Download Storms at Sea - Boone County Schools

Storms at Sea
The scenes from hurricane disaster areas are
shocking. Homes are destroyed. Towns and
cities are in disarray. Floods often displace
people. How do these powerful storms, with
pouring rain, flooding, and winds at speeds
of more than 250 kilometers per hour, get
started? Scientists have spent a great deal of
time and energy answering this question.
The more scientists know, the better they
can predict when and where a hurricane
might hit.
Hurricane research often focuses on how to
How Do Hurricanes Begin?
predict and prepare for hurricanes.
Hurricanes are huge tropical storms with
high winds. Hurricanes form over warm water in tropical areas.
In these areas, the Sun’s energy produces warmer temperatures
due to Earth’s tilt. During late summer, these tropical areas
reach their warmest temperatures. The air over the ocean in
these tropical areas is also quite warm. In fact, hurricane
conditions require the water to be at or above 26.5 °C. The warm
temperatures also cause evaporation. Thus, the air above the
ocean becomes extremely humid.
Convection currents in the air form when warm air rises and
cooler air moves in underneath. This cooler air, now close to the
warm water, gains thermal energy from the water and, in turn,
rises. As the warmed air rises, it cools and
falls, rushing in to fill the space left by the
rising warmer air. This pattern sets up a
continual cycle of heating and cooling, as
the air rises and falls.
Eventually, the convection-powered
weather system takes on a spinning motion.
In the Northern Hemisphere, the motion is
counterclockwise due to the rotation of
Earth on its axis. This phenomenon is
known as the Coriolis effect. (Similar
storms in the Southern Hemisphere, known
as cyclones, spin clockwise.) The Coriolis
effect causes air to rotate around a central
area in which the air pressure is lower than
Discovery Education Science
The Coriolis effect causes storms to rotate
counterclockwise in the Northern Hemisphere.
© Discovery Communications, LLC
Storms at Sea
its surroundings. Continuing to gain energy
from the warm ocean, wind speeds increase.
When winds reach 61 kilometers per hour, the
system is called a tropical depression. If wind
speed increases to 63 kilometers per hour, the
system becomes a tropical storm. At 119
kilometers per hour, the storm is considered a
hurricane. Hurricanes can have diameters as
large as 2,100 kilometers.
Hurricane Motion
With its counterclockwise spin and high wind
speeds, a hurricane develops the familiar
When Hurricane Hugo hit the continental
United States in 1989, it was the strongest
spiraling appearance. The low-pressure area
hurricane to strike in 20 years.
that forms in the center of the storm is known
as the eye. The eye of a hurricane can span up to 48 kilometers.
When the eye is calm, it is bordered by the eyewall, an area
where air moves upward at a very fast rate, carrying large
amounts of moisture into the storm. Due to the increased
moisture, the eyewall often appears to be a wall of clouds. The
eyewall is a turbulent area that contains the most destructive
power of the storm.
Hurricanes travel across the ocean.
Sometimes the hurricane path approaches
islands and coastal areas, bringing danger to
people and animals. Hurricanes move by
currents in the atmosphere known as
steering winds. For forecasting purposes, it
is better when the hurricane is carried by
strong steering winds because this makes
the hurricane path easier to predict. Weak
steering winds cause a more unpredictable
path.
Hurricane Francis, 2004, approaches the
coast of Florida.
Naming and Classifying Hurricanes
Once a tropical depression becomes a tropical storm, it is
named. Naming storms helps make communication clear among
researchers as well as to the public. During hurricane season,
there may be more than one tropical storm. The World
Meteorological Organization (WMO) is responsible for naming
tropical storms. Each year, the WMO uses an alphabetized list of
Discovery Education Science
© Discovery Communications, LLC
Storms at Sea
names. The first tropical storm of the
year gets an “A” name, the second gets a
“B” name, the third gets a “C” name, and
so forth throughout the season. The
following year, the WMO begins with a
new list. There are 6 years’ worth of lists
prepared at any given time. These name
lists are used on a rotating basis. If a
hurricane is very destructive, however,
the WMO will not reuse the name.
Hurricanes are classified by wind
speeds. The Saffir-Simpson Hurricane
The Saffir-Simpson scale classifies
Wind Scale (SSHWS) categorizes
hurricanes by wind speed.
hurricanes into five different levels. A
category 1 hurricane is the least destructive. A category 5 can
cause catastrophic damage to homes, trees, and buildings.
Discovery Education Science
© Discovery Communications, LLC