Sumatras are lines of thunderstorms which usually occur between March and November each year. These squalls (lines of thunderstorms) develop at night over Sumatra or the Malacca Straits and move east towards Singapore and Peninsular Malaysia usually during the pre-dawn and early morning. They are often characterised by sudden onset of strong gusty surface winds and heavy rain lasting from 1 to 2 hours as they move across the island. Maximum gusts of up to 26 metres per second (93 km/h) have been recorded during the passage of a Sumatra squall (Gusts are temporary increase in wind speed).
A Sumatra Squall approaching eastern Singapore on 4 July 2005
Radar images showing the passage of a Sumatra Squall
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