Hurricane season starts today in Eastern Pacific

Hurricane Center 2011

The winds attain maximum force close to the point of lowest pressure—about 724 torr (unit of pressure), or about 28.5 in. of mercury. The diameter of the area affected by winds of destructive force may exceed 240 km (150 mi). Gale winds prevail over a larger area, averaging 480 km (300 mi) in diameter. The strength of a hurricane is rated from 1 to 5. The mildest, Category 1, has winds of at least 120 km/hr (74 mph). The strongest (and rarest), Category 5, has winds that exceed 250 km/hr (155 mph). Within the eye of the storm, which averages 24 km (15 mi) in diameter, the winds stop and the clouds lift, but the seas remain very violent.

Hurricanes generally move in a path resembling the curve of a parabola. In the northern hemisphere the storms usually travel first in a north-westerly direction and in the higher latitudes turn towards the north-east. In the southern hemisphere the usual path of the hurricane is initially to the south-west and subsequently to the south-east. Hurricanes travel at varying rates. In the lower latitudes the rate ranges from 8 to 32 km/hr (5 to 20 mph) and in the higher latitudes it may increase to as much as 80 km/hr (50 mph). Those areas in which the hurricane winds blow in the same direction as the general movement of the storm are subjected to the maximum destructive violence of the hurricane.
Since 1943 military aircraft have been flying into hurricanes to measure wind velocities and directions, the location and size of the eye, the pressures within the storms, and their thermal structure. A coordinated system of tracking hurricanes was developed in the mid-1950s, and periodic improvements have been made over the years. Radar, sea-based recording devices, geosynchronous weather satellites, and other devices now supply data to hurricane monitoring centres, such as the National Hurricane Center in Florida, which follows each storm virtually from the beginning.
Improved systems of prediction and communication have been able to help minimize loss of life in hurricanes, but property damage is still heavy, especially in coastal regions. The strongest hurricane to hit the western hemisphere in the 20th century, called Gilbert, devastated Jamaica and parts of Mexico in 1988 with winds that gusted up to 350 km/hr (218 mph). The United States is often affected by destructive hurricanes, the most recent being Andrew (1992), with an estimated $12 billion in damage, more than 50 dead, and thousands left homeless. Agnes (1972), with $3 billion in damage and 134 deaths and Hugo (1989), with more than $4 billion in damage and more than 50 deaths, also wreaked tremendous destruction. In Britain, one of the world’s windiest countries, but one not normally affected by hurricanes, the hurricane that swept the southern half of the country in 1987 exceeded 100 mph and felled millions of trees.

 

Hurricane Stages

Hurricanes and typhoons usually move westward at about 10 mph (16 kph) during their early stages and then curve poleward as they approach the western boundaries of the oceans at 20° to 30° lat., although more complex tracks are common.
In the Northern Hemisphere, incipient hurricanes usually form over the tropical Atlantic Ocean and mature as they drift westward; hurricanes also form off the west coast of Mexico and move northeastward from that area.
An average of 3.5 tropical storms per year eventually mature into hurricanes along the east coast of North America, usually over the Caribbean Sea or the Gulf of Mexico between June and November; one to three of these approach the U.S. coast annually, some changing their direction from west to northeast as they develop. Hurricanes and typhoons of the N Pacific usually develop sometime between May and December; typhoons and tropical cyclones of the Southern Hemisphere favor the period from December through April; Bay of Bengal and Arabian Sea tropical cyclones occur either between April and June or September and December, the times of the onset and retreat of the monsoon winds.

Formation of HurricanesA cyclone that eventually reaches hurricane intensity first passes through two intermediate stages known as tropical depression and tropical storm. Hurricanes start over the oceans as a collection of storms in the tropics. The deepening low-pressure center takes in moist air and thermal energy from the ocean surface, convection lifts the air, and high pressure higher in the atmosphere pushes it outward. Rotation of the wind currents tends to spin the clouds into a tight curl; as the winds reach gale force, the depression becomes a tropical storm. The mature hurricane is nearly circularly symmetrical, and its influence often extends over an area 500 mi (805 km) in diameter.
As a result of the extremely low central pressure (often around 28.35 in./72 cm but sometimes considerably lower, with a record 25.91 in./65.8 cm registered in a 1958 typhoon) surface air spirals inward cyclonically (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), converging on a circle of about 20 mi (30 km) diameter that surrounds the hurricane’s “eye.” The circumference of this circle defines the so-called eye wall, where the inward-spiraling, moisture-laden air is forced aloft, causing condensation and the concomitant release of latent heat; after reaching altitudes of tens of thousands of feet above the surface, this air is finally expelled toward the storm’s periphery and eventually creates the spiral bands of clouds easily identifiable in satellite photographs.
The upward velocity of the air and subsequent condensation make the eye wall the region of heaviest precipitation and highest clouds. Because the outward increase in pressure is greatest there, the eye wall is also the region of maximum wind speed. By contrast, the hurricane eye is almost calm, experiences little or no precipitation, and is often exposed to blue sky. Temperatures in the eye are 10°F to 15°F (5°C–8°C) warmer than those of the surrounding air as a result of sinking currents at the hurricane’s core.

Damage Caused by Hurricanes