What is a hurricane?
A hurricane is a huge storm! It can be up to 600 miles across and have strong winds
spiraling inward and upward at speeds of 75 to 200 mph. Each hurricane usually
lasts for over a week, moving 10-20 miles per hour over the open ocean. Hurricanes
gather heat and energy through contact with warm ocean waters. Evaporation from
the seawater increases their power. Hurricanes rotate in a counter-clockwise
direction around an "eye." The center of the storm or "eye" is the calmest part. It
has only light winds and fair weather. When they come onto land, the heavy rain,
strong winds and large waves can damage buildings, trees and cars.
How do hurricanes form?
Hurricanes only form over really warm ocean water of 80°F or warmer. The
atmosphere (the air) must cool off very quickly the higher you go. Also, the wind
must be blowing in the same direction and at the same speed to force air upward
from the ocean surface. Winds flow outward above the storm allowing the air
below to rise. Hurricanes typically form between 5 to 15 degrees latitude north
and south of the equator. The Coriolis Force is needed to create the spin in the
hurricane and it becomes too weak near the equator, so hurricanes can never form
there.
What is storm surge?
Storm surges are frequently the most devastating element of a hurricane. As a
hurricane’s winds spiral around and around the storm, they push water into a mound
at the storm’s center. This mound of water becomes dangerous when the storm
reaches land because it causes flooding along the coast. The water piles up, unable
to escape anywhere but on land as the storm carries it landward. A hurricane will
cause more storm surge in areas where the ocean floor slopes gradually. This
causes major flooding.
As you watch the storm-surge animations, notice the effect that the physical
geography of each coastline has on storm surge. Also, note the waves on top of the
ocean's surface. Wind, waves, and sea-level rise all contribute to storm-surge
damage.
With technology the way it is, there are computer models that allow forecasters to
predict the amount of storm surge that will affect a coastal area. These are called
Slosh Models and take into account a storm’s strength, its path, how the ocean
shallows, and the shape of the land. Then it calculates how much storm surge a
hurricane will probably cause.
When does hurricane season start?
The Atlantic hurricane season is from June 1 to November 30, but most hurricanes
occur during the fall months. The Eastern Pacific hurricane season is from May 15
to November 30. (Below is a graphic that shows you when hurricanes are most
active across parts of the world.)
Who names hurricanes?
From 1950 to 1952, tropical cyclones of the North Atlantic Ocean were identified
by the phonetic alphabet (Able-Baker-Charlie-etc.), but in 1953 the US Weather
Bureau switched to women's names. The rest of the world eventually caught on, and
naming rights now go by the World Meteorological Organization, which uses
different sets of names depending on the part of the world the storm is in. Around
the U.S., only women's names were used until 1979, when it was decided that they
should alternate a list that included men's names too. There's 6 different name
lists that alternate each year. If a hurricane does significant damage, its name is
retired and replaced with another.
What is the difference between a hurricane and a typhoon?
Nothing except geography. Tropical storms occur in several of the world's oceans,
and except for their names, they are essentially the same type of storm. In the
Atlantic Ocean, Gulf of Mexico, and the Eastern Pacific Ocean, they are called
hurricanes. In the Western Pacific Ocean, they are called typhoons. In the Indian
Ocean, the Bay of Bengal, and Australia, these types of storms are called cyclones.
How do Hurricanes form
Hurricanes are the most awesome, violent
storms on Earth. They form near the
equator over warm ocean waters.
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the Atlantic Ocean or eastern Pacific
Ocean.
The generic, scientific term for these storms, wherever they occur, is tropical cyclone. Other
names they are given, depending on where in the world they are born, are typhoons, cyclones,
severe tropical cyclones, or severe cyclonic storms. Whatever they are called, the same forces
and conditions are at work in forming these giant storms, which all can cause damage or
devastation when they hit land where people live.
Tropical cyclones are like engines that require warm, moist air as fuel. So the first ingredient
needed for a tropical cyclone is warm ocean water. That is why tropical cyclones form only in
tropical regions where the ocean is at least 80° F for at least the top 50 meters (about 165 feet)
below the surface.
The second ingredient for a tropical cyclone is wind. In the case of hurricanes that form in the
Atlantic Ocean, the wind blowing westward across the Atlantic from Africa provides the
necessary ingredient. As the wind passes over the ocean’s surface, water evaporates (turns into
water vapor) and rises. As it rises, the water vapor cools, and condenses back into large water
droplets, forming large cumulonimbus clouds. These clouds are just the beginning.
Meteorologists have divided the development of a tropical cyclone into four stages: Tropical
disturbance, tropical depression, tropical storm, and full-fledged tropical cyclone.
