Hurricanes are tropical storms with wind speeds above 119 km/hr (74 mph).
Hurricanes are divided into 5 categories in the Saffir-Simpson scale.
Tropical storms form over warm ocean surfaces; normally at least 27˚C for a tropical depression to develop into a hurricane strength storm.
Warm ocean surfaces provide two things for the air above them:
This is important as the heat warms the air and the air starts to rise.
However to really understand what goes on we need to know that there is two types of heat:
Both of these provide the energy for a hurricane to form.
Tropical Storm Tracks 1985-2005
The map above shows the tracks of tropical storms and hurricanes.
There is a band with virtually no storm tracks over the Equator.
This is because there is not enough Coriolis force (the spinning force of the Earth) close to the equator.
The main things needed for Tropical Storms/Hurricanes are:
1. As the air above a sufficiently warm sea surface warms and rises
2. Rising air forms lower air pressure – see linked article 2.
3. Air moves in from surrounding areas winds.
4. Coriolis force spins the moving air, water vapour and clouds.
5. The spinning pushes more air, water vapour and clouds into the centre where the storm gets stronger and stronger.
6. As long as the hurricane stays over warm water it will continue to get stronger.
Hurricanes lose strength if the move over land or cooler ocean areas.
7. Hurricanes form bands of clouds rotating around a central eye.
8. Inside the eye conditions are calm.
9. The strongest winds are found within the eye wall.