Breeding Grounds:Hurricanes are products of a tropical ocean and a warm, moist atmosphere.Powered by heat from the sea, they are typically steered by high - level easterly winds while in the tropics, generally south of 25 degrees south latitude and by high-level easterly winds north of 25 degrees north latitude.When hurricanes become very strong, they can create their own steering winds.
The Atlantic Hurricane season starts on June 1.For the United States, the peak hurricane threat exists from mid - August to late October, although the official hurricane season extends through November.Over other parts of the world, such as the western North Pacific, typhoons can occur year-round.
Storm Structure:The process by which a disturbance forms and strengthens into a hurricane depends on at least three conditions.First, a disturbance gathers heat and energy through contact with warm ocean waters.Next, added moisture evaporated from the sea surface powers the infant hurricane like a giant heat engine.Third, the hurricane forms a wind pattern near the ocean surface that spirals air inward.Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere.If the winds at these higher levels are relatively light, this structure can remain in tact and further strengthen the hurricane.
The center, or eye, of the hurricane is relatively calm with sinking air, light winds and a few clouds.The most violent winds and rain take place in the eyewall, the ring of thunderstorms immediately surrounding the eye.At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion.Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.
How do the weather forecasters determine the category of any specific hurricane?
The Saffir-Simpson Hurricane Scale is a 1 to 5 rating based on the hurricane's intensity. This scale estimates potential property damage. Hurricanes or typhoons reaching Category 3 and higher are considered MAJOR hurricanes because of their potential for loss of life and damage. Category 1 and 2 storms are still very dangerous and warrant preventative measures. In the western North Pacific, the term "Super Typhoon" is used for tropical cyclones with sustained winds exceeding 150 mph. For more information on the Saffir-Simpson Hurricane Scale, go to:www.nhc.noaa.gov/aboutsshs.html.
- Category 1: Wind Speeds (MPH) 74 - 95: Minimal: Damage primarily to shrubbery, trees, foliage and unanchored mobile homes.No real damage to other structures. Example of Category 1: Hurricane Irene in 1999.
- Category 2: Wind Speeds (MPH) 96 - 110: Moderate: Some trees blown down.Major damage to exposed mobile homes.Some damage to roofing materials, windows and doors. Examples of Category 2: Hurricanes Georges in 1998 and Hurricane Floyd in 1999.
- Category 3: Wind Speeds (MPH) 111 - 130: Extensive: Large trees blown down.Mobile homes destroyed.Some structural damage to roofing materials of buildings.Some structural damage to small buildings. Examples ofCategory 3: Hurricane Betsy in 1965 and Hurricane Alicia in 1983.
- Category 4: Wind Speeds (MPH) 131 - 155: Extreme: Trees blown down.Complete destruction of mobile homes.Extensive damage to roofing materials, windows and doors.Complete failure of roofs on many small residences. Examples of Category 4:Hurricane Andrew in 1992.
- Category 5: Wind Speeds (MPH) >155: Catastrophic: Complete failure of roofs on many residences and industrial buildings.Extensive damage to windows and doors.Some complete building failure. Example of Category 5: Hurricane Camille in 1969.