Elisha Nuttall, of Hillview Christian School, asks :-

Why do hurricanes occur not very often and why only in some parts of the world?

Andy Sturman, a climatologist at the University of Canterbury's Geography Department, responded.

The reason that tropical cyclones (called hurricanes in the Atlantic and typhoons in the Pacific) do not occur very often is that several things need to happen at the same time for them to form. These include that the sea-surface temperature must be higher than 26C, wind speed and direction should be mostly constant with height, and the atmosphere should be unstable (when a strong decrease of temperature with height encourages the air to rise). It should also be noted that they cannot develop within about 5 degree latitude of the equator, as it is difficult for the air to rotate in this region. The final thing required is an existing large weather disturbance, such as a group of thunderstorms, which can grow together into a hurricane. It is not often that all of these things occur at the same time to create a hurricane, so that they are not common.

These factors also control where hurricanes can occur. Hurricanes are very strong storms that need a lot of energy to form. They get this energy from the warm air and ocean surface in the tropics, so that they only form over the sea and when they move over land they start to die. That is why they occur mostly over the tropical ocean areas of the world. Sometimes they wander from the tropics towards New Zealand, but because the sea surface temperatures are colder in our region, they weaken very quickly. However, we can sometimes be affected by old tropical cyclones that have enough energy left to cause damage to our buildings, roads and farmland, such as Cyclone Bola (1988) and Wahine (1968).