From Glossary of Meteorology


A squall, with wind speeds occasionally exceeding 13 m s-1 (30 mph), in the Malacca Strait between Malay and Sumatra during the southwest monsoon (April through November).

It usually blows from the southwest, sometimes from the west or northwest, raising a heavy sea on the Malay coast. The wind veers and strengthens and a heavy bank or arch of cumulonimbus arcus passes overhead (arched squall) with heavy rain and often thunder. Sumatras usually occur at night; they bring a sudden drop in temperature and are generally due to the descent of air cooled by radiation on the high ground of northern Sumatra. In a few cases they mark an air mass boundary during the advance of the monsoon. They are said to occur simultaneously along a line of 320 km (200 miles) or more that advances in a direction between southeast and northeast at about 9 m s-1 (20 mph).

Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.