Synoptic systems have a horizontal