This page covers the following weather related phenomenon: severe
thunderstorms, tornadoes, tropical cyclonic storms (hurricanes, typhoons,
and cyclones), nor'easters, drought, and the causes and effects of El
Water and Heat
Water has one of the highest heat capacities of all known
substances. This means that it takes a lot of heat to raise the
temperature of water by just one degree. Water thus absorbs a
tremendous amount of heat from solar radiation, and furthermore, because
solar radiation can penetrate water easily, large amounts of solar energy
are stored in the world's oceans.
Further energy is absorbed by water vapor as the latent heat of
vaporization, which is the heat required to evaporate water or change it
from a liquid to a vapor. This latent heat of vaporization is given
up to the atmosphere when water condenses to form liquid water as
rain. If the rain changes to a solid in the form of snow or ice, it
also releases a quantity of heat known as the latent heat of fusion.
Thus, both liquid water and water vapor are important in absorbing heat
from solar radiation and transporting and redistributing this heat around
Due to general atmospheric circulation patterns, air masses containing
differing amounts of heat and moisture move into and across North
America. Polar air masses, containing little moisture and low
temperatures move downward from the poles. Air masses that form over
water are generally moist, and those that form over the tropical oceans
are both moist and warm. Because of the Coreolis effect due to the Earth's
rotation, air masses generally move from west to east. But, because
of the differences in moisture and heat, the collision of these air masses
can cause instability in the atmosphere.
Fronts and Mid-latitude
Different air masses with different temperatures and moisture content, in
general, do not mix when they run into each other, but instead are
separated from each other along boundaries called fronts.
When cold air moving up from the poles encounters warm moist air moving
down from the tropics, a cold front develops and the warm moist air rises
above the cold front. This rising moist air cools as it rises
causing the condensation of water vapor to form rain or snow. Note
that the cold air masses tend to circulate around a low pressure center in
a counterclockwise fashion in the northern hemisphere and clockwise in the
southern hemisphere. Such circulation around a low pressure center
is called a mid-latitude cyclone.
When warm air moving northward meets the cooler air to the north, a warm
front forms. As the warm air rises along a gently inclined warm
front, clouds tend to form, and can also cause rain, but rain is less
likely because the warm front is not as steep as a cold front.
If the rapidly moving cold front overtakes the warm front, an occluded
front forms, trapping warm air above a layer of cold and cool air.
Mid-latitude cyclones and their associated fronts are responsible for such
severe weather conditions as thunderstorms, snow storms and associated
hail, lightening, and occasional tornadoes.
Thunderstorms occur anywhere that warm moist air has absorbed enough heat
to make the air less dense than the surrounding air. This commonly occurs
along cold fronts, but can occur in other places as well, particularly
where daytime heating forms hot air near the Earth's surface. As the warm
moist air rises it begins to cool and water begins to condense into tiny
droplets that form clouds.
Condensation of the water droplets in the clouds releases the latent heat
of evaporation, adding heat to the rising air, thus decreasing its density
and allowing it to rise to higher levels in the atmosphere. This rising
air, called an updraft, starts to build clouds to heights of up to 6
Further rising and cooling within the clouds causes more condensation, as
well as the formation of ice crystals which release further latent heat
and build cloud heights up to 12 km. Eventually the water droplets
and ice crystals in the clouds become so large that they can no longer be
supported by the uprising air mass, and they begin to fall forming rapid
downdrafts on the leading edge of the cloud.
In the mature stage of thunderstorm development updrafts and downdrafts
operate side by side within the cloud. This is the most dangerous
stage of a thunderstorm because of the high winds accompanying the
downdrafts, the heavy rain, as well as thunder, lightening, and possible
hail and tornado development.
Eventually the cloud reaches the dissipating stage as the downdrafts drag
in so much cool dry air that it prevents further updrafts of warm moist
air. With lack of updrafts of warm moist air, the cloud begins to
dissipate and eventually it stops raining
Thunderstorms can form as single cells, with only one cloud mass, or as
multiple cells, with several clouds moving along a similar path.
