Transcript Hurricanes - Geog

Hurricanes
What Are They?
• Hurricanes are intense hazards that bring heavy
rainfall, strong winds and high waves, and cause other
hazards such as flooding and mudslides.
• Hurricanes are characterised by enormous quantities
of water.
• This is due to their origin over moist tropical seas.
• High intensity rainfall and large totals of up to 500 mm
in 24 hours invariably cause flooding.
• The path of the hurricane is erratic; hence it is not
always possible to give more than 12 hours´notice.
• This is insufficient for proper evacuation measures.
Development
• Hurricanes develop as intense low-pressure systems over
tropical oceans.
• Winds spiral rapidly around a calm central area known as the
eye.
• The diameter of the whole hurricane may be as much as 800
km, although the very strong winds that cause most of the
damage are found in a narrower belt, up to 300 km wide.
• In a mature hurricane, pressure may fall to as low as 880-970
millibars.
• This, and the strong contrast in pressure between the eye
and the outer part of the hurricane, lead to strong gale-force
winds.
Movement
• Hurricanes move excess heat from low latitudes to higher
latitudes.
• They normally develop in the westward flowing air just
north of the equator (known as the easterly wave).
• They begin life as small-scale tropical depressions, localised
areas of low pressure that cause warm air to rise.
• These trigger thunderstorms which persist for at least 24
hours and may develop into tropical storms, which have
greater wind speeds of up to 117 km/h (73 mph).
• However, only about 10% of tropical disturbances ever
become hurricanes, storms with wind speeds above 118
km/h (above 74 mph).
Formation
• Sea temperatures must be over 27 degrees C.
• Warm water gives off large quantities of heat
when it is condensed – this is the heat which
drives the hurricane.
• The low-pressure area has to be far enough away
from the equator so that the Coriolis Force (the
force caused by the rotation of the earth) creates
rotation in the rising air mass – if it is too close to
the equator, there is insufficient rotation and a
hurricane would not develop.
Tornadoes
IB SL
What Are They?
• Most tornado activity (70%) is found in the
Great Plains of the USA.
• They form as a result of intensive convective
rainfall systems and are highly unpredictable.
Formation
• Different air types meet and due to their different
properties, do not mix. In the USA warm moist air
with its origins in the Caribbean meets colder air that
has originated in the northern Rockies. There is a
vast contrast between the two, and massive
instability results. A tornado can form if:
• The two meeting air masses are conditionally
unstable.
• Low level air has a relative humidity of over 65%.
• A low level southerly jet stream exists in the humid
air.
Characteristics
They are unpredictable, have extreme
low pressure at their centre, winds of
over 50m per sec.
Impacts
• These are considerable.
• The drop in pressure van lead to explosions,
trains are de-railed roofs removed, and flying
debris leads to further damage.
• Entire communities can be flattened.
Tropical Cyclones
IB SL
These Include...
•
•
•
•
Hurricanes
Typhoons
Cyclones
Willy Willies
• And they are pretty much the same storm but
are given different (local) names due to their
geographical location.
Location
What Are They?
• They are extremely powerful low-pressure
systems, and are believed to be responsible for a
greater number of deaths than any other natural
hazard.
• They are generated over the Atlantic and Pacific
oceans and are most common in autumn when
ocean temperatures are at there highest.
• There is some evidence to suggest that their
frequency and severity is increasing as a result of
global warming.
Formation Of Hurricanes
• Warm oceans with surface temperatures in excess of 27
degrees, and a deep layer of water to 60m.
• A location between 5degrees north and south of the equator.
(Without this the corriolis force is not sufficient to create the
spinning motion characteristics of hurricanes).
• Relatively stable and uniform atmospheric conditions of
temperature, humidity and pressure. In the upper
troposphere air drawn in at lower altitudes must be able to
escape.
• Relative humidity of over 60% to provide sufficient energy to
power the hurricane.
• Little change of horizontal wind with height.
• Existing cyclonic spinning of winds in the lower troposphere.
Hurricane Structure
• The eye is 20-60km across, existing in centre
of the hurricane where conditions are calm
and clear.
• Intense rainfall due to rapidly cooling and
rising air. Amount can be up to 25cm a day.
• Strong winds. In order to be classified as a
hurricane they must average 119km per hour.
Impact Of Hurricanes
This can be huge although
MEDC's are usually more
equipped to cope.
Activity 1
Using Advanced Geography P170-172, answer
the following questions…
1. What conditions are needed for hurricanes to
develop?
2. Why is the threat of hurricanes decreasing?
(Use the “Increase In Hurricanes” Times
Article).
3. What areas are at risk of hurricane damage
and why?
Activity 2
Using Advanced Geography P170-172, answer
the following questions…
1. What are the differences between tornadoes
and hurricanes?
2. What conditions are needed for them to
develop?
3. How are tornadoes distributed throughout
the U.S.A?
4. How can they be managed?