TR PICAL CYCL NES

Figure 1 Destruction caused by Typhoon Haiyan

A tropical cyclone is the generic term for a low pressure system over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and winds at low levels circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere). The whole storm system may be five to six miles high and 300 to 400 miles wide, although sometimes can be even bigger. It typically moves forward at speeds of 10-15 mph, but can travel as fast as 40 mph. At its very early and weak stages it is called a ‘tropical depression’. When the winds reach 39 mph it is called a ‘tropical storm’. If the wind should reach 74 mph or more the tropical storm is called a ‘hurricane’ in the Atlantic and the north-east Pacific or a ‘typhoon’ in the north-west Pacific. In other parts of the world, such as the Indian Ocean and South Pacific the term ‘cyclone’ or ‘tropical cyclone’ is used.

How do tropical cyclones form?

In the tropics there is a broad zone of low pressure which stretches either side of the equator. The winds on the north side of this zone blow from the north-east (the north-east trades) and on the southern side blow from the south-east (south-east trades).

Within this area of low pressure the air is heated over the warm tropical ocean. This air rises in discrete parcels, causing thundery showers to form. These showers usually come and go, but from time to time, they group together into large clusters of thunderstorms. This creates a flow of very warm, moist, rapidly rising air, leading to the development of a centre of low pressure, or depression, at the surface.

Figure 3: Where tropical cyclones (hurricanes) form and the direction they move in.

There are various trigger mechanisms required to transform these cloud clusters into a tropical cyclone. These trigger mechanisms depend on several conditions being ‘right’ at the same time. The most influential factors are:

a source of warm, moist air derived from tropical oceans with sea surface temperatures normally in the region of, or in excess, of 27 °C;winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form;winds which do not vary greatly with height – known as low wind shear. This allows the

Figure 2 Typhoon Haiyan

storm clouds to rise vertically to high levels;sufficient distance from the equator to provide spin or twist.

The Coriolis force caused by the rotation of the Earth helps the spin of this column of rising air. The development of the surface depression causes an increase in the strength of the trade winds. The spiraling winds accelerate inwards and upwards, releasing heat and moisture as they do so.

As the depression strengthens it becomes a tropical storm and then a hurricane or typhoon. A mature hurricane or typhoon takes the form of a cylinder of deep thundercloud around a centre that is relatively free from clouds. There is a relatively small area of intense horizontal winds at the surface, often well over 100 m.p.h., while air rises strongly above, maintaining the deep cumulonimbus clouds.

Further aloft at about six miles, the cloud tops are carried outwards to give thick layer clouds due to the outward-spiraling winds leaving the tropical cyclone core. At the centre of the tropical cyclone, air is subsiding, which makes it dry and often cloud free, and there is little or no wind at the surface. This is called the eye of the storm.

Textbook – pages 70 – 74

In your own words explain how a tropical cyclone (hurricane) forms?

Tropical cyclones require water of over 27OC to form. The warm water heats up the air above to create an area with very low pressure. The air rises and quickly cools creating thick, dense clouds and heavy rain.

Tropical cyclones require energy from the warm water so they weaken once the reach land

The rising air is spun by the Coriolis Force which is caused by the Earth rotating.

They are first classified as tropical storms until they reach wind speeds of 119 km/h when they are called tropical cyclones, hurricanes or typhoons.

Draw diagram 3.10 (page 71) labels must be included.

The Saffir-Simpson scale

Category Wind speed

(Km/h)

Pressure (mb)

Storm surge (m)

Damage

1 119-153 <980 1.0-1.7 Minor – trees, mobile homes

2 154-177 979-965 1.8-2.6 Roofs and windows of buildings, small boats broken from moorings, flooding

3 178-209 964-945 2.7-3.8 Structural damage to buildings, flooding over a metre up to 10km inland

4 210-249 944-920 3.9-5.6 Major – destroys building, beaches and floods up to 10km inland

5 <250 >920 Over 5.7 Catastrophic – destruction up to 5 metres above sea level, mass evacuation needed.

Characteristics

On average, tropical storms last for 10 days, but the bigger ones last for 4 weeks. 3 main types of damage are wind, storm surges and floods. The winds can reach over 250 km/h and destroys trees, crops, buildings, transport, power and communications. The storm surges can reach over 5.7m along the coast and are huge waves that hit the land. The floods are caused by torrential rainfall which can last hours or days. The flood and rain can also cause landslides and mudslides. They can reach over 100km wide and travel at 50 kph. The outside of the storms are the dangerous bits, however the eye (center) is calm because of the hot air rising.

Damage caused by the tropical cyclone also depends on whether the country is an HIC or LIC. HIC civilians can protect their house and evacuate in time. Warnings are often picked up and broadcasted and recovery after the hurricane. LIC’s often have disastrous consequences with poor infrastructure, less money to rebuild and a slower economy to recover.

Figure 4 Hurricane Sandy

Figure 5 Hurricane Matthew 2016 Haiti