Kalboishakhis – Bangladesh’s deadly storms

On Sunday night, what was reported as a ‘severe storm’ tore through northern Bangladesh, killing 16 people and destroying 1,000 homes.

Those familiar with the climatology of the region will not have been entirely surprised by the occurrence of such a massive storm system.

April is the peak of the ‘dry storm’ season ahead of the monsoon rain which usually arrives in June.

Tornadoes pose the deadliest threat at this time of year and hundreds or even thousands of fatalities may occur.

Yet Sunday night’s deaths were not the result of a tornado outbreak but a kalboishakhi.

Although not well understood, kalboishakhis tend to form in hot humid air ahead of the monsoon season. The warm, moist air at low levels is carried aloft where it meets cold, dry air which originates over Iran and Turkmenistan, and which is forced around the southern side of the Himalayas.

The contrasting air masses provide the energy required for these storms to form, often to the northwest of Kolkata in India, before moving across the border into Bangladesh during the evening. The storms are characterized by strong northwesterly winds, and they tend to move in a southeasterly direction.

Sunday’s storm seems to have followed this pattern.  The north of the country was worst hit with nine people being killed in Netrokona, six in neighbouring Sunamganj and one killed by lightning in Naogaon.

The winds were apparently strong enough to derail a train carrying 500 passengers across a railway bridge, although they appear to have had a lucky escape as there were no reports of any fatalities.