Although thunderstorms can occur nearly everywhere, they show an unequal
distribution over Australia. Areas that receive the highest number
of thunderstorms are areas where warm moist air moves southward from the
Hail is a rain of semi-spherical, concentrically layered ice balls th`t
are dropped from some thunderstorms. Hail rarely kills people, but
it does heavy damage to agriculture, roofs, and automobiles. The
conditions necessary to form hail during a thunderstorm are:Large
thunderstorms with high cloud tops formed from hot moist rising air. Upper
level cold air with a large temperature contrast between the upper level
air and the rising moist air. Strong updrafts within the thunderstorm to
keep hailstones suspended in the cloud while layers of ice are added to
the stones. When the stones become too large to be suspended by the
cloud they fall to the surface as hail. Although thunderstorms are most
common along the Gulf coast, thunderstorms that produce hail are more
common in the mid-continent region where temperature contrasts between
upper air masses and the rising hot air are greater. Hail can range
from pea-size stones to grapefruit size stones.
Lightning is the electrical discharge from clouds that causes thunder.
Thus, lightning occurs from all thunderstorms. lightning is the
major cause of forest fires and results in many deaths. Deaths from
lightning have a similar distribution to the occurrence of thunderstorms,
with Florida having both the most thunderstorms and the most lightning
Deaths from lightning usually occur outdoors as seen in the following
Locations of Lightning Strikes (USA)
Open fields, sport fields
On boats & in water-related activity
On tractors & heavy road equipment
Lightning is caused by an imbalance of electrical charge between and
within clouds and the ground. Most of you have simulated lightning
by walking across a carpet on a dry day and then touching a metal object
like a door knob. Static electrical charge builds on your body and is
discharged to the door knob as a bolt of electricity. During the buildup
of a thundercloud, charged particles of water droplets and ice become
separated in the cloud. Positively charged particles are moved to the top
of the cloud and negatively charged particles are moved to the bottom of
The thundercloud then begins to interact with the ground. The
negatively charged part of the cloud induces a build-up of positive
charges on the ground. Similarly beneath the upper positively
charged part of the cloud negative charges are induced in the ground
below. When the difference in voltage between the oppositely charged parts
of the cloud and the ground become great enough, the electricity is
discharged as a bolt of lightning.
Note that lightning can travel from the cloud to the ground, from the
ground to the cloud, and within the cloud itself. lightning travels
at speeds of about 160,000 km/h (100,000 miles per hour), and usually
includes several strokes that all occur within about 1/2 of a
second. The discharge of electricity during a bolt of lightning
heats up the air surrounding the bolt causing rapid expansion of the
air. It is this rapid expansion of the air that causes the sound we
Tornadoes are funnel shaped clouds that are associated with
thunderstorms. Tornadoes have wind velocities higher than hurricanes
(up to 500 km/hr [318 miles per hour]), but affect a much smaller area
than hurricanes. Tornadoes are mostly found in inland areas with high
- A tornado develops within a severe thunderstorm when there is an
excessive amount of vertical wind shear. Vertical wind shear is when
upper level winds are blowing at a high velocity relative to lower level
winds. Prior to the development of the thunderstorm strong high
level winds blowing to the west initiates a spinning flow near the Earth's
surface. This spinning flow takes the form of an invisible
horizontally oriented cylinder. As the thunderstorm develops, strong
updrafts of warm air lifts this rotating air into a more vertical position
within the thundercloud, causing part of the thundercloud to rotate around
the a vortex in a counterclockwise direction. Tornadoes form within
this rotating air, usually at the rear flank of the thunderstorm, and
extend down from the thundercloud occasionally reaching the surface.
They travel at velocities between stationary and 110 km/hr, with cyclonic
wind speeds up to 500 km/hr as noted above. The diameters of
tornadoes range from a few tens of meters up to 1.5 km. They do not
often remain in contact with the ground for long periods of time, but can
skip across the surface as the thunderstorm moves along.
- The intensity of a tornado is classified by the Fujita tornado intensity
scale shown in the table below
Fujita Tornado Intensity Scale
F0 - F1
F2 - F3
F4 - F5
Gale tornado Light damage. Some damage to chimneys; break
branches off trees; push over shallow-rooted trees; damage sign
Moderate tornado Moderate damage. The lower limit is the
beginning of hurricane wind speed; peel surface off roofs; mobile
homes pushed off foundations or overturned; moving autos pushed
off the roads.
Significant tornado Considerable damage. Roofs torn off frame
houses; mobile homes demolished; boxcars pushed over; large trees
snapped or uprooted; light-object missiles generated.
