Cyclones in Detail

Features and energy

Definitions Cyclones - mix of multiple thunderstorms forming a huge spiraling system

Cyclone is the generic name for a massive rotating storm system, at their strongest they are called hurricanes (Caribbean and America), Typhoons (Pacific Ocean), Cyclones (India and Australia) and Willy Willies in Australia

Hurricanes, Cyclones and Typhoons

Deep areas of low pressure

In the tropics Forms over Warm sea surfaces

Strong and violent winds and heavy rain to adjacent land

Name dependant on location! Essentially the same

Cyclones/hurricanes/Typhoons

Formation - very little evidence as to exact method of formation

Must have - Warm tropical seas (>26 degrees) and considerable depth

In autumn - highest water temp

In trade wind belt - surface winds warm as the blow towards equator

Between 5 and 20 north or south

Basics

0. A tropical depression becomes a hurricane when its sustained recorded winds reach 74 mph.

Note: These systems are rated from 1 – 5 on the Saffir-Simpson Scale ("S/S Scale") developed in the early 1970s by Herbert Saffir, Consulting Engineer, and Robert Simpson, then director of the National Hurricane Center.

Once formed Always move westwards - unpredictable courses swing towards poles when hit land (global heat transfer mechanism)

Can have diameter of 650 KM Needs temp, pressure and humidity to be uniform over a large area in the lower troposphere for a long period of time

Leads to strong surface winds and intense low pressure near the surface

To enable movement, must have continual heat to maintain rising air currents and moisture

Once mature develops the eye (30 -50KM, subsiding air, light winds, clear skies and high temperatures)

Once onto a land surface, temperature drops and the hurricane loses energy, has to overcome more friction and does not have enough moisture. Most hurricanes last 7 to 14 days

Pressure characterized by a centre of characterized by a centre of

extremely low surface air extremely low surface air pressure with a steep pressure pressure with a steep pressure gradient from the centre gradient from the centre outward producing a series of outward producing a series of closed concentric isobars . .

Sea-level air pressure varies Sea-level air pressure varies from near the atmospheric mean from near the atmospheric mean value of 1013 mb in the value of 1013 mb in the environment (at a radius of environment (at a radius of 1000 km from the storm centre) 1000 km from the storm centre) to below 900 mb in extreme to below 900 mb in extreme cases. cases.

The lowest minimum pressure The lowest minimum pressure ever recorded in the centre of ever recorded in the centre of a hurricane was 870 mb.a hurricane was 870 mb.

Hurricane Anatomy The main parts of a hurricane are the rain bands on its outer edges, the eye, and the eye wall.

Air spirals in toward the centre in a counter-clockwise pattern, and out the top in the opposite direction.

In the very centre of the storm, air sinks, forming the cloud-free eye of high pressure

Storm Surges Are associated with typhoons/hurricanes/cyclones

Pressure drop n the weather system allows the sea water to rise above normal

Water piled up in large waves by high winds

Can be up to 6 meters above usual levels

Hurricane Size Typical hurricanes are about 300 miles wide

Size is not necessarily an indication of hurricane intensity

Hurricane-force winds can extend outward to about 25 miles from the storm centre of a small hurricane and to more than 150 miles for a large one.

The area over which tropical storm-force winds occur is even greater ranging as far out as almost 300 miles from the eye of a large hurricane.

Hurricane Circulation In the northern hemisphere, hurricane winds circulate around the centre in a counter-clockwise fashion.

This means that the wind direction at your location depends on where the hurricane's eye is.

A boat on the northern edge of the orange area in Hurricane Fran (right) would experience winds from the east, while a boat on the southern edge would have westerly winds

Hurricane path Depends on complex interactions

between the storm with its own internal circulations and the earth's atmosphere.

The air in which the hurricane is embedded is a constantly moving and changing "river" of air.

Other features in that flow, such as high and low pressure systems, can greatly alter the speed and the path of the hurricane.

In turn, it can modify the environment around the storm.

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Hurricane speed Typically, a hurricane's forward speed averages around 15-20 mph.

However, some hurricanes stall, often causing devastatingly heavy rain.

Others can accelerate to more than 60 mph. Hurricane Hazel (1954) hit North Carolina on the morning of 15 October; fourteen hours later it reached Toronto, Canada where it caused 80 deaths.

Some hurricanes follow a fairly straight course, while others loop and are difficult to forecast

Prediction Regular occurrence , clearly defined hurricane seasons

America - Aug to October Australia and pacific India/Bay of Bengal Therefore clear expectation of when will occur

Once formation has started clearly visible on satellite imagery

As they are so easy to spot, why are there such huge differences?