Download Hurricanes and the ITCZ

Hurricanes and the ITCZ
Hurricanes
Hurricanes have two main parts. The first is the eye of the hurricane, which is a calm area
in the centre of the storm. Usually, the eye of a hurricane measures about 20 miles in
diameter, and has very few clouds. The second part is the wall of clouds that surrounds
the calm eye. This is where the hurricane's strongest winds and heaviest rain occur.
Hurricanes are born over warm, tropical oceans. The top 50 meters of the ocean surface
needs to be 26.5o C. The air above the ocean must be cooler than the water temperature,
allowing thunderstorms to form. Hurricanes are fuelled by water vapour that is pushed up
from the warm ocean surface, so they can last longer and sometimes move much further
over water than over land. The combination of heat and moisture, along with the right
wind conditions, can create a hurricane.
Hurricanes are enormous, and they can range in size from 300-600 miles wide and about
10 miles high. They typically have a lifespan of about 10 days. The wind speed of a
hurricane is 75 miles per hour or more. Between 40 mph and 74 mph winds, the storm is
called a tropical storm.
How a Hurricane Forms
Hurricanes form in tropical regions where there is warm water (at least 80 degrees
Fahrenheit / 27 degrees Celsius), moist air and converging equatorial winds. Most
Atlantic hurricanes begin off the west coast of Africa, starting as thunderstorms that
move out over the warm, tropical ocean waters. A thunderstorm reaches hurricane status
in three stages:
 Tropical depression - swirling clouds and rain with wind speeds of less than
38 mph (61.15 kph)
 Tropical storm - wind speeds of 39 to 73 mph (54.7 to 117.5 kph)
 Hurricane - wind speeds greater than 74 mph (119 kph)
It can take anywhere from hours to several days for a thunderstorm to develop into a
hurricane. Although the whole process of hurricane formation is not entirely understood,
three events must happen for hurricanes to form:
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A continuing evaporation-condensation cycle of warm, humid ocean air
Patterns of wind characterized by converging winds at the surface and strong,
uniform-speed winds at higher altitudes
 A difference in air pressure (pressure gradient) between the surface and high
altitude
Warm, moist air from the ocean surface begins to rise rapidly. As this warm air rises, its
water vapour condenses to form storm clouds and droplets of rain. The condensation
releases heat called latent heat of condensation. This latent heat warms the cool air
aloft, thereby causing it to rise. This rising air is replaced by more warm, humid air from
the ocean below. This cycle continues, drawing warmer, moist air into the developing
storm and continuously moving heat from the surface to the atmosphere. This exchange
of heat from the surface creates a pattern of wind that circulates around a centre. This
circulation is similar to that of water going down a drain.
"Converging winds" are winds moving in different directions that run into each other.
Converging winds at the surface collide and push warm, moist air upward. This rising air
reinforces the air that is already rising from the surface, so the circulation and wind
speeds of the storm increase. In the meantime, strong winds blowing at uniform speeds at
higher altitudes (up to 30,000 ft / 9,000 m) help to remove the rising hot air from the
storm's centre, maintaining a continual movement of warm air from the surface and
keeping the storm organized. If the high-altitude winds do not blow at the same speed at
all levels -- if wind shears are present -- the storm loses organization and weakens.
High-pressure air in the upper atmosphere (above 30,000 ft / 9,000 m) over the storm's
centre also removes heat from the rising air, further driving the air cycle and the
hurricane's growth. As high-pressure air is sucked into the low-pressure centre of the
storm, wind speeds increase.
ITCZ
Near the equator, from about 5° north and 5° south, the northeast trade winds and
southeast trade winds converge in a low pressure zone known as the Intertropical
Convergence Zone or ITCZ. Solar heating in the region forces air to rise through
convection which results in an over-supply of precipitation. The ITCZ is a key
component of the global circulation system.
Weather stations in the equatorial region experience precipitation up to 200 days each
year, making the equatorial and ITC zones the wettest on the planet. The equatorial
region lacks a dry season and is constantly hot and humid.
The location of the ITCZ varies throughout the year and while it remains near the
equator, the ITCZ over land ventures farther north or south than the ITCZ over the oceans
due to the variation in land temperatures. The location of the ITCZ can vary as much as
40° to 45° of latitude north or south of the equator based on the pattern of land and ocean.
In Africa, the ITCZ is located just south of the Sahel at about 10°, dumping rain on the
region to the south of the desert. The Intertropical Convergence Zone has been called the
doldrums by sailors due to the lack of horizontal air movement (the air simply rises with
convection). The ITCZ is also known as the Equatorial Convergence Zone or
Intertropical Front.
There's a diurnal cycle to the precipitation in the ITCZ. Clouds form in the late morning
and early afternoon hours and then by 3 to 4 p.m., the hottest time of the day,
convectional thunderstorms form and precipitation begins.
Position of the ITCZ during July and January
Variation in the location of the Intertropical convergence zone drastically affects rainfall
in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than
the cold and warm seasons of higher latitudes. Longer term changes in the inter-tropical
convergence zone can result in severe droughts or flooding in nearby areas.
Within the ITCZ the average winds are slight, unlike the zones north and south of the
equator where the trade winds feed in. Early sailors named this belt of calm the
doldrums because of the inactivity and stagnation they found themselves in after days of
no wind.
Hurricane Hazards
One of the most dramatic, damaging, and potentially deadly events that occur is a
hurricane.
Hurricanes are products of the Tropical Ocean and atmosphere. Powered by heat from the
sea, they are steered erratically by the easterly trade winds and the temperate westerly
winds, as well as by their own energy. As they move ashore, they bring with them a
storm surge of ocean water along the coastline, high winds, tornadoes, torrential rains,
and flooding.
Each year on average, ten tropical storms develop over the Atlantic Ocean, Caribbean
Sea, or Gulf of Mexico. About six of these typically strengthen enough to become
hurricanes. Many of these remain over the ocean with little or no impact on the
continental United States. However, about five hurricanes strike the United States
coastline every three years. Of these five, two will be major hurricanes measuring a
category 3 or higher (defined as having winds above 111 miles per hour) on the SaffirSimpson Scale. The Saffir-Simpson Hurricane Scale is a 1-5 rating based on the
hurricane's present intensity. This is used to give an estimate of the potential property
damage and flooding expected along the coast from a hurricane landfall. Wind speed is
the determining factor in the scale, as storm surge values are highly dependent on the
slope of the continental shelf and the shape of the coastline, in the landfall region. Note
that all winds are using the U.S. 1-minute average. These storms can end up costing our
nation millions, if not billions, of dollars in damages.
During a hurricane, homes, businesses, public buildings, and infrastructure may be
damaged or destroyed by many different storm hazards. Debris can break windows and
doors, allowing high winds and rain inside the home. In extreme storms (such as
Hurricanes Hugo, Andrew and Katrina), the force of the wind alone can cause
tremendous devastation, as trees and power lines topple and weak elements of homes and
buildings fail. Roads and bridges can be washed away and homes saturated by flooding.
Destructive tornadoes can also be present well away from the storms centre during
landfall. Yet, storm surge alone poses the highest threat to life and destruction in many
coastal areas throughout the United States and territories. And these threats are not
limited to the coastline -- they can extend hundreds of miles inland, under the right
conditions.
Minimising the effects of hurricanes:
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Seeding – the addition of ice in the form of iodide
Hazard resistant designs
Planning and preparedness
Public education on hurricanes
Proper designing of settlements