Nature's lethal weapons

BBC, Monday, 18 September, 2000

Different names for the storms - depending where you are

Hurricanes, typhoons and cyclones are the world's most violent and destructive storms.

They are giant whirlwinds where air moves around a centre of low pressure - the eye of the storm - reaching high speeds in circular waves extending 20 or 30 miles from the middle.

The word "hurricane" probably comes from the Mayan storm god Hunraken, or other Caribbean terms for evil spirit and big wind. And the terms "hurricane", "typhoon" and "tropical storm" all describe the same type of swirling wind activity.

How they start
How much harm do they do?
Can they be stopped?
What hurricane ratings mean

When cyclonic winds reach speeds of more than 64kmh, they are officially "tropical storms" and are assigned a name.

If the winds reach 118 kmh then they are redefined as hurricanes or typhoons, depending on location.

The storm is a "hurricane" if it is in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E. Air circulation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

"Typhoons" occur in the Northwest Pacific Ocean west of the dateline.

Tropical storms are the only natural disasters with their own names.

Until 1978, they were given women's names, but then US weather forecasters bowed to political correctness and agreed to alternate them with men's names.

                  How do they start?

            Wait the changing of the picture!   

Tropical cyclones need a warm layer of ocean water and an unstable atmosphere to develop.

Cool ocean winds at low pressure force hot, humid air high into the atmosphere forming a column. Moisture is forced up the column which condenses and releases latent heat energy, the primary source of fuel for tropical cyclones.

At the centre is a calm sunlit eye, measuring tens of kilometres across.

In simple terms, hurricanes are giant machines that convert heat energy from tropical ocean water into wind.


How much harm can they do?
Violent winds, rain, waves, and storm tides make hurricanes one of the most dangerous natural disasters, accounting for an eighth of weather-related deaths.

They have the potential to devastate the world's insurance markets.

A large hurricane can stir up more than a million cubic miles of the atmosphere every second, typically dumping 15-30cm of rain on landfall.

Some hurricanes bring lots more rainfall and cause major flooding with storm surges and high surf. As they can move rapidly and erratically, the path the winds will take is hard to predict precisely.

Hurricanes also affect the depths of the ocean. In 1975, instruments dropped from research planes in the Gulf of Mexico showed that Hurricane Eloise disturbed the ocean hundreds of feet below the ocean's surface and created underwater waves that persisted for weeks.

Can nature's weapon be stopped?
In 1962 the US government began to look at ways of how to weaken hurricanes, but the project ended without results in 1983.

However, one of the world's leading hurricane scientists, Hugh Willoughby, still believes there just might be a way of disrupting hurricanes.

One idea which scientists are looking at is to put black soot into the air by burning petroleum on ships near a hurricane.

Black absorbs heat from the sun which would then create updrafts to break up the hurricane's normal wind patterns.

Mr Willoughby has even considered a big tin foil mirror in space to reflect sunlight to heat the ocean in just the right spot to divert a hurricane.

Scientists are taking these ideas seriously because they think if they could cut hurricane winds by 10% or 15 % it could prevent many billions of dollars of damage.


Hurricane ratings
The Saffir-Simpson hurricane intensity scale is most commonly used for Atlantic and Pacific storms to estimate of the potential flooding and damage to property.

Category 1: 118 - 154kmh
Some damage, mainly affecting trees, foliage, and mobile homes. Low-lying coastal roads may be inundated. Average cost of damage - $24,000,000.

Category 2: 155 - 177kmh
Trees can be blown down and exposed mobile homes and vehicles may be badly damaged. No major damage to buildings. Coast roads and low-lying inland roads may be cut off. Small craft can be torn from moorings. Shore evacuation is required. Average cost of damage - $218,000,000.

Category 3: 178 - 209kmh
Many large trees and signs are likely to be torn down. There will be some structural damage to small buildings and mobile homes can be destroyed. Waves will pound the coast and flooding rises above one metre. Evacuation of residences within a kilometre of shoreline may be required. Average cost of damage - $1,108,000,000.

Category 4: 210 - 248kmh
Trees, signs and small residences will be severely damaged, roof materials on larger buildings may come loose. First floor flood damage will occur. Escape routes inland can be cut off 5 hours before the eye of the storm hits and evacuation of shore residences is vital. Average cost of damage - $2,274,000,000.

Category 5: 249kmh +
Trees, shrubs and buildings may be blown down and there is considerable damage to roofs of buildings and signs. Glass in windows and doors is unlikely to escape from harm; many roofs will fail and some buildings can be completely blown away. Massive evacuation of residential areas within 5 to 10 miles of shore is probably required. Average cost of damage - $5,933,000,000.

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