Hurricanes are deadlier than tornadoes, but anyone who has experienced a tornado will need some convincing. It is largely a question of scale.
So how are they different? They differ in the way they are formed and in the areas which they affect, but the chief difference is one of size.
Hurricanes are intense tropical storms, normally affecting the islands of the Caribbean, the Gulf coast of America and its eastern seaboard. They are formed over the western mid-Atlantic, normally 5 to 20 degrees N, and they generally move towards the NW or WNW before making landfall. Technically they are closed low pressure cells, which, according to the Beaufort Wind Scale, develop winds up to Force 12, or more than 75 mph (121 kph) – sometimes much more.
These winds spiral round in an anti-clockwise direction as they are drawn into the hurricane’s centre (the eye), before rising. It is the speed of the wind which causes most damage, but there is the added effect of the associated low air pressure which can allow the sea’s surface to rise, sometimes to unusual levels.
Around the eye will grow enormous towers of cumulo-nimbus clouds, fed by the rising air, from which very heavy rain and perhaps hail will fall, accompanied by thunder and lightning. The eye, of perfectly still air, can be anything up to 30 miles (50 kms) across, and the whole hurricane may have a diameter of 460 miles (800 kms).
Hurricanes can move at 10-15 mph (16-24 kph), but rarely last longer than three or four days. An average hurricane season can last between late July and early October. So during this period the effects of a hurricane – violent winds, heavy rain and hail, electrical storms and a rise in sea level – produce a deadly combination, and any affected coastal area is likely to face some level of devastation. Once over land hurricanes begin to dissipate, so their effects are restricted almost entirely to coastal regions.
Similar storms also occur in the Pacific Ocean, where they are called cyclones and, in Australia, willy-willies.
Tornadoes, by contrast, are local in occurrence and affect inland areas. They form, during spring and early summer, along squall lines between cool northerly air and warm, damp air from the tropics, and occur most often in the centre and midwest of the USA. However they can also form over the UK and the western plains of Europe.
Local intense ground heating causes a sudden uprush of air in a vortex which appears to pull heavy, dark, funnel-shaped clouds down to the earth. A tornado is only likely to be about 100 metres or so across, and will only last for about an hour. However, the intensity of its winds, which can be up to nearly 200 mph (300 kph), can cause complete devastation, particularly if it cuts a swathe through a built up area.
Most often, and fortunately, tornadoes only affect rural areas, and therefore simply cause interest and excitement, but if they happen to pass over a village or town they can wipe the place out completely. Damage will include loss of buildings, and sometimes of lives, so their effect cannot be minimised.
It is possible to give warning of an approaching hurricane, so that effects can be minimised to a certain extent, whereas tornadoes are entirely unpredictable, and occur so suddenly that there is little or no chance of escape for someone in the wrong place at the wrong time. This suddenness can make tornadoes appear more deadly, but taken as a whole, the effects of hurricanes can be so widespread and long lasting, that their death toll is often much greater.
Crops on Caribbean islands have been wiped out by hurricanes, causing economic problems lasting several years. Coastal towns have been damaged by winds and needed wholesale rebuilding programs, and of course, who can forget the ravages of hurricane Katrina as it caused the waters around New Orleans to rise and breach the leves? These effects are long lasting, both for property and for people, and death tolls can continue to grow long after the event.
Perhaps it is a question of subjective judgement. Both are deadly in their own way and tornadoes give no warning, but hurricanes are so huge and slow moving that their effects are normally much greater and certainly more varied and long lasting. On balance they are the deadlier of the two.