1. Tropical disturbance
When the water vapor from the warm ocean condenses to form clouds, it
releases its heat to the air. The warmed air rises and is pulled into the
column of clouds. Evaporation and condensation continue, building the
cloud columns higher and larger. A pattern develops, with the wind
circulating around a center (like water going down a drain). As the moving
column of air encounters more clouds, it becomes a cluster of
thunderstorm clouds, called a tropical disturbance.
2. Tropical depression
As the thunderstorm grows higher and larger, the air at the top of the cloud
column is cooling and becoming unstable. As the heat energy is released
from the cooling water vapor, the air at the top of the clouds becomes
warmer, making the air pressure higher and causing winds to move
outward away from the high pressure area. This movement and warming
causes pressures at the surface to drop. Then air at the surface moves
toward the lower pressure area, rises, and creates more thunderstorms.
Winds in the storm cloud column spin faster and faster, whipping around
in a circular motion. When the winds reach between 25 and 38 mph, the
storm is called a tropical depression.
Tropical storm
When the wind speeds reach 39 mph, the tropical depression becomes a tropical storm.
This is also when the storm gets a name. The winds blow faster and begin twisting and
turning around the eye, or calm center, of the storm. Wind direction is counterclockwise
(west to east) in the northern hemisphere and clockwise (east to west) in the southern
hemisphere. This phenomenon is known as the Coriolis effect.
Tropical cyclone
When the wind speeds reach 74 mph, the storm is officially a tropical cyclone. The storm
is at least 50,000 feet high and around 125 miles across. The eye is around 5 to 30 miles
wide. The trade winds (which blow from east to west) push the tropical cyclone toward
the west—that is, toward the Caribbean, the Gulf of Mexico, or the southeastern coast of
the U.S. The winds and the low air pressure also cause a huge mound of ocean water to
pile up near the eye of the tropical cyclone, which can cause monster storm surges when
all this water reaches land.
Tropical cyclones usually weaken when they hit land, because they are no longer being
“fed” by the energy from the warm ocean waters. However, they often move far inland,
dumping many inches of rain and causing lots of wind damage before they die out
completely.
Tropical cyclone categories:
Categor Wind Speed
y
(mph)
1
74-95
2
96-110
3
111-130
4
131-155
5
Over 155
Damage at
Landfall
Minimal
Moderate
Extensive
Extreme
Catastrophic
Storm Surge (feet)
4-5
6-8
9-12
13-18
19+
How do hurricanes form?
Hurricanes are mind-boggling in their size and strength. Modern technology lets us see how big
hurricanes are and track them through satellite imagery, while storm-penetrating aircraft measure
their strength, in terms of wind speeds and atmospheric pressure. That technology gives us
detailed warnings that people a century ago never had.
Hurricanes usually form in the tropical zones north and south of the equator, where warm waters
offer ample fuel for storm formation. That's where you see convection, where warm air rises and
forms clusters of thunderstorms. The Coriolis effect, created by the Earth's spinning motion, may
cause this cluster of storms to rotate (counter-clockwise above the equator, or clockwise below),
but the system has to be a few hundred miles from the equator for the Coriolis force to exert an
effect. If the system becomes organized and strong enough, with sustained winds of 39 miles per
hour, it becomes a tropical storm. At 74 miles per hour or more, it becomes a hurricane.
A hurricane can be hundreds of miles across. It's like a big engine, with upper-level winds acting
as a vent, pulling the rising warm air away from the storm. Rain bands spiral around the center of
the system. Air sinks in the clear center, or eye, which can be from 5 miles to more than 100
miles across.
Because the atmospheric pressure is so low in the eye, the water rises up there in a sort of
mound. This contributes to the devastating storm surge when a hurricane hits land, but most of
the surge comes from winds pushing the sea water ahead of the storm, raising ocean levels by
several feet.
When a hurricane hits land, it tends to weaken mostly because it has lost its source of energy the warm ocean waters. Friction might slow down the surface wind, but it also can enhance
turbulence and therefore gusts, scientists have found.
One of the most important ways of understanding hurricanes is understanding how they form. If
scientists can figure out why hurricanes form in some conditions and not in others, then they can
better predict when and where they will form.
There are several things that must be together for a hurricane to form:
1) Hurricanes only form over really warm ocean water of 26.5° C or more. That’s warm! Ocean
water in the Maritimes never reaches this temperature so hurricanes cannot form in these waters.
2) The atmosphere (the air) must cool off very quickly the higher you go. Have you ever noticed
that it gets really cold when you climb to the top of a mountain? Same idea.
3) The wind must be blowing in the same direction and at the same speed from the ocean surface
right up to 9,000 metres above sea level.
4) A hurricane will not form any closer than 500 kilometres to the equator. The Coriolis Force is
needed to create the spin in the hurricane and it becomes too weak near the equator.
