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Pressure system
A pressure system is a region of the Earth's atmosphere where air pressure is
unusually high or low. High and low pressures form (and die) constantly due to
thermodynamic interactions of the atmosphere and water in oceans, lakes, and other
bodies of water.
Low pressure system
The components of storms are attracted to regions of low pressure. For this reason,
heavy precipitation and overcast conditions are often associated with low-pressure
systems. Due to the Coriolis Effect, low-pressure systems often develop cyclonic
properties: in the northern hemisphere, winds around the system move counter
clockwise, and in the southern hemisphere they move clockwise. Low pressure
systems, additionally, often become junctures of fronts. You may have seen a weather
map with a red L on it. This red L means there is a low pressure system over that area
of the map.
Most of history's most powerful storms, such as the 1993 North American storm
complex and all tropical cyclones have been low-pressure cyclonic systems.
Tornadoes invariantly have very strong local low-pressure systems at their vortices.
Air moves into a Low pressure system. It pushes any air that was there upwards.
High-pressure system
High pressure systems are associated with clear, cool weather. Around high-pressure
systems, winds flow clockwise in the northern hemisphere, counter clockwise in the
southern hemisphere. You may have seen a weather map with a blue H on it. This
blue H means there is a high pressure system over that area of the map.
In the northern winter, high-pressure systems (called Canadian highs or Arctic air
masses) often migrate to mid-latitude regions such as the North American upper
Midwest, New England, and northern Europe. These create cold snaps where
unseasonably cold and sunny weather are observed. Cold snaps often follow winter
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warm spells, where temperatures may be as high as 10°C to 20°C (50-68 °F), and
often happen suddenly. The most dramatic Arctic cold snaps, observed in the central
regions of North America involve temperature drops of 25°C (45°F) or more in a few
hours.
Arctic highs, alone, rarely trigger precipitation because of the cloudless weather they
produce. However, in combination with other weather-making systems, the cold air
they bring can produce massive snowstorms.
High pressure usually means good weather.
The air moving away from the High pressure system leaves a "hole" to be filled,
so air from above sinks into that "hole".
Anticyclones
The cloudy rainy weather of low-pressure depressions is due to rising air, which is
most pronounced near frontal regions. The anticyclone on the other hand is produced
by a large mass of descending air. This subsidence takes place throughout a depth of
the atmosphere up to 12km. Such subsidence means that the air is very stable and
atmospheric pressure is high. In addition, winds associated with an anticyclone are
usually very light if present at all, especially close to the centre of the high-pressure
system.
Subsidence warms the air by compression. Any clouds present quickly evaporate as
the temperature of the air rises above its dew point. For this reason, anticyclones
usually bring fine, dry and settled weather, particularly in the summer.
Sometimes, subsidence and compression of the air can produce a temperature
inversion at one or two thousand metres above the ground. Such phenomena act as
caps to rising air heated by the ground under the influence of the Sun, preventing
extensive air cooling and cloud formation. Unfortunately, if the air is moist below the
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temperature inversion, a dreary formless layer of cloud can form which becomes
difficult to disperse owing to the light winds. Such debilitating weather is common in
winter when the Sun’s radiation is too weak to burn off the cloud layer.
Winter anticyclones, if clear of cloud, bring with them further problems. A short
cloudless day is the forerunner of a long night with more radiation cooling than a lowangle Sun can counteract the next day. The second night of cooling therefore starts
with a lower air temperature than the first. Such conditions, if persistent, can lead to
successive nights of frost, which become progressively harder. When the air is
particularly moist, cooling at night soon results in fog. Britain in particular can
experience episodes of anticyclonic fog from late September through to May.
Anticyclones move, but not quite in the same purposeful way as travelling
depressions. They nudge their way into position and can be incredibly stubborn about
leaving, perhaps persisting for weeks, diverting depressions to different routes. Such
persistent anticyclones are known as "blocking highs". In winter they can lead to long
spells of very cold weather, especially if their airflow comes from Russia and Siberia.
