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.

This can be huge although MEDC's are usually more equipped to cope. 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… What conditions are needed for hurricanes to develop? Why is the threat of hurricanes decreasing? (Use the “Increase In Hurricanes” Times Article). What areas are at risk of hurricane damage and why?

Activity 2 Using Advanced Geography P170-172, answer the following questions… What are the differences between tornadoes and hurricanes? What conditions are needed for them to develop? How are tornadoes distributed throughout the U.S.A? How can they be managed?