Severe tornado. Severe damage. Roofs and some walls torn off
well-constructed houses; trains overturned; most trees in forest
uprooted; heavy cars lifted off the ground and thrown.
Devastating tornado Devastating damage. Well-constructed houses
leveled; structures with weak foundations blown off some distance;
cars thrown and large missiles generated.
Incredible tornado. Incredible damage. Strong frame houses
lifted off foundations and carried considerable distance to
disintegrate; automobile sized missiles fly through the air in
excess of 100 meters(109 yds); trees debarked; incredible
phenomena will occur.
319 mph - MACH 1
(the speed of sound)
The maximum wind speeds of tornadoes are not expected to reach
the F6 wind speeds.
- Tornadoes have occured throughout Australia. Because they are
associated with strong thunderstorms, the frequency of tornadoes is
closely related to the areas that have lots of thunderstorms, but also
occur predominantly in the interior and southern plains where cold
air from Anarctica encounters warm air moving southward. It is
these contrasting air masses and their common paths of circulation which
give rise to both the thunderstorms and the tornadoes. Tornadoes are most
common during the spring and summer months. They often occur as
swarms associated with cold fronts.
Tornado Damage - Tornado damage is caused by the high wind speed
and high difference in atmospheric pressure between the tornado and
its surroundings. The rotating winds can knock down weaker
structures, and the extremely low pressure inside the tornado
generates strong pressure differences between the inside and outside
of buildings. This pressure difference causes roofs to be lifted
and removed. The high winds pick up smaller objects including
small structures, animals, people, cars, and especially mobile homes,
and can carry these objects up to several kilometers. The debris
picked up by the winds become rapidly moving projectiles that can
become lethal when hurled against a human body.
Tornado Prediction and Warning - Tornadoes cannot be predicted with
precision. However, when strong thunderstorm activity is
detected, a tornado watch is generally issued for all areas that may
fall in the path of the thunderstorm. Doppler radar can detect
rotating motion within a thunderstorm and when this is detected, or a
tornado is actually observed, a tornado warning is issued for all
areas that may fall in the path of the thunderstorm.
Go at once to the basement, storm cellar, or the lowest level of
If there is no basement, go to an inner hallway or a smaller inner
room without windows, such as a bathroom or closet.
Get away from the windows.
Go to the center of the room. Stay away from corners because they
tend to attract debris.
Get under a piece of sturdy furniture such as a workbench or heavy
table or desk and hold on to it. Use arms to protect head and neck.
If in a mobile home, get out and find shelter elsewhere.
If at work or school:
Go to the basement or to an inside hallway at the lowest
Avoid places with wide-span roofs such as auditoriums, cafeterias,
large hallways, or shopping malls.
Get under a piece of sturdy furniture such as a workbench or heavy
table or desk and hold on to it. Use arms to protect head and neck.
If possible, get inside a building.
If shelter is not available or there is no time to get indoors,
lie in a ditch or low-lying area or crouch near a strong building.
Be aware of the potential for flooding. Use arms to
protect head and neck.
If in a car:
Never try to out drive a tornado in a car or truck. Tornadoes can
change direction quickly and can lift up a car or truck and toss it
through the air.
Get out of the car immediately and take shelter in a nearby
building. If there is no time to get indoors, get out of the car and
lie in a ditch or low-lying area away from the vehicle. Be aware of
the potential for flooding.
After the tornado:
Help injured or trapped persons Give first aid when
appropriate. Don't try to move the seriously injured unless they are
in immediate danger of further injury. Call for help.
Turn on radio or television to get the latest emergency
Stay out of damaged buildings. Return home only when authorities
say it is safe.
Use the telephone only for emergency calls.
Clean up spilled medicines, bleaches, or gasoline or other
flammable liquids immediately.
Leave the buildings if you smell gas or chemical fumes.
Take pictures of the damage--both to the house and its
contents--for insurance purposes.
Remember to help your neighbors who may require special
assistance--infants, the elderly, and people with disabilities.
Inspecting utilities in a damaged home:
Check for gas leaks--If you smell gas or hear a blowing or
hissing noise, open a window and quickly leave the building. Turn
off the gas at the outside main valve if you can and call the gas
company from a neighbor's home.
If you turn off the gas for any reason, it must be turned back on
by a professional.