In summer they can lead to long hot spells and sometimes drought.
A ridge of high pressure is a wedge-shaped extension of an anticyclone or belt of high
pressure. The weather associated with ridges is similar to that in an anticyclone. In
temperate latitudes as in the British Isles, ridges of high pressure often occur between
two depressions and move with them. They give rise to intervals of fair weather
between the cloud and rain of the low-pressure systems.
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Depressions
Depressions, sometimes called mid-latitude cyclones, are areas of low pressure
located between 30° and 60° latitude. Depressions develop when warm air from the
sub-tropics meets cold air from the Polar Regions. There is a favourite meeting place
in the mid-Atlantic for cold polar air and warm sub-tropical air. Depressions usually
have well defined warm and cold fronts, as the warm air is forced to rise above the
cold air. Fronts and depressions have a birth, lifetime and death; and according to the
stage at which they are encountered, so does the weather intensity vary.
A depression appears on a synoptic (weather) chart as a set of closed curved isobars
with winds circulating anticlockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere due to the rotation of the Earth. The warm and cold fronts
associated with depressions bring with them characteristically unsettled weather.
Depressions vary from between 200 and 2,000 miles in diameter; they may be deep
when pressure at their centre is very low and the isobars are tightly packed, or shallow
when less well developed.
A depression develops like the propagation of a wave in water. Initially, a uniform
boundary or front exists between cold air pushing southwards and warm air pushing
northwards (in the Northern Hemisphere). A wave-shaped distortion may appear on
the front, and a small low-pressure centre develops at the crest of the wave. In the
immediately surrounding area the pressure begins to fall. A disturbance of this kind is
called a wave depression. As the "wave" develops, a warm sector of air forms
bounded by the warm and cold fronts, which begins to tie over the engulfing cold air.
Both the warm and cold fronts originate from the centre of the depression. On the
ground, sudden changes in the wind direction may be experienced when fronts pass
by.
Wave depressions can grow off the tail ends of primary cold fronts. The depression so
formed is then called a secondary depression. New centres may also develop from
occluded fronts within the primary depression. The secondary system can then
become the main system, and the primary occluded front becomes caught up in the
developing circulation, effectively becoming a third front.
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Schematic development of a Depression
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:
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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.
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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.
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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
Saffir-Simpson 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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

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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
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ATTEMPTS TO CONTROL HURRICANES
Prior to the work done in the area of storm modification by the South Florida
Company Dyn-O-Mat, there had not been any attempts to modify hurricanes since
1980, when the NOAA decided to end Project Stormfury.
However, for a period of over 30 years, scientists attempted to find a solution to the
hurricane problem through storm modification. Early projects included one led by Dr.
Irving Langmuir and a group of scientists at General Electric. The project, called
Project Cirrus, focused on seeding hurricanes with dry ice.
Despite some apparent success with the project, not a lot of research done until after
the stormy years of 1954 and 1955 when President Eisenhower appointed a special
committee to look into storm modification.
Overcoming a lack of enthusiasm and interest in the subject from some scientists, the
program took off in the early 1960s with Project Stormfury, which was headed by Dr.
Robert H. Simpson, director of the National Hurricane Research Labs.
PROJECT CIRRUS
Efforts in storm modification go as far back as the late 1940s when Dr. Irwin
Langmuir began exploring the idea of using ice crystals to weaken storms such as
hurricanes. In 1947, he organized a team of scientists from General Electric, and
brought his idea to fruition with Project Cirrus.
The highlight of this effort occurred on October 13, 1947 when Langmuir and his
team had a Navy plane fly into a hurricane off the coast of Georgia, and drop a
payload of dry ice into it.
The hope of using dry ice was to release energy from the storm's inner core. and thus
weaken the hurricane itself. It was originally considered a success. After the cloud
seeding had taken place, the hurricane suddenly changed direction, and made landfall
near the coastal city of Savannah, Georgia.
