Weather
Watching: Hurricanes 101
This
year has been the most active and devastating hurricane season on
record. What is this awesome force of nature?
Simply
put, hurricanes are part of the earth’s mechanism of global heat
transfer. The Earth is perpetually trying to reach a temperature
equilibrium. In much the same manner as Arctic Outbreaks bring cold
frigid and freezing temperatures to the subtropics in the winter,
hurricanes transfer an enormous amount of heat to the northern latitudes
in the summer and fall.
Latent
Heat of Condensation
Hurricanes
are known in different parts of the world as Typhoons, Tropical
Cyclones, Willy-Willys, and Medicanes (Mediterranean Hurricanes).
No matter what their name, the mechanism that creates these massive
storms is known as the “Latent Heat of Condensation”. This is a
process of transferring the sun’s energy, collected in the warm
ocean waters, back into the atmosphere.
1.
Water retains heat
The
sun’s energy heats the earth. Land heats and cools faster than water
– land also experiences more extreme temperatures than water. Conversely,
water heats and cools more slowly and to less extreme, but the heat
collected in water is more persistent and is retained longer. Oceans
act as a heat collector, much like a solar panel, but on a global
scale. Consider the fact that our world is three-fourths water and
you get an idea of how much heat our oceans collect and exchange
in a given summer.
2.
Energy to change state
Water
has three different states of mass: liquid, ice (solid), and gas
(water vapor). Heating and cooling water is much different than
air. Scientifically speaking, it takes one calorie of energy to
heat (or cool) one gram of water 1° Celsius (C) - one gram is approximately
.035 fl oz. This is true up and down the temperature scale, except
when water passes through one of the changes of state – ice to water,
water to gas, gas to water, or water to ice.
In these instances it takes more energy – much more – to change
the water’s state, without changing its temperature. A pan of water
takes only a few minutes to heat to the boiling point 212°F (100°C)
and takes relatively little energy (heat). But when the water reaches
the boiling point, it takes longer and much more energy for the
water to boil at 212°F, creating water vapor (steam).
It takes one calorie of energy to raise the temperature of one gram
of water 1°C. As the water temperature reaches 212°F it slowly converts
from liquid water to water vapor, as evidenced by the bubbles that
form at the bottom of the pan. In order to change the state from
water to vapor, it takes an additional 540 calories of energy per
gram to just change the water’s state without changing the temperature.
3.
Condensation
Evaporation
is a cooling process — it removes heat from its surroundings.
When
a swimmer gets out of the water, the air flows over their body and
removes heat to fuel the evaporative process of water on the skin.
This is why the body shivers; heat is actually being removed from
the body. This principle is also why evaporative coolers work well
in dry Southwest desert areas.
Conversely,
condensation is a warming process because heat is released back
into the atmosphere as water vapor condenses back into liquid water.
Rainfall is a good example of condensation. Hurricanes spread enormous
amounts of rain over hundreds of thousands of square miles.
For
this reason, hurricanes are fueled by the Latent Heat of Condensation.
If it takes 540 calories to convert 1 gram of water to vapor, just
think of how many trillions upon trillions of energy calories are
generated by the enormous rainfall amounts hurricanes produce. It
has been said that one hurricane produces enough energy to power
the entire United States for a full year. If only we could harness
this unbridled power of nature.
Development
of a Hurricane
Most
Atlantic hurricanes start as small disturbances off the West Coast
of Africa. Created by weak atmospheric perturbations, these disturbances
become Tufts, small clusters of thunderstorms. At those latitudes,
predominant wind pattern is from east to west in the equatorial
regions around the southern edge of the Atlantic High Pressure Cell.
These winds are known as “Trade Winds.”
As
Tufts move on the Trade Winds, they move out over warmer
water, which starts the fueling process. Numerous Tufts develop
and die over the course of any given summer; however, when upper
atmospheric dynamics are just right, barometric pressure drops with
the thunderstorm activity and any given Tuft can turn into a Tropical
Depression.