Mitigation of Tornado Disasters
Because tornadoes can strike anywhere and anytime there are thunderstorms,
the best mitigation is for an educated populace to be aware of the
conditions under which tornadoes develop and heed any tornado watches or
warnings that are issued by a responsible agency, and practice the tornado
safety tips listed above. The only other mitigation that can reduce
the damage produced by tornadoes is building codes that require structures
to be constructed with extra reinforcing of wood frames and masonry.
Hurricanes are massive tropical cyclonic storm systems with winds
exceeding 119 km/hr (74 miles/hour). The same phenomena is given
different names in different parts of the world. In the western
Pacific they are called typhoons, and in the southern hemisphere they are
called cyclones. But no matter where they occur they represent the
same process. Hurricanes are dangerous because of their high winds,
the storm surge produced as they approach a coast, and the severe
thunderstorms associated with them. Although death due to hurricanes has
decreased in recent years due to better methods of forecasting and
establishment of early warning systems, the economic damage from
hurricanes has increased as more and more development takes place along
coastlines. It should be noted that coastal areas are not the only
areas subject to hurricane damage. Although hurricanes loose
strength as they move over land, they still carry vast amounts of moisture
onto the land causing thunderstorms with associated flash floods and
Origin of Tropical Cyclones
When a cold air mass is located above an organized cluster of tropical
thunderstorms, an unstable atmosphere results. (This is called a tropical
wave). This instability increases the likelihood of convection, which
leads to strong updrafts that lift the air and moisture upwards, creating
an environment favorable for the development of high, towering clouds. A
tropical disturbance is born when this moving mass of thunderstorms
maintains its identity for a period of 24 hours or more. This
is the first stage of a developing hurricane.
Surface convergence (indicated by the small horizontal arrows in the
diagram below) causes rising motion around a surface cyclone (labeled as
"L"). The air cools as it rises (vertical arrows) and condensation
occurs. The condensation of water vapor to liquid water releases the
latent heat latent heat condensation into the atmosphere. This heating
causes the air to expand, forcing the air to diverge at the upper levels
(horizontal arrows at cloud tops)
Since pressure is a measure of the weight of the air above an area,
removal of air at the upper levels subsequently reduces pressure at
the surface. A further reduction in surface pressure leads to
increasing convergence (due to an higher pressure gradient), which
further intensifies the rising motion, latent heat release, and so on.
As long as favorable conditions exist, this process continues to build
upon itself. When cyclonic circulation begins around the central
low pressure area, and wind speeds reach 62 km/hr (39 mi/hr) the
disturbance is considered a tropical storm and is given a name.
When wind speeds reach 119 km/hr (74 mi/hr) it becomes a
hurricane. Note that all tropical waves, disturbances, or
storms do not necessarily develop into hurricanes.
To undergo these steps to form a hurricane, several environmental
conditions must first be in place:
Warm ocean waters (of at least 26.5°C [80°F]) throughout about the
upper 50 m of the tropical ocean must be present. The heat in
these warm waters is necessary to fuel the tropical cyclone.
The atmosphere must cool fast enough with height, such that it is
potentially unstable to moist convection. It is the thunderstorm
activity which allows the heat stored in the ocean waters to be
liberated for tropical cyclone development.
The mid-troposphere (5 km [3 mi]), must contain enough moisture to
sustain the thunderstorms. Dry mid levels are not conducive to the
continuing development of widespread thunderstorm activity.
The disturbance must occur at a minimum distance of at least 500 km
[300 mi] from the equator. For tropical cyclonic storms to occur,
there is a requirement that the Coreolis force must be present.
Remember that the Coreolis effect is zero near the equator and
increases to the north and south of the equator. Without the
Coreolis force, the low pressure of the disturbance cannot be
There must be a pre-existing near-surface disturbance that shows
convergence of moist air and is beginning to rotate. Tropical cyclones
cannot be generated spontaneously. They require a weakly organized
system that begins to spin and has low level inflow of moist air.
There must be low values (less than about 10 m/s [20 mph]) of
vertical wind shear between the surface and the upper troposphere.
