A tropical cyclone is an intense low pressure area or a whirl in the atmosphere over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and winds at low levels, circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere). From the centre of a cyclonic storm, pressure increases outwards. The amount of the pressure drop in the centre and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds.
As per the criteria adopted by the World Meteorological Organisation (W.M.O.), India Meteorological Department classifies the low pressure systems in the Bay of Bengal and in the Arabian Sea into 7 classes. Low pressure areas with maximum sustained surface winds of speed between 31 & 61 km.p.h. (17 to 33 knots) are called tropical depressions.
|Once the winds around the low pressure area reach at least 62 km.p.h, it is called a tropical cyclone and is assigned a name. When wind speed is between 89 & 118 km.p.h (48 & 63 kt) it will be a Severe Cyclonic Storm (SCS), between 119 & 221 km.p.h (64 & 119 kt) it is Very SCS and when exceeds 221 km.p.h (119 knots), the cyclone is called a Super Cyclonic Storm. Kerala coast have been affected by low pressure systems upto the category of severe cyclonic storm (max. wind speed 118 km.p.h)|
Tropical cyclones require certain conditions for their formation. These are
- A source of warm, moist air derived from tropical oceans with sea surface temperature normally near to or in excess of 27 °C
- Winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form
- Winds which do not vary greatly with height - known as low wind shear. This allows the storm clouds to rise vertically to high levels;
- Coriolis force / spin induced by the rotation of the Earth. The formation mechanisms vary across the world, but once a cluster of storm clouds starts to rotate, it becomes a tropical depression. If it continues to develop it becomes a tropical storm, and later a cyclone/ super cyclone.
Tropical cyclones are named to provide ease of communication between forecasters and the general public regarding forecasts and warnings. Since the storms can often last a week or even longer and more than one cyclone can be occurring in the same region at the same time, names can reduce the confusion about what storm is being described.
Names were first used in World War II and were subsequently adopted by all regions. In most regions pre-determined alphabetic lists of alternating male and female names are used. However, in the north-west Pacific the majority of names used are not personal names. While there are a few male and female names, majority are names of flowers, animals, birds, trees, foods or descriptive adjectives. By the mid-1960s names were used for all tropical storms except those in the North Indian Ocean . The names currently in use and those to be used in future years are listed. Various meteorological organisations have responsibility of naming them.
The names of cyclones in Indian Seas are not allocated in alphabetical order, but are arranged by the name of the country which contributed the name. It is usual practice for a storm to be named when it reaches tropical storm strength (winds of 34 knots).The list of names to be used for the North Indian Seas is given below:The names selected for North Indian Ocean cyclones from 2004 onwards
|Oman||Baaz||Sidr (2007)||Ward (2009)||Mujan|