Depression² a region of low atmospheric pressure. A depression forms as warm, moist air from the tropics mixes with cold, dry polar air, producing warm and cold boundaries and unstable weather low cloud and drizzle, showers or fierce storms. The warm air being less dense, rises above the cold air to produce the area of low pressure on the ground. Air spirals towards the centre of depression in an anticlockwise direction in the northern hemisphere, clockwise in the southern hemisphere. Depression occasionally develops in the deep-sea area of the indian ocean or in the bay of bengal. The timing of such depressions coincides with the southwest monsoon wind. During such depressions, continuous heavy rainfall occurs in Bangladesh and adjoining areas for 7 to 10 days. At times more than two to three weeks of rainfall may occur in sylhet and cox's bazar areas. [Mohd Shamsul Alam]