Vertical wind shear is the rate of change of wind velocity with
altitude. Large values of vertical wind shear disrupt the
incipient tropical cyclone by removing the rising moist air too
quickly, preventing the development of the tropical cyclone. Or, if a
tropical cyclone has already formed, large vertical shear can weaken
or destroy it by interfering with the organization around the cyclone
Hurricanes thus commonly develop in areas near, but not at the equator, as
shown in the diagram below. As they move across the oceans their
paths are steered by the presence of existing low and high pressure
systems, as well as the Coreolis force. The latter force causes the storms
to eventually start turning to the right in the northern hemisphere and to
the left in the southern hemisphere.
Note that about 12% of all tropical cyclones develop in the Atlantic
Ocean. Those that begin to form near the coast of Africa are often
referred to as "Cape Verde" hurricanes, because the area in which they
develop is near the Cape Verde Islands. 15% of all tropical cyclones
develop in the eastern Pacific Ocean, 30% develop in the western Pacific
Ocean, 24% in the Indian Ocean both north and south of the equator, and
12% develop in the southern Pacific Ocean. It is notable that
essentially no tropical cyclones develop south of the Equator in the
Hurricane Structure (Northern
Because the converging winds spiral inward toward the central low pressure
area, the winds rotate in a clockwise direction around the central low in
the southern hemisphere (counterclockwise in the northern
hemisphere). As these winds spiral inward they draw in the
thunderclouds around the storm, creating the spiral rain bands that are
clearly visible on satellite images
As the winds converge toward the central core, they spiral upwards,
sending warm moist air upwards. As this air rises, it cools and releases
its latent heat into the atmosphere to add further energy to the storm.
The winds spiraling around this central core create the eye of the
tropical cyclone and eventually spread out at high altitudes.
Eventually, cool air above the eye begins to sink into the central core.
This dry descending air within the eye gives the core a clear, cloud free
sky, with little to no wind.
Since the main source of energy for the storm is the heat contained in the
warm tropical and subtropical oceans, if the storm moves over the land, it
is cut off from its source of heat and will rapidly dissipate.
(Northern Hemisphere)Winds spiraling counterclockwise (in the northern
hemisphere) into the eye of the hurricane achieve high velocities as they
approach the low pressure of the eye. The velocity of these winds is
called the hurricane-wind velocity. The central low pressure center
of the eye also moves across the surface of the Earth as it is pushed by
regional winds. The velocity at which the eye moves across the
surface is called the storm center velocity. Thus, when we consider
the velocity of winds around the hurricane we must take into account both
the wind velocity and the storm center velocity. Depending on the side of
the hurricane, these velocities can either add or subtract. In the
example at the left, the hurricane is traveling north with a storm center
velocity of 30 km/hr, and a hurricane-wind velocity of 150 km/hr. On
the right hand side of the storm both velocities are to the north so the
total wind velocity is 180 km/hr (30 + 150). On the left hand side
of the storm, however, the wind is blowing to the south. Thus, since
the storm is moving in the opposite direction to the winds, the velocities
subtract and the total wind velocity is 120 km/hr (150 - 30). This
is an important point. Winds are always stronger on the right side
of a moving hurricane in the northern hemisphere. The opposite is
true in the southern hemisphere, since winds circulate in a clockwise
direction, the winds are stronger on the left-hand side of the storm in
the southern hemisphere).
Tropical Cyclone Size
Since winds spiral inward toward the central low pressure area in the eye
of a hurricane, hurricane-wind velocity increases toward the eye.
The distance outward from the eye to which hurricane strength winds occur
determines the size of the hurricane. Winds in the eye wall itself
have the highest velocity and this zone can extend outward from the center
to distances of 16 to 40 km. Hurricane force winds (winds with
velocities greater than 119 km/hr) can extend out to 120 km from the
center of the storm. The largest tropical cyclone recorded, Typhoon
Tip, had gale force winds (54 km/hr) which extended out for 1100 km
in radius in the Northwest Pacific in 1979.
The smallest, Cyclone Tracy, had gale force winds that only extended
50 km in radius when it struck Darwin, Australia, in 1974. There is
very little association between hurricane intensity (either measured
by maximum sustained winds or by central pressure) and size.
Hurricane Andrew is a good example of a very intense tropical cyclone of
small size. It had 922 mb central pressure and 230 km/hr sustained
winds at landfall in Florida, but had gale force winds extending out to
only about 150 km from the center.
Hurricane Intensity and Frequency
Once a hurricane develops, the Saffir-Simpson Scale is used to classify a
hurricane's intensity and damage potential. There are five possible
categories. Category 1 storms are more common than category 5
storms. In a typical year, there may be many category 1
storms, but category 5 storms occur very infrequently.
