Hurricanes, typhoons and monsoons
16 Oct 2003
Izzy puts US in a Tizzy was a Geography in the News article in a
Let's take the month of September 2003, ADRC reported the world grabbing headlines of the
Why is it important to know and understand the effects of such major natural events?
Firstly, because extreme weather events seem to be getting more frequent and worse, whilst secondly, whether we like it or not what happens in one part of the global village impacts on the rest of us. As a case study, lets compare the effects of hurricane Isabel in the
But before we look at these case studies, are you clear about the difference between hurricanes, typhoons and monsoons?
Basically there isn't a difference between a hurricane and a typhoon they are regional names for an intense tropical cyclone with a well-defined circulation and maximum sustained surface winds in excess of 74mph which develops over tropical waters. So in the Atlantic Ocean, the Pacific Ocean east of the International Dateline they are hurricanes, west of it they are typhoons. However east and west of the Indian subcontinent they are called cyclones. All a little confusing.
So what about monsoons the major cause of devastating floods on the Indian subcontinent?
In contrast they are very different. A monsoon low-pressure system develops over the landmass of southern
What environmental conditions are needed for tropical cyclones to develop?
Crucial to tropical cyclone development is a sea surface temperature of 27ºC, the dominant wind direction must be east to west with some deflection due to the influence of the Coriolus* force producing rotation or spin. Given these conditions tropical disturbances or waves develop with about 100 or so detectable each year.
As the waves move westwards the wind speed increases to 38mph, once the circulation becomes closed a tropical depression develops. Then gradually shower and thunderstorm activity increases as the gusting winds stir up the warm ocean waters causing moisture to rise and condense as wind speeds increase up to 73 mph. It has now become a tropical storm. The accompanying heavy downpours release heat and energy and with increased wind speed to over 74mph a more definite eye (check out the satellite photo below) is formed. Gradually more and more air is forced to rise upwards and outwards with a significant barometric pressure drop at the ocean surface thus a tropical cyclone is born.
Annually about 10 tropical storms develop of which about 5 or 6 develop into full-blown hurricanes measured on the Saffir-Simpson (S-S) scale. The categories of 1 to 5 measures wind speed, barometric pressure and possible damage. So category 1 has wind speeds of 74-95mph, > 28.94 inches of mercury and minimal damage whereas category 5 has winds >155mph, <27.17 inches of mercury and catastrophic damage.
So now lets consider the two September tropical cyclones on either side of the world. Izzy in spite of the hype just about made it into category 2 on the S-S scale on the
This view taken just below Grand Falls shows the high level of the river before the hurricane event so it was the very wet spring and summer aggravated by higher than usual winds, which caused all the flooding not the hurricane per se.
A friend emailing from
In contrast, just five days before the
So the Guardian editor certainly had a point worth remembering when hearing news of disastrous natural events that perhaps British broadcasters have given more time and prominence to Isabel than it deserved.