Hurricanes

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Hurricanes

• Most everything you always wanted to know but
  were afraid to ask

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• Topics
• How do hurricanes get started?
• How much energy is involved?
• Why do they get stronger over the Gulf of Mexico?
• Why do they spin?
• What is the impact of climate change?

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Technically called tropical cyclone (or tropical
disturbance, tropical depression, tropical storm,
typhoon, or hurricane, depending on strength and
geographical context). The word hurricane is
derived from the name of a native Caribbean
Amerindian storm god, Huracan, via Spanish
huracán. Intense low pressure systems with
sustained winds of 64 knots (74 mph or
120kph) Water must be warm (26.5C) and deep (50
m) The air has to be humid. Must be at about 10
north or south of the equator so Coriolis effect
can cause rotation. Hurricanes grow from tropical
depressions or thunderstorms. Hurricane season is
year round on a global basis but in the North
Atlantic, it lasts from June to November.
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How do hurricanes form?
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It all starts with Mr. Sun, the source of energy
for all of climatic events. Through the long
summer, the sun is more directly over the
northern hemisphere and incoming solar radiation
is more intense. In addition, the daylight hours
are of longer duration.
Ocean
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Intensity of solar radiation
A
B
Summer in Northern Hemisphere Sunlight at A
strikes Earth more directly or intensely than at
B.
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Duration of solar radiation
Similarly, the Northern Hemisphere has in the
summer more daylight in a 24 hour rotation than
the length of time it is in darkness. The yellow
line (day) is longer than the red line
(night). This means more solar radiation is
received and temperatures are higher. This is the
duration of solar radiation received.
Sunlight
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So, throughout the long summer, the sun is more
intense and of longer duration. These two factors
result in a great deal of solar radiation being
absorbed by the waters of South Atlantic
Ocean. The water gets warmer.
Warmer
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The warm water warms the air above it. The warm
air starts to rise. As the air rises it creates
low pressure at the surface. .
Low pressure
Warm
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The warm water warms the air above it. The warm
air starts to rise. As the air rises it creates
low pressure at the surface.
Nature abhors a vacuum. Air flows into the area
of low pressure. It rises and the cycle
continues.
Low pressure
Warm
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As the air flows across the ocean surface it
evaporates water. This is because the solar
radiation absorbed by the ocean provides the
energy needed to speed up the motion of the water
molecules so that they will change state, that is
become the invisible gaseous form of water, water
vapour.
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The energy is called latent heat and the
evaporated water is carrying it up into the
atmosphere.
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Think of what happens when you sweat. Water
evaporates off your body and removes heat from
your body. You feel cooler. Not as cool as this
guy, however.
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Now, as air rises, it cools. Cools at a steady
rate that averages 1C/100 m. (called the lapse
rate) As the air cools, the motion of the water
vapour molecules slows down. Eventually they slow
down to the point where they change into a drop
of liquid water. This is condensation. Clouds
forms. When this happens, the kinetic energy of
the water vapour molecules becomes heat energy,
and it warms the air around it. The same amount
of heat that was used to evaporate water is
released to the air. This causes the air to be a
little bit warmer and it rises that much
more. This creates even more low pressure and
draws in even more air. If the air that is
cooling has a lot of water vapour in it, there
are a lot of molecules condensing, and therefore
a lot of heat is released. This makes the low
pressure very intense, and the air blows in
really fast.
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Wind shear There cant be a lot of wind shear or
hurricanes wont form..
See this page for a great animation of wind
shear. http//ww2010.atmos.uiuc.edu/(Gh)/guides/mt
r/hurr/grow/home.rxml
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This release of latent heat is what causes .
convectional storms thunderstorms and
hurricanes
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And this is why hurricanes quickly degrade when
they move over land. The amount of water being
evaporated drops dramatically, and the hurricanes
energy is depleted. Winds die and the storm
breaks up.
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Intensity is ranked on the Saffir-Simpson
Hurricane Scale http//www.usatoday.com/weather/s
torms/hurricane/ghurricane_scale/flash.htm
Storm surges cause a lot of the damage not just
the wind http//www.usatoday.com/graphics/weather/
gra/gsurge/flash.htm
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How much energy?

• (Lots. Lots and lots and lots.)

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So, how much energy are we dealing with
here? When 1 litre of water is condensed, theres
enough energy released to power a 60 watt light
bulb for 20 hours
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The average hurricane condenses 20 trillion
litres of water in a day. Thats 20 with 12
zeros! 20 000 000 000 000 Thats the same as
all the worlds electrical generating plants
produce in 200 days.
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Why do hurricanes get stronger over the Gulf of
Mexico?

• Are you in the loop (current)?

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The loop current flows into the Gulf of Mexico
from the Caribbean Sea past the Yucatan
Peninsula. It loops around in the Gulf and exits
pass the tip of Florida to join the Gulf Stream.
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The loop current
This map shows the actual generalized pattern of
surface currents.
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The loop current breaks off into eddies of
(comparatively) very warm water, which circulate
around in the Gulf of Mexico.
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Hurricane Katrina passes over the eddy of the
loop current, September 2005
In so doing, Katrina gained an enormous amount of
energy because of the vast amounts of water that
were evaporated.
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Katrina, building strength over the loop current
area of the Gulf of Mexico, Sept. 2005
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Why do they spin?
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Hurricanes, tornadoes, and mid-latitude cyclonic
storms all spin counter-clockwise in the Northern
Hemisphere because of the rotation of the
earth. This is called Coriolis Force or Effect
Objects in motion (in the Northern Hemisphere)
appear to bend to the right of their path of
motion (and to the left in the Southern
Hemisphere.) The Paris Gun
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View this slide show on Coriolis Force. See this
movie on Coriolis Force. View this movie of
hurricanes and Coriolis Effect
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Low pressure
Air flows into the low pressure area and because
of Coriolis Force, it is deflected to the right
of its path of motion. This starts the circular
motion.
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What is the impact of climate change?
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If the earth is getting warmer (and it has and it
is), then it stands to reason that hurricanes
should be worse and there should be more of
them. More heat is absorbed more water
evaporated more water vapour condensed more
latent energy released more hurricanes. Evidence
does actually support that at least, not
yet. The frequency of storms in the major
tropical storm spawning areas has NOT
increased. The frequency of Category 4 and 5
storms has, however, increased in most of these
areas.
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Wilma

• Category 5
• Need we say more?

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Click here for a hurricane tracker site.