Tropical cyclones can produce intense vortices of air with winds spiralling in towards the low-pressure centre, variously called hurricanes, typhoons and tornadoes. Enormous destruction results from both winds and tidal surges in coastal areas, and to a lesser extent from effects of heavy rains. Damage and loss of the life caused by tropical cyclones vary widely from year to year. On the occasion when a violent tornado strikes a heavily populated area, it may cause millions of dollars of damage, and the loss of many lives.
The diameter of the cyclonic circulation is generally less than 1000 km, with hurricane force winds (120 kph) restricted to a 100-300 km central ring. Typhoons, the term used in the north-western Pacific, sometimes reach 2000 km in diameter. The hurricanes themselves are rapidly rotating columns of air (160-800 km/hour) of small diameter (averaging 300-400 meters) which travel along a path up to 80 km (but occasionally up to 480 km) and cause considerable damage to buildings and crops encountered by the base of the column, which sucks dust, water and debris up into its rotating spiral. They tend to occur in the vicinity of a severe thunderstorm, in the warm sector of a cyclone, and are most frequent in late spring and early summer.
Typhoons or twisters are smaller in diameter and with shorter lives, often being born out of "supercells" associated with large storms.
In an average year, about 80 tropical cyclones form over the warm ocean waters on the surface of the globe; tornadoes in these conditions are referred to as water spouts. About 20 of them form in the north-western Pacific, and about 7 in the western North Atlantic and the Caribbean. Whilst these events are relatively few, each may have serious results and, on average, about 20,000 people lose their lives each year because of them; the damage caused may reach US$ 6,000 to US$ 7,000 million< The tornado occurs in its most violent form in the south-eastern USA, in the USSR and in Australia. During 1961-1970 alone the damage sustained amounted to US$ 6,750 million, equivalent to about 0.5% of the GNP of the countries concerned. Some 14 million people were affected, 12,000 lives were lost, 7.6 million hectares of land were flooded or otherwise damaged and 4 million buildings were partially or totally damaged. The property damage and loss of life from Hurricane Betsy in the mid-1960s threatened the stability of the entire global insurance market. The freak hurricane which struck the southern UK on October 19, 1987 cost insurance companies in the UK a total of over £860 million, uprooted some 2-3 million trees and killed 19 people. Hurricane Gilbert struck the Caribbean in September 1988 killed over 250 people, left over 1 million homeless and caused at least $10 billion of property damage. The impact of Hurricane Mitch in 1998 upon countries in Central America, notably Nicaragua, Honduras, Guatemala, and El Salvador, was been enormous. Over 3 million people in the region lost their homes and livelihoods. All four countries suffered major damage to agriculture, a primary generator of the foreign exchange these countries require to survive in the global economy, which is estimated to have set back set back regional development prospects by two to five decades.
Experts have suggested that global warming could give hurricanes 50% more power.
A cyclone in the Indian province of Gujarat in June 1998 killed more than 10 000 people.
Hurricane George caused damage estimated at US$10 million in the Caribbean in September 1998. Hurricane Mitch led to more than 9 000 deaths in Nicaragua and Honduras in October 1998, and caused major setbacks to development plans.