Hurricane Sandy, which hit the US recently, was a Category 1 hurricane. It developed as a tropical depression on 22 October, but was upgraded as a Category 1 hurricane three days later. The hurricane’s impact was exceptionally severe because it collided with an ‘Arctic cold front’ just as it was hitting the US, creating an unusually large and dangerous storm system spanning nearly 1,000 miles.
Hurricanes are a type of tropical cyclone. There are three main groups of tropical cyclones, based on their intensity—tropical depressions, tropical storms and a third group of more intense storms, whose name depends on the region where it is formed. While these intense storms are called ‘hurricanes’ in the North Atlantic and Northeast Pacific Basin, in the Northwestern Pacific, they are called ‘typhoons’. In the Indian Ocean and Southern Hemisphere, they are referred to as ‘cyclones’.
Hurricanes are further classified into five categories depending on the intensity of their sustained winds. A Category 1 hurricane has sustained winds between 74 and 95 miles per hour (mph). The highest classification, Category 5, is reserved for storms with winds exceeding 156 mph. Hurricane Katrina in 2005, which caused 1,833 deaths, was a Category 5 hurricane.
Delhi monitors tropical cyclones that develop in the Northern Hemisphere between 100°E and 45°E. Depending on their sustained windspeeds, they are classified as: Depression (under 31 mph), Deep Depression (31 to 38 mph), Cyclonic Storm (39 to 55 mph), Severe Cyclonic Storm (55 to 73 mph), Very Severe Cyclonic Storm (74 to 139 mph) and Super Cyclonic Storm (140 mph and above). Cyclone Nargis that led to 138,000 deaths in Myanmar in 2008 was a Very Severe Cyclonic Storm, while Cyclone Nilam that hit Tamil Nadu and Andhra on Wednesday was a cyclonic storm.