Cyclones (Tropical) – Introduction


A hurricane is a type of tropical cyclone that is a warm-core, low pressure system without any “front” attached, that develops over the tropical or subtropical waters, and has an organized circulation.

The word hurricane comes from the Carib Indians of the West Indies, who called this storm a huracan. Supposedly, the ancient Tainos tribe of Central America called their god of evil “Huracan”. Spanish colonists modified the word to hurricane. Depending upon location, tropical cyclones have different names around the world. The various names are as follows:[1]

  • Atlantic/Eastern Pacific Oceans – hurricanes
  • Western Pacific – typhoons
  • Indian Ocean – cyclones

Regardless of what they are called, there are several favorable environmental conditions that must be in place before a tropical cyclone can form. These conditions are:[2]

  • Warm ocean waters (at least 80°F / 27°C) throughout a depth of about 150 ft. (46 m).
  • An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection.
  • Relatively moist air near the mid-level of the troposphere (16,000 ft. / 4,900 m).
  • Generally a minimum distance of at least 300 miles (480 km) from the equator.
  • A pre-existing near-surface disturbance.
  • Low values (less than about 23 mph / 37 km/h) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the change in wind speed with height.

Formation Regions of Tropical Cyclones

Radar Animation of Tropical Cyclone[9]

Given that sea surface temperatures need to be at least 80°F (27°C) for tropical cyclones form, it is natural that they form near the equator.

Tropical cyclones form between 5 and 30 degrees North latitude and typically move toward the west. This is due to the sufficient Coriolis Force, the force that causes the cyclone to spin. Sometimes the winds in the middle and upper levels of the atmosphere change and steer the cyclone toward the north and northwest. When tropical cyclones reach latitudes near 30 degrees North, they often move northeast.

There are seven tropical cyclone “basins” where tropical cyclones form on a regular basis:[3]

Tropical Cyclone Formation Regions with Mean Tracks[4]

These are seven tropical cyclone “basins” with dates between which the most activity occurs:[5]

Basin Dates of Activity
Atlantic basin
North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea
The Hurricane season is “officially” from 1 June to 30 November. Peak activity is in early to mid September. Once in a few years there may be a tropical cyclone occurring in May or December.
Northeast Pacific basin
Mexico to about the dateline
A broad peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September.
Northwest Pacific basin
From the dateline to Asia including the South China Sea
Occur all year round regularly though there is a distinct minimum in February and the first half of March. The main season goes from July to November with a peak in late August/early September.
North Indian basin
Including the Bay of Bengal and the Arabian Sea
A double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>74 mph / 119 km/h winds) occur almost exclusively from April to June and again in late September to early December.
Southwest Indian basin
From Africa to about 100°E
Beginning in late October/early November, reaching a double peak in activity-one in mid-January and one in mid-February to early March, and then ending in May.
Southeast Indian/Australian basin
100°E to 142°E
Beginning in late October/early November, reaching a double peak in activity-one in mid-January and one in mid-February to early March, and then ending in May. The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin’s lull.
Australian/Southwest Pacific basin
142°E to about 120°W
Begins in late October/early November, reaches a single peak in late February/early March, and then fades out in early May.

Sources of Tropical Cyclones

The seedlings of tropical cyclones, called “disturbances”, can come from:[6]

Easterly Waves: Also called tropical waves, this is an inverted trough of low pressure moving generally westward in the tropical easterlies. A trough is defined as a region of relative low pressure. The majority of tropical cyclones form from easterly waves.

West African Disturbance Line (WADL): This is a line of convection (similar to a squall line) which forms over West Africa and moves into the Atlantic Ocean. WADL's usually move faster than tropical waves.

Tropical Upper Tropospheric Trough (TUTT): A TUTT is a trough, or cold core low in the upper atmosphere, which produces convection. On occasion, one of these develops into a warm-core tropical cyclone.

Old Frontal Boundary: Remnants of a polar front can become lines of convection and occasionally generate a tropical cyclone. In the Atlantic Ocean storms, this will occur early or late in the hurricane season in the Gulf of Mexico or Caribbean Sea.

Other Types of Cyclones

An extra-tropical cyclone is a storm system that primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones (also known as mid-latitude or baroclinic storms) are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. Tropical cyclones, in contrast, typically have little to no temperature differences across the storm at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics.[7]

A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources, and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (usually greater than 60 n mi), and generally have a less symmetric wind field and distribution of convection.[8]



  1. NOAA – NWS – JetStream – Online School for Weather – Tropical Weather – Introduction:
  2. NOAA – NWS – JetStream – Online School for Weather – Tropical Weather – Introduction:
  3. NOAA – NWS – JetStream – Online School for Weather – Tropical Weather – Introduction:
  4. Image Source: [Accessed July 14, 2013]
  5. NOAA – NWS – JetStream – Online School for Weather – Tropical Cyclone Formation Regions:
  6. NOAA – NWS – JetStream – Online School for Weather – Tropical Weather – Introduction:
  7. NOAA Hurricane Research Division:
  8. NWS National Hurricane Center – Glossary of NHC Terms:
  9. Image Source: [Accessed: February 21, 2014]