However, about a decade later, a scientist named Mook recognized that the storm's
sudden change in direction was not caused by the seeding, but rather the upper level
steering winds in the vicinity of the hurricane.
MID FIFTIES STORMINESS
Nevertheless, Langmuir's work had generated some enthusiasm. This enthusiasm was
particularly strong among officials in the United States Government. In the years
following Langmuir's experiment, a number of powerful hurricanes made landfall in
the United States.
They included Carol, Edna, and Hazel in 1954, and the first billion dollar hurricane,
Hurricane Diane in 1955. All four of these storms were Category Four strength
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according to the Saffir-Simpson Scale, and caused extensive damage from Florida all
the way to New England and even Canada.
In response to these devastating storms, President Eisenhower appointed a
Presidential Commission to investigate the idea of storm modification. Despite the
lack of enthusiasm for the idea, Congress extended the life of this special committee
for another two years in 1956, and by the end of the decade there were scientists that
were ready to take another stab at attacking the hurricane.
PROJECT STORMFURY
In the early 1960s, two significant developments in the area of storm modification
energized the quest to weaken and eradicate the hurricane. The first was the
development of a new cloud seeding process by Dr. Robert H. Simpson, who was the
director of the National Hurricane Research Labs in Miami, Florida.
Simpson, who eventually went on to develop the classification system known as the
Saffir-Simpson Scale, theorized that hurricanes could be weakened by releasing
frozen nuclei or particles of Silver Iodide compound (AgI) into the wall clouds of a
hurricane or tropical storm, and imbalance the forces within the storm system.
Simultaneously, a group at the Navy Weapons Center in California improved seeding
technology by developing new seeding generators that would be able to release large
amounts of crystals into tropical storms and hurricanes.
As a result, Project Stormfury was born in 1962. Prior to that, a test case had already
been done on Hurricane Esther in September, 1961, and with some success. The
project team of workers from both the U.S. Weather Bureau and the Navy was able to
decrease the sustained winds in the storm by ten percent.
After the creation of Project Stormfury, the new team assembled put together a
reformulated idea on how to attack a hurricane. They proposed that a hurricane could
be weakened by converting the supercooled water within the deep clouds of the storm
is converted to ice, the hurricane's vertical column of air would be warmed and the
storm would be weakened.
The team didn't have many chances to work on this new idea over the next eight years
because there weren’t any storms that were far enough from land, and the team didn't
want to risk litigation in case something went wrong. The team did have several
chances though.
First, in 1963, they were able to conduct tests on Hurricane Beulah, but with only
marginal success. Then, in 1965, the team considered seeding Hurricane Betsy, but
due to the close proximity to Puerto Rico and other Caribbean islands, and the storm's
erratic motion, the team did not go through with it, and Betsy ended up slamming into
South Florida and causing severe damage.
Betsy was the last major hurricane to make a direct hit on South Florida before 1992
when Hurricane Andrew devastated Homestead, Florida and ended up causing $27
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billion dollars in damage. Finally, in 1969, Project Stormfury was going to have a
significant test case.
It was on the heels of Hurricane Camille barrelling into the Gulf Coast regions of
Mississippi and Alabama when Hurricane Debby was seeded on a couple of occasions
over the two day period of August 19-20, 1969. Each time the storm was seeded,
sustained winds were reduced significantly.
The first time, winds dropped 31 percent while the second time, they only dropped 15
percent. The apparent success with Debby helped fuel new projects, and
improvements in technology. In particular, Hurricane Hunter aircraft, which went up
dramatically during the 1970s.
Ultimately though, Project Stormfury was cancelled in 1980 since the team was
unable to clearly ascertain whether or not the seeding efforts were really causing
storms to weaken, or the systems just became victims of the environment around
them. Nevertheless, the work done did bear some fruit as forecasters and scientists
alike were able to learn a great deal from their research, and it has helped them
improved forecasting accuracy.
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