Saffir-Simpson Hurricane Damage-Potential Scale
(inches of mercury)
some damage to trees, shrubbery, and unanchored mobile
major damage to mobile homes; damage buildings' roofs, and blow
destroy mobile homes; blow down large trees; damage small
completely destroy mobile homes; lower floors of structures near
shore are susceptible to flooding
extensive damage to homes and industrial buildings; blow away
small buildings; lower floors of structures within 500 meters of
shore and less than 4.5 m (15 ft) above sea level are
Ships at sea transmit weather reports that help meteorologists
locate centers of low pressure that may develop into tropical
Images from weather satellites, which are collected every 30
minutes, are then scanned to look for any development or growth of the
disturbance. In particular, the images are examined to detect
any rotational development of the storm, an indication that it may be
approaching tropical storm strength.
If a tropical storm or hurricane is detected and appears to pose a
threat to land areas, observation airplanes are sent to examine the
storm. Such planes fly into the storm at an altitude of about
3,000 meters or lower. The plane collects data on wind speed,
air pressure, and moisture content by dropping devices called
dropsondes into the storm. These dropsondes transmit the
meteorological data continuously has they fall to the ocean surface,
and thus provide information on the vertical structure of the
storm. In addition, radar devices carried on the plane collect
data about the intensity of the rainfall and wind velocities.
The planes fly completely through the storm, passing though the eye,
sometimes making several passes. The data collected give
meteorologists a 3 dimensional picture of the structure of the storm.
Satellite images and radar from land based stations allow
scientists to track the position of the storm and report it to all
agencies that may be affected if the storm makes landfall
Changes in Cyclone
Tracks and Intensities
Because cyclones are influenced by large-scale air masses, they sometimes
move along rather erratic paths. Cyclones are especially influenced
by the strength and direction of upper level winds. As noted above,
strong upper level winds create a vertical wind shear that cause the top
of the cyclone to be sheared of and result in the loss of strength of the
storm. The erratic nature of a hurricanes path often make it
difficult to predict where and when it will make landfall prior to several
hours before it actually does make landfall. In the lower latitudes,
near the equator, cyclones generally are pushed by the westerly (easterly
in Northern Hemisphere) trade winds and have storm center velocities that
are relatively low (8 to 32 km/hr). As they move southward
(northward in the Northern Hemisphere), storm center velocities generally
increase to greater than 50 km/hr. If the cyclone encounters a low
pressure trough between two high pressure centers, it is steered into the
trough and follows it along a southwestward trend (northeastward in the
Northen Hemisphere), increasing its velocity as it does so.
Angle of Cyclone Approach to Coast
The amount of damage that occurs when a hurricane approaches a coast
depends on the angle of approach.
Two extreme examples of Hurricanes illustrate this point.
A hurricane that moves along the coast has a coast-parallel hurricane
track. From such a track extensive damage would occur along the coastline
closest to the storm, with bands of lesser damage extending inland.
Since this track (upper diagram) has the most intense winds offshore (on
the right side of the hurricane), the coast would not feel the highest
wind velocities. Thunderstorm activity associated with this track
would be most severe on the northern side of the storm, since the spiral
rain bands would be feeding off the moist air above the ocean.
A hurricane that approaches the perpendicular to the coast has a
coast-normal track. Such a storm would produce extreme damage all
along the right-hand side of its track, with bands of decreasing damage
occurring both to the left and right of the track. Furthermore, as the
storm approached the coast areas to right hand side of the storm would
receive the heaviest thunderstorm activity, since the rain bands would be
feeding off the moist oceanic air.
Enhanced by High Tides
Cyclones cross the Queensland coast most often in January, Februrary and
a time that corresponds to the highest tides of the year. The
combination of Storm Surges caused by the wind and pressure differences
and high tides results in exceptionally hig water
Heavy winds produced by hurricanes push the ocean in front of them. As
this water gets pushed into the shallow zones along the coastline sea
level rises. Since the storm surge is driven by the winds, the
height of the rise in sea level is related to the velocity of the
wind. For a moving storm the greater winds occur on the right side
of the storm (in the northern hemisphere). Sea level also rises
beneath the eye of the storm due to the low pressure in the eye.
