Cyclone Lehar could have just blasted off the southeast coastal areas of Andhra Pradesh with heavy rain and strong winds of more than 170 kmph, gusting at 200 kmph. It could have caused severe damage to standing crops and horticulture as well. But the cyclone has eased out as a deep depression and the threat that was looming over the coastal areas of the state seems to be over.
Rain and winds associated with the weakened weather system will not cause much damage as expected earlier. In fact the steady rain will now prove beneficial for the crops that really needed rain during this time- northeast monsoon season.
Tropical cyclones have a history of decaying or dissipating over the sea waters. Here are the possible reasons why Cyclone Lehar fizzled out.
Wind Shear – is one of the biggest reasons for cyclones to die down. Wind shear is a difference between speed and direction of the wind. It has two components- vertical and horizontal. Horizontal wind shear is seen across fronts and near the coast, and vertical shear typically near the surface.
Latent Heat: the energy inside the weather system usually remains vertical and therefore the strength of the storm remains intact. But the moment it becomes horizontal, it starts falling apart and loses steam.
Cold waters – If a weather system moves from warm to cold waters, it could degenerate. Usually in higher latitudes during winters, sea surface temperatures in north Bay of Bengal remain on lower side of 26.5 degrees (a temperature necessary for formation of tropical storm) and if the storm reaches these latitudes (above 15 degree North ) cold water will make them shed the energy.
Proximity to land – is another reason for cyclones to dissipate as they interact with the prevailing weather of the nearby land, as was the case with Helen.
Entrainment – is the interference of dry and cold winds with the weather system. The winds heading from land to the Bay of Bengal are cooler and may affect the system.
Incidentally, the last cyclone that had an Indian name before Lehar was Cyclone Jal that had met with the same fate. After weakening into a deep depression from a severe Cyclone Jal had hit Chennai coast on 7th of November, 2010.
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