Linear weather disturbances frequently occur over the tropical Indian Ocean and are usually identified by convergence of the opposing airstreams and by cyclonic activity. The associated weather is concentrated along ' lines' or narrow zones lying either along or across the general windflow. Intrusion of Antarctic Air into the Indian Ocean produces such disturbances in either half of the region—the Trough Lines of the Southwest Monsoon in the Northern Hemisphere and the Meridional Fronts of the South Indian Ocean.

Cold fronts from high latitudes south of Madagascar are sometimes swept along the East African coastline into the tropical airstream. Within the tropics they undergo considerable modification and are transformed into dynamic fronts as distinct from the cold fronts of the high latitudes. They are recognised as discontinuities in the windfield with equatorward bulges in the isobaric patterns. On an average three or four Trough Lines reach the Ceylon area each year. Their characteristics and their influence on the Ceylon weather are examined in this paper.

The Meridional Fronts of the South Indian Ocean are the boundary zones between successive high pressure cells of the sub-tropical anticyclonic belt of the Southern Hemisphere. They develop as a result of convergence of the tropical airstream moving southwards behind one particular cell with Antarctic Air induced northwards ahead of the succeeding cell. Convergence does not arise purely from any concentration of temperature

gradients. Consequently, these fronts appear like the usual tropical convergence zones rather than the high latitude cold fronts. They move regularly from West to East across the ocean. Some interesting cases are discussed in this paper.