How do hurricanes form?

It's hurricane season, the time of year when countries in the tropics are at risks of hurricanes causing devastation.

Hurricane Katrina satellite image -  Gulf of Mexico 2005

For a hurricane to form, a mixture of conditions is needed:
- Warm ocean water - the water in the ocean must be at least 27' C (80'F) to substantial depths.
- Westward moving area of low air pressure - these are called tropical disturbances
- Constant wind-speeds between 0-22km above sea-level

Hurricane Formation:
1. The warm-sea water evaporates and rises, causing low-pressure areas, and as this water condenses it form clouds. The condensation releases heat energy to power the storm.
2. As the air rises, it cools and sinks in the middle and this forms the eye (a period of calm which typically spans a diameter of 30-50km in the centre of the hurricane)

3.  The force which causes moist air in the hurricane to spin is called the 'Coriolus Effect', a phenomenon related to the earth's spin which means that fluids swirl anti-clockwise in the Northern hemisphere and clockwise in the Southern. It is for this reason that hurricanes do not form directly on the equator, because the Coriolis effect is too weak for the hurricane to spin, so they usually form 5-20' north and south of the equator.
4. As hurricanes move across warm ocean, they suck in more moist air, making the hurricane grow. On the right the is a scale which shows how hurricanes are classified and as a hurricane gains more power, it can be reclassified to a higher category hurricane which has the ability to cause more damage.

Hazards of Hurricanes:

• Winds - category 5 hurricanes can cause wind speeds higher than 156mph and the highest ever recorded sustained winds caused by cyclone (the name for a hurricane in India and Australia) Olivia with wind speeds of 253.5. Extremely high wind speeds can cause extensive structural damage and the risk of flying debris. 
• Storm surges -  the storm's low pressure can cause water to rise well above predicted tides (8cm rise in level for every 10mb fall in pressure). Winds then drive these waves onshore, where the storm surges causes flooding, extreme damage to infrastructure and buildings, and salt-water contamination. Hurricane Katrina (2005) caused storm surges of 25 to 28 ft above normal tide level along the coast of Mississippi and New Orleans. 
• Heavy Rainfall - this is due to rapidly cooling moist air and can cause flooding and mudslides. The heavy flooding can make rescue much more difficult, putting many people at risk.

Storm surge hitting Atlantic City cause by hurricane Sandy 2012

The dissipation of a hurricane is caused by a number of factors:

- High winds above the hurricane's vertical system are called shearing winds as they can break up and disorganize the hurricane, weakening it.

- If the hurricane passes of colder sea water, it loses power supplied by warmer water and so the hurricane begins to lose momentum and decay.

- One a hurricane makes landfall it ultimately begins to dissipate because the landmass causes friction and sucks out the hurricanes moisture.