But, the surge generated by this low pressure is usually much less than
the wind-driven surge. The height of the storm surge depends on wind
speed, the shape of the coastline, and variations in the water depth along
the coast line. Height also depends on tidal cycles. If a
storm approaches the coast during high tide, the storm surge will be
higher than if it approaches during low tide. Category 5 tropical
cyclones can produce storm surges in excess of 6m (20 feet).
The highest storm surge measured, 12.8 m (42 feet) occurred in 1899 in
Because the storm surge occurs ahead of the eye of the storm, the surge
will reach coastal areas long before the hurricane makes landfall.
This is an important point to remember because flooding caused by the
surge can destroy roads and bridges making evacuation before the storm
Since thunderstorms accompany hurricanes, and these storms can strike
inland areas long before the hurricane arrives, water draining from the
land in streams and estuaries may be impeded by the storm surge that has
pushed water up the streams and estuaries.
It is also important to remember that water that is pushed onto the land
by the approaching storm (the flood surge) will have to drain off after
the storm has passed. Furthermore after passage of the storm
the winds typically change direction and push the water in the opposite
direction. Damage can also be caused by the retreating surge, called
the ebb surge.
Along coastal areas with barrier islands offshore, the surge may first
destroy any bridges leading to the islands, and then cause water to
overflow the islands. Barrier islands are not very safe places to be
during an approaching hurricane!
Hurricanes cause damage as a result of the high winds, the storm surge,
heavy rain, and tornadoes that are often generated from the thunderstorms
as they cross land areas. Strong winds can cause damage to
structures, vegetation, and crops, as described in the Saffir-Simpson
scale discussed previously. The collapse of structures can cause
death. The storm surge and associated flooding, however, is what is most
responsible for casualties. Extreme cases of storm surge casualties have
occurred as recently as 1970 and 1990 in Bangladesh.Bangladesh is an area
with high population density and with over 30% of the land surface less
than 6 m above sea level. In 1970 a cyclone struck Bangladesh during
the highest high tides (full moon). The storm surge was 7 m (23 ft.)
high and resulted in about 400,000 deaths. Another cyclone in 1990
created a storm surge 6 m high and resulted in 148,000 deaths.
The amount of damage caused by a tropical cyclone is directly related to
the intensity of the storm, the duration of the storm (related to its
storm-center velocity, as discussed above), the angle at which it
approaches the land, and the population density along the coastline. The
table below shows how damages are expected to increase with increasing
tropical storm category. Like the Richter scale for earthquakes,
damage does not increase linearly with increasing hurricane category.
Median Damage (1990 Dollars)
Predicting Hurricane Frequency and Intensity
As discussed above, modern methods of weather forecasting involving
satellites, radar, etc. allow accurate tracking of the development and
paths of hurricanes. In addition, computer models have been
developed that enable the prediction of storm surge levels along the
coast, given data on wind velocity, wind distribution, and storm center
velocity. Computer models have also been developed to predict the
paths the storms will take and have met with moderate success. Accurate
forecasting of storm tracks is more problematical because of the numerous
variables involved and the erratic paths hurricanes sometimes take.
Drought and Famine
In contrast to the exceptional weather conditions we have discussed so
far, which tend to bring high quantities of rainfall, a drought is a
period of time of abnormal dryness in a region. Droughts are slow onset
hazards that may lead to secondary effects like famine In Australia,
droughts are caused when upper level air flow creates a long lasting high
pressure ridge over the central region. The high pressure
causes cyclonic flow bringing dry air down to the surface. As this
air sinks in the high pressure areas it warms and the relative humidity
decreases further. Thus the air is so undersaturated with water that
it sucks up even more water from the surface. Such persistent high
pressure zones block the flow of warm moist air preventing storms that
would normally bring rain.
One of the dramatic manifestations of the interaction between the oceans
and the atmosphere and its effects on both climate and weather is the
Southern Oscillation, one of the consequences of which is El Niño.
The Southern Oscillation is a back and forth variation in atmospheric
pressure between a high pressure system normally located off the west
coast of South America and a low pressure system normally located in the
western Pacific near Indonesia and Australia. In the U.S. El Niño
conditions result in heavy rains, flooding, landslides, and
tornadoes in greater than normal amounts because El Niño conditions drive
abnormal amounts of moist warm air across North America. El Niño causes
flooding in Peru, as well as drought and fires in Indonesia and Australia.
