A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, strong winds, and heavy rain. The term “hurricane” is used specifically for these powerful storms in the Atlantic Ocean and the Northeast Pacific Ocean. Because these systems require large amounts of heat and moisture, many assume they can form anywhere over warm tropical oceans. However, the geographical location where these storms begin is specific, governed by a delicate balance of atmospheric and oceanic conditions. This article explains why these rotating storms avoid the precise center of the planet and where they generally begin their journey.
The Coriolis Effect and Equatorial Formation
Tropical cyclones do not form precisely on the equator, or within approximately 5 degrees latitude north or south of it. The reason for this equatorial “dead zone” is the absence of a sufficient rotational force known as the Coriolis effect. This effect is an apparent force resulting from the Earth’s rotation, which acts to deflect moving air and water masses.
This deflective force is absolutely necessary to initiate and maintain the tight, organized spin of a developing low-pressure system. The strength of the Coriolis effect is zero at the equator and steadily increases toward the poles. Without this force, the air flowing into the low-pressure area near the surface cannot be forced into a rotating circulation pattern, preventing the formation of a cohesive storm structure.
While the waters near the equator are warm, the lack of sufficient Coriolis force means that any cluster of thunderstorms that forms cannot organize into a rotating system. Generally, the minimum distance required is about 300 miles from the equator for the Earth’s rotation to impart the necessary spin.
Necessary Environmental Conditions
The Coriolis force is only one part of the equation, as several other environmental factors must align for a tropical cyclone to develop. The most significant fuel source is warm ocean water, which must be at least 80°F (26.5°C). This temperature must also extend down to a depth of about 150 feet (46 meters) to ensure the storm does not pull up cooler water from below as it churns the ocean surface.
Another factor is low vertical wind shear, which describes how much the wind changes speed and direction with increasing altitude. High wind shear would tilt the storm’s vertical structure, tearing apart the developing core of thunderstorms and preventing the release of heat that powers the system. A storm requires winds to be relatively uniform throughout the atmosphere so the energy-releasing convection can rise straight up.
Finally, a tropical cyclone needs a pre-existing weather disturbance to act as a starting point for organization. This disturbance is often a tropical wave, a slow-moving area of low pressure and convergence. This initial circulation provides the platform for warm, moist air to begin rising and condensing, an action that releases latent heat to further warm the atmosphere and deepen the low-pressure center.
Where Tropical Cyclones Actually Form
Tropical cyclones typically form in a band located between 5 and 30 degrees latitude in both the Northern and Southern Hemispheres. This geographical zone represents the area where all the necessary ingredients are present. The water is consistently warm enough, and the distance from the equator is great enough to benefit from the Coriolis effect.
This latitude band also often features low wind shear and the presence of atmospheric disturbances, such as those associated with the Intertropical Convergence Zone. Once formed, these systems are steered by large-scale wind patterns, often tracking westward in the lower latitudes before curving toward the poles.
The regional name for these storms varies based on where they form. The term “hurricane” is used for storms in the Atlantic and Northeast Pacific. Storms in the Northwest Pacific are called “typhoons,” and systems in the Indian Ocean and South Pacific are referred to as “tropical cyclones.”