What Is The Coriolis Force?
The Coriolis Force is a force that deflects moving objects to one side because of the Earth’s
rotation. The object is still going straight but the Earth moves underneath it, making it look like it
is moving to one side. In the Northern Hemisphere, the Coriolis Force deflects objects to the
right.
Have you ever tried rolling a ball to a friend across the floor of a spinning merry-go-round? The
ball looks like it is deflected to one side because of the spinning merry-go-round. This is the
same idea as the Coriolis Force. Try it out!
So what happens when all of these conditions are there?
Sometimes nothing. Sometimes a hurricane or tropical storm (a weaker form of storm) is formed.
The warm ocean water warms the air above it. This air rises because it is lighter than the cooler
air above. When the parcel of warm air reaches the cooler air above, the water vapor turns into
water drops and warms the surrounding air. When the warm air rises, the cool air replaces it and
wind is created. The wind will start spinning because of the Coriolis Force and a tropical storm
or hurricane is formed!
HOW MANY CATEGORY FOUR AND FIVE STORMS HAVE HIT?
-- Since 1900, just three Category 5 storms have hit the continental U.S. There have been 15
Category 4 storms in that time, including Hurricane Charley.
-- The last time the U.S. was hit by two hurricanes of Category 4 or above in the same year was
1915. That year a Category 4 storm hit Galveston and another one hit New Orleans.
-- Category 5 storms since 1900 (in order of intensity): 1) Unnamed storm hit Florida Keys in
1935, killing 408 people, 2) Hurricane Camille hit Mississippi in 1969 and 3) Hurricane Andrew
hit southeast Florida in 1992.
A hurricane in the making
A hurricane can be up to 600 miles in diameter and can reach eight miles into the air. Warm
water is the engine that creates and sustains the storm.
Water vapor, sucked upward around a tropical depression, creates heat energy when it condenses
to form rain. The high-altitude heat triggers exaggerated updrafts, which suck up even more
water vapor.
Once the chain is started, nothing but land and cooler water or high-altitude shearing winds can
stop the fierce
To be a hurricanes the winds must be at least 74 miles per hour. The terms hurricane
and typhoon are both other names for a strong tropical cyclones. A hurricane is the
worlds greatest storm. The word hurricane comes from the word Huracan, the god of big
winds and evil spirits that was once worshiped by the Maya people.
Hurricanes have a distinctive feature called an eye. The eye of a hurricane is in the middle of the
spiral. The eye is produced by the spiraling action of the storm and it is the area where the air is
slowing sinking. When the eye of a hurricane passes over a region the winds decrease to just a
gentle breeze, and the rain stops. You may even be able to see the sun during the day or the stars
at night. Then, the rest of the storm passes and the wind suddenly changes directions and
becomes ferocious again. The eyewall is the area inside the hurricane where the winds and rain
are the worst. The eyewall is the area surrounding the eye.
Some hurricanes are much stronger then others. Scientists have come up with a way to categorize
hurricanes to give the public a better idea of what should be expected in the coming storm. This
measurement is called the Saffir-Simpson scale and hurricanes are ranked from 1 (least
dangerous) to 5 (extremely dangerous):
Category
Wind Speed (km/h)
Storm surge (metres)
Description
1
118 - 153
1.2 - 1.7
weak
2
154 - 177
1.8 - 2.7
moderate
3
178 - 210
2.8 - 3.9
strong
4
211 - 249
4.0 - 5.5
very strong
5
> 250
> 5.5
devastating
 Storm surge is the height that ocean water reaches above the normal tide.
Category 1: This is the least damaging category of hurricane. These hurricanes can damage trees
and shrubbery and poorly constructed signs. There also might be minor flooding to low-lying
areas.
Category 2: In this type of hurricane, some trees might be blown down and lots of damage
might occur to signs. There might also be damage to roofs of houses and piers and marinas near
the water. People that live along the coast might need to be evacuated.
Category 3: Practically all signs will be torn down in this type of hurricane. There will be
structural damage to some buildings and all mobile homes will be destroyed. There will also be
serious flooding along the coast and structures near the water might be damaged because of
floating debris. People that live near the coast and within several blocks of the coast might need
to be evacuated.
Category 4: Extensive damage to roofs, windows, and door. All mobile homes are destroyed.
There will be flooding as far inland as 9.5 kilometres. All residents within 500 metres of the
shore might need to be evacuated.
Category 5: This is the worst category of hurricanes. Luckily, they are rare. There has never
been a category 5 storm in Atlantic Canada in recorded history. There will be severe damage to
all buildings including extensive shattering of glass due to the extreme winds. A massive
evacuation of residents within 16 kilometres of the shore might be necessary.