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Also spelled Kalbaishakhi, a storm occasionally accompanied by thunder that takes place a few times every year during or slightly before the Bengali month of Baishakh (early April) in Bangladesh and West Bengal, following the hot and humid Choitra month ending an old year and symbolising the washing away of the grime of the old year.
Yeah, must be a Kalboishakhi cloud.
by A.Beng Ali April 02, 2011