Tropical
Depressions are classified as storms with winds less than 38mph.
Tropical Depressions form a cyclonic low-pressure center, which
begins the organization process necessary for hurricane development.
Tropical Depressions can also form in the Caribbean and/or Gulf
of Mexico, as well.
When
Tropical Depressions reach the warmer shallower waters of the Caribbean/Gulf
of Mexico, their intensity usually increases. Tropical Storms
are classified as storms with winds 38-73mph. Because of the proximity
of the South American continent, the Trade Winds start deflecting
more northerly out of the tropical latitudes. When this happens,
the earth’s rotation influences the directional flow through Coreolis
Force.
Coreolis
is a spherical force which is caused by the curvature of the earth.
The further north from the equator, the smaller the earth’s diameter
becomes. This natural curve tends to deflect wind flow to the right
in the Northern Hemisphere. Generally speaking, this is why the
further north a hurricanes goes, the more it will re-curve its path
towards the North and sometimes East. This is a primary reason why
the United States’ Gulf Coast and Eastern Seaboard are prime targets
for hurricane landfall. The dynamics, both atmospheric and geographical,
are near perfect.
Hurricanes
become a self-generating process when warm ocean currents reach
80°F or more. As long as hurricanes remain over warm waters, they
will continue to regenerate themselves. Once a storm makes landfall,
or moves over relatively cooler ocean waters, it becomes cut off
from its energy source and rapidly weakens and dies.
If
a weakened storm moves back over warm ocean waters, it may regenerate
to be as strong, if not stronger, than it was previously. This happened
with Hurricane Wilma – it weakened over the Yucatan Peninsula before
it strengthened again and made landfall in south Florida as a strong
Category 3 hurricane.
According
to the Saffir-Simpson Hurricane Scale, there are five categories
of hurricanes: Category 1 storms measure winds 74-95 mph; Category
2, 96-110 mph; Category 3, 111 to 130 mph; Category 4, 131 to 154
mph; and Category 5, winds greater than 155 mph.
Hurricane
Season
As
the earth rotates from winter to spring and summer, the days become
longer and nights shorter. The Summer Solstice marks the longest
day of the year – normally June 21. It’s also where the sun reaches
its maximum tilt northward. This is known as the Tropic of Cancer
(23.5°N) and runs through the Straits of Florida between Cuba and
the U.S. mainland. After the solstice, the seasonal days get shorter
and the earth’s apex to the sun slowly moves southward back toward
the equator.
The Summer Solstice also roughly correlates with the start of the
annual hurricane season. July and August are historically the warmest
months of the year even though the days are getting shorter. This
is, in part, because the atmosphere has been warmed by months of
summer sunshine collected in the oceans and the lack of cold in
the Polar Regions, which are now in constant sunlight. There’s relatively
little cooling in the Arctic to counter-balance summer’s warmth.
As
summer wanes into August and September, the Fall Equinox (September
21) signals that day and night are of equal length and the sun is
directly back over the Equator. As the weather cools, Fall begins,
but the oceans still team with massive amounts of heat energy collected
over the summer. The potential for hurricanes exists as long as
the oceans remain warm. October and November may produce devastating
hurricanes as the oceans release their heat energy.
Components
of a Hurricane
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courtesy
photo
An average
hurricane is about 300 miles in diameter, but larger storms
can be as much as 500 miles across. In the Northern Hemisphere,
these storms spin counter-clockwise, or from right to left.
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The
hurricane’s Eye is normally 20 to 40 miles in diameter, but
can be much larger. Eyes are distinguished by relatively calm and
clear weather in midst the maelstrom. At the edge of the Eye is
the Eye Wall, which is the most intense and destructive part
of the storm. Winds are their strongest and the storm surge can
reach as much as 30 feet high or more, as what was witnessed with
Hurricane Katrina.
