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).