Skip Navigation


Tropical cyclones are cyclonic whirls 80 to 800 kilometres (50 to 500 miles) in diameter; the winds near the centre form an almost circular vortex with a slight inward motion toward the centre near the ocean surface. Because of the Earth's rotation, the vortex circulation is clockwise in the Southern Hemisphere and counterclockwise in the Northern. The areal extent of tropical cyclones is small compared with storms outside the tropics, but the violence of the weather within the disturbed zone is usually far greater. Sustained winds in excess of 160 kilometres (100 miles) per hour are common near the centre, and winds twice that strong have occurred. Very close to the centre of mature cyclones, however, the winds drop abruptly from their extreme maximum to light breezes or even to complete calm. This central circular calm area bears the name eye of the storm and has an average diameter of 24 kilometres (15 miles). The lowest sea-level pressures on Earth occur in or near the eye of a hurricane.
All tropical storms develop over water that is warm enough to supply appreciable quantities of vapour to the air. Most storms develop along the equatorial convergence zone, where the trade winds of both hemispheres meet. This zone changes position with the seasons, penetrating to about latitude 15 N between July and October and to latitude 10 -15 S from January to March or April. These months represent the principal cyclone seasons of the respective hemispheres. The formation of the storms is most frequent farthest from the Equator, where the Coriolis effect (clockwise or counterclockwise rotation due to the rotation of the Earth) is greater (the effect is zero at the Equator).
As the warm water evaporates, moist air is carried aloft, where it condenses, releases latent heat, and is warmed further. This warming strengthens the updraft, and a low-pressure area is created in the lower atmosphere. Surrounding air moves into this low-pressure region and feeds the updraft. The Coriolis effect causes the developing storm to spiral, and it continually draws energy from the condensation of lifted water vapour. After several days of intensification, this process produces a mature traveling cyclone.
Dissipation of the storms over the tropical oceans is rare. When removed from their oceanic moisture source, however, they lose intensity; even passage over smaller islands of the tropics can result in great reductions in strength.

Hurricane Force Winds
    Category 1 - Winds 74-95 mph
    Category 2 - Winds 96-110 mph
    Category 3 - Winds 111-130 mph
    Category 4 - Winds 131-151 mph
    Category 5 - Winds 155+ mph

Hurrican Watch - Hurricanes may threaten within 36 hours. Be prepared to take action if a warning is issued by the National Weather Service. Keep informed of the storm's progress.

Hurricane Warning - Leave beachfront and low-lying areas. Leave mobile homes for more substantial shelter. Stay in your home if it is sturdy, on high ground and not near the beach. But if you're asked to evacuate, do so. Stay tuned to the radio for hurricane advisories and safety information.

Did You Know?

Puerto Rico used to have as many as 25 military installations.