The phenomena is manifested by the arrival of warm water off the coast of
Peru around Christmas time, and thus is called El Niño (Spanish for the
boy child) because it arrives at this time. An El Niño event occurs every
2 to 7 years with various degrees of strength. Some El Niño events
are more intense than others, and the condition lasts from 18 to 24
months. The following table lists the years of El Niño events.
El Niño Years
- Under "normal" conditions the easterly trade winds, driven
by the pressure difference between the eastern Pacific high and the
western Pacific low and blowing toward the equator, push warm water
toward the equator and across the Pacific Ocean toward Australia and
Indonesia. This causes a pool of warm water to form near the equator in
the western Pacific. It also causes the thermocline (the boundary
between warm waters in the upper layers of the ocean and the cold
deep waters below) to move closer to the surface off the coast of South
America, bringing nutrient-rich waters to surface by upwelling. Such
nutrient-rich waters help sustain large fish populations. The
upwelling cold water cools the atmosphere above, and prevents rain clouds
from forming off the coast of Peru.
The warm water pushed to the west by the trade winds, heats as it flows
along the equator, so that on arrival in the western Pacific heat is added
to the overlying atmosphere causing it to rise, form clouds, and produce
extensive rainfall. The moisture depleted upper atmosphere then circulates
back to east where it descends off the coast of South America contributing
to the dry conditions. During periods of exceptionally strong trade winds
the upwelling of cold water off the South America cools the water even
further creating a condition called La Niña (girl child).
"El Niño Conditions"
- During El Niño periods there is a weakening of the easterly trade
winds and the warm waters of the western Pacific are pushed toward the
east. This causes the thermocline in the eastern Pacific to sink,
preventing the upwelling of cold waters from below, depleting the waters
in nutrients, and thus leading to starvation of fish populations. As
the warm water shifts eastward so does the development of atmospheric
disturbances that lead to upwelling of the atmosphere to form
thunderstorms. Rising bodies of moist air thus occur closer to
the coast of the Americas, leading to increased storminess, not only in
South America, but in North America as well. These low pressure
systems that develop in the eastern Pacific can move over the continent
and cause severe weather as noted above.
In addition, they create upper level winds that tend to shear the tops off
of developing tropical storms and hurricanes in the Atlantic Ocean and
Gulf of Mexico, leading to a decrease in the number of intense tropical
cyclones that develop in these regions.
La Niña conditions have about the opposite effect of El Niño
conditions. i.e. better fishing harvests off the west coast of
South America, drier conditions in North and South America, more
hurricanes in the Atlantic, and wetter conditions in Australia and
Over the past 50 years, the oscillation of warm water back and forth
across the tropical Pacific Ocean has created El Niño conditions 31% of
the time, La Niña conditions 23% of the time, and "normal" conditions 46%
of the time.
Prediction of El Niño
As can be seen from the data presented above, the southern oscillation
which creates the El Niño condition has operated throughout the last
century. Archeological evidence from South America indicates that
the oscillation has been operating for thousands of years. Still, it
has only been in recent years that atmospheric and ocean scientists have
become aware of the phenomenon, and then only because particularly strong
El Niños occurred in the years 1982-83 and 1997-98 causing considerable
damage from natural disasters in North America.
The frequency of El Niño events and the intensity of the events is not
statistically predictable. In other words we do not as yet know when
the next El Niño will occur. This is due to the relatively
short amount of historical data currently available. Still, the 1997-98
event and its intensity were predictable several months beforehand because
measurements of sea surface temperature from satellites and instrument
buoys in the Pacific Ocean were able to identify the movement warm surface
waters from east to west across the Pacific Ocean.
Thus, future El Niño events will likely be predictable several months
before they actually develop. This could have important economic
consequences. For example, knowing that an El Niño event is coming
could result in farmers in normally dry parts of South America preparing
the soil for a good crop months in advance because of the expected wetter
weather in the months ahead. Fishermen could begin preparing for a
poor fishing harvest off the coast of South America. In terms of
Natural disasters, Peru was much better prepared for the 1997-98 El Niño
and constructed storm drains and stockpiled emergency supplies, probably
saving thousands of lives.
It is important to remember that because the southern oscillation shifts
back and forth, some areas receive beneficial aspects of the phenomenon
while other areas receive adverse aspects.