Storm
surge is caused by the extreme low-pressure in the Eye. The
lack of atmospheric pressure allows the ocean surface to rise higher
than the surrounding ocean. This rise in sea surface is whipped
by the violent winds and creates massive violent swells. As the
Eye Wall comes ashore, it encounters shallow coastal waters, which
catapults the storm surge to greater heights.
Storm
surge is often the most devastating part of a hurricane and claims
more lives due to the extreme flooding and violent forces of raging
water. Low-lying areas become inundated with angry seas and the
shear force of wind-whipped water turns sturdy homes into matchsticks.
Katrina’s storm-surge caused catastrophic damage as the storm moved
ashore. The hurricane’s eye passed just east of New Orleans, and
brought an enormous storm surge into the Mississippi Sound, which
breached Lake Ponchatrain. Levees broke and massive flooding inundated
the city.
A
hurricane’s right-front quadrant can be equally destructive, as
it possesses severe Squall-Line thunderstorms, torrential rains,
and deadly tornadoes in its spiral bands. Spiral bands also
rotate counter-clockwise around the storm. They are the storm’s
leading edge as it begins to make landfall. Katrina’s Eye Wall and
storm surge actually made landfall near Bay St. Louis, MS, and literally
destroyed the small coastal communities of Waveland and Pass Christian,
among others. Here, the winds were estimated at greater than 140
mph and the storm surge was nearly 30 feet. The storm surge flooded
many miles inland, leaving complete devastation in its path.
Theories
This
was the most active hurricane season on record. There are many different
theories as to why, some of which tie into the concept of Global
Warming. It makes sense that if the oceans are warming and collecting
solar heat, then the planet’s atmosphere will also warm as a result.
Another
theory, which also makes sense, is the earth moves and rotates in
cyclic patterns: day to night, season to season, year to
year. The earth also has a 22-year solar cycle where the earth reaches
it’s solar maximum every 11 years then rotates to a solar minimum
11 years later. In the year 2000, we reached our “Solar Max”. In
the years leading up to the 2000 Solar Max, weather records indicated
an increase in global temperatures, which seems natural. We also
saw a greater activity in the El Nino and La Nina oceanic patterns.
These
anomalies affect and change global weather patterns, as well. In
the 1980s, the meteorological mantra was “The Next Ice-Age”. This
was a period when the solar cycle was approaching it’s minimum and
the earth and oceans were experiencing relatively cooler temperatures.
Cooler oceans would indicate slower hurricane seasons. Only time
will tell how these cyclic patterns truly affect the earth’s climate
and hurricane cycles.
Tracking
the Storm
Today’s
world is much more technically prepared to warn its citizens of
a hurricane’s approach. Satellite imagery and Doppler Radar provide
ample warning days in advance.
The
famous Hurricane Hunters fly C-130 aircraft missions through hurricanes
to collect and provide critical weather information. Radiosonde
data – weather instruments with a radio transmitter are dropped
from the plane at regular intervals to paint a vertical cross-section
of wind, temperature, pressure, and moisture contained in the storm.
This
information gives scientists at the National Hurricane Center in
Coral Gables, FL much needed data to plot and forecast a hurricane’s
development and projected path. Their forecasts have saved innumerable
lives through the years. They also provide emergency managers advanced
warning and landfall projections to allow the necessary time to
pre-stage and stockpile water, food, and other emergency supplies.
They also give the public and businesses adequate time to board
up and secure their buildings and belongings.
Hurricanes
are the most dynamic and destructive of all atmospheric storms.
They are also an integral part of the planet’s heat exchange. Without
them parts of the northern latitudes would be uninhabitable.
We
cannot stop hurricanes, so we must learn to get out of their way.
Advanced warning is the most crucial part of the mitigation process,
and learning more about these dynamic forces helps us to better
understand and contend with nature’s most devastating storms.
John
Gerrish, CWS’s Sales Manager and Certified Meteorologist, assisted
hurricane relief efforts at his local church which provided the
photos.
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