3.2+Characteristics,+distribution+and+causes+of+tropical+storms

2. What are the distribution, characteristics and causes of tropical revolving storms?
In other words:
 * Where do you find them?
 * What are they like?
 * What makes them happen?
 * **Also how do you measure the ferocity of a storm? - see animation at the bottom of the page**

Tropical revolving storms which have a wind speeds reaching 119 km/hour have a variety of names depending on where you find them.
 * Where do you find them?.**
 * They are **hurricanes** of the United States and the Caribbean
 * They are **tropical cyclones** on the India Ocean
 * They are **Typhoons** in the Pacific

They only usually occur within a band from 20 Deg North to 20 Deg South of the equator. They do not happen at all times of the year but are most common between mid summer and autumn. So for the Southern Hemisphere this is between December and April while in the Northern Hemisphere, it is between June and October. Those storms whose wind speed do not reach these higher limits but have wind speeds reaching 60 km/hr are usually called tropical storms.

Tropical storms are a type of severe spinning(rotating) storm that occurs over the ocean near the tropics. As they gather speed, the spinning of the earth, the Coriolis effect, pushes them westward and away from the equator. If they reach land there are several things that you would notice
 * What are they like?**
 * High Winds** : The extreme wind speeds in tropical cyclones are directly related to the steep pressure gradients near the cyclone centre. Wind damage increases exponentially with increased wind speed. In severe cyclones the maximum sustained winds can approach 200 km/h with short period gusts closer to 300 km/h. Such violent winds can devastate natural vegetation and all but the strongest man-made structures.
 * [[image:3.2B_Before_After.png align="left"]]Storm Surge** :As a tropical cyclone crosses the coast the combination of low pressure near the centre and strong onshore winds can produce a large increase in sea level, called a storm surge. This can bring about sea waves breaking into areas not normally affected, producing absolute destruction of buildings or other facilities. In highly vulnerable areas such as the Ganges River Delta of Bangladesh, where a large population inhabits the fertile flats close to sea level, the results can be disastrous. Storm surges in this area have resulted in death tolls exceeding 100,000.
 * Flood Rains and Landslides** : In some areas of the world (especially the Philippines, China and Japan), torrential rain brought by tropical cyclones can produce landslides in mountainous terrain, sometimes with disastrous results. In early November 1991 on the Philippines island of Leyte an estimated 6,000 people died and a further 20,000 were left homeless when flood rains in the wake of Typhoon Thelma caused flash flooding, landslides and a burst dam. Hardest hit was the port city of Ormoc where a huge mudslide occurred on nearby hillsides made much worse by extensive logging operations that had removed the protection of the trees.

Hurricanes all start over the sea where, for reasons that we do not fully understand, a patch of ocean has a surface temperature of more than 26 Deg C, that is at least 50 metres deep and at least 500 km from the equator.
 * What causes them?**

The hot air, being lighter than the cooler air around it, rises taking a lot of water vapour with it. As the water vapour rises up it cools to form big cumulus clouds. This creates low pressure at sea level. Wherever you have low pressure, air with higher pressure tends to move in to replace it. The winds that travel around the Earth (called the Trade Winds) at this point are pulled in to fill the gap left by the rising air. But due to the turning effect of the earth, air does not move straight into a low pressure zone but whirls in around and towards it – just like the water going down the plug hole – for the same reason. Remember the world is turning at about 1700 km an hour! We have the hot air rising, the air coming in from outside and coming under the cloud in a spin, picking up more water vapour and spiralling upwards as it warms. This has 2 effects; the first is that the storm clouds begin to be pulled into a spin by the incoming wind. The second effect is that the spinning storm is pulled outward – leaving a low pressure funnel at the centre. Now way up high there is cold air which is under higher pressure so this is sinks down into the centre – at the bottom this begins to warm and gets pulled in to the warm spinning stormy mass, and so more cold air follows in after it. As everything is spinning faster and faster the storm begins to drift sideways because of the trade winds. Not only that but this huge bundle of energy depresses the sea level under it, so there is a backup ridge of water all around it – this gives rise to surges both before and after the hurricane has passed. So the hurricane brings surges and high winds and heavy rain all together! This why they cause so many problems to the countries they cross. Now that the animation! ** Make sure you explore the last slide fully, to find out about the strength of a rotating storm and the damage that it can do! ** media type="file" key="BBC hurricane animation.swf" width="468" height="468" align="center"