Importance of storms -- a few wrap-up comments before we leave the
weather section.
1. Storms liberate enormous amounts of latent heat into the
atmosphere.
2. As such they are part of the global circulation carrying surplus
heat away from the low latitudes to the heat-deficient higher
latitudes, where the storms release the heat through condensation
and freezing.
3. Storms supply precipitation and so largely determine fresh water
distribution and water resources.
4. They create many environmental hazards:
a. Lightning and lightning fires
b. Hail and crop loss
c. Tornadoes and hurricanes are just hazards by definition whenever
people or their "stuff" are in their paths.
d. Floods (a little too much of that water resource!)
e. Winds
5. People try to minimize the hazardous aspect of storms (e.g.,
hurricane seeding to drop wind velocity or dam and levee
building), but such efforts often backfire unpredictably.
Later, we'll see a common theme in research is that society
responds to recurrent, low level hazards in a way that sets it
up for much more catastrophic losses later, when the much rarer,
higher magnitude event happens along. More on that later.
Well, th-th-that's about all for storms, folks. Make sure you understand the
basic idea of air mass analysis (air masses and fronts) and know that it was a
major breakthrough in the understanding of mid-latitude wave cyclones, the
most common storm in the mid-latitudes. Be able to associate the major storm
types discussed in this lecture with the general latitudes in which they occur
(mid-latitude, tropical, equatorial, and high latitude). Make sure you are
familiar with the main characteristics and dynamics of the major storm types
and that science still does not understand high latitude storm genesis and
development. Be aware of the various ways that storms are important in the
earth system (and to us humans).
The next lecture will summarize major climate types.
Document and © maintained by Dr.
Rodrigue
First placed on web: 10/22/00
Last revised: 06/19/07