What is a hurricane?

A hurricane is an area of low pressure over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and sustained winds near the surface of at least 74 m.p.h. (and stronger gusts) 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 m.p.h., but can travel as fast as 40 m.p.h.

The term 'hurricane' is usually restricted to the Atlantic and north-east Pacific region. In the north-west Pacific they are known as 'typhoons' and elsewhere simply as 'cyclones'. If sustained wind speeds are between 39 m.p.h. and 73 m.p.h. they are known as a 'tropical storms'. Collectively, they are often referred to as 'tropical cyclones'.

Tropical cyclones

Causes of hurricanes

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.

There are various trigger mechanisms required to transform these cloud clusters into a tropical storm and eventually into a hurricane. These trigger mechanisms depend on several conditions being 'right' at the same time. The most influential factors are:

  1. 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;
  2. winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form;
  3. winds which do not vary greatly with height - known as low wind shear. This allows the storm clouds to rise vertically to high levels;
  4. sufficient distance from the equator to provide spin or twist.

Tropical cyclone facts

How do hurricanes get their names?

Various meteorological organisations have responsibility for names and employ different conventions. Some cycle through a set number of lists starting at "A" each year. Some continuously cycle through names without reverting to a new list each year and some use a unique list each year. The names of all tropical storms, hurricanes, typhoons and cyclones and are known in advance. In the areas with cyclical lists a name will be retired if the cyclone bearing it was particularly notorious and caused loss of life.

Tropical cyclone names

Do hurricanes occur in the United Kingdom?

Hurricanes are tropical features and require sea temperatures much higher than those around the UK, even in the summer. Hence, hurricanes cannot form at our latitudes. However, we are sometimes affected by deep depressions that were originally hurricanes which have moved to higher latitudes, such as ex-Hurricane Ophelia in 2017. Such depressions are classified as 'ex-hurricanes' or 'extra-tropical cyclones' since they have changed their prime energy source from the warm ocean surface to the clash of warm tropical and cold polar air - a process known as extratropical transition. An ex-hurricane can sometimes still have hurricane strength winds (greater than 73 m.p.h.) even though it is no longer classified as a hurricane. Also, hurricanes have their strongest winds close to their centre whilst the strongest near surface winds in ex-hurricanes can often be far removed from the centre of the depression.

More on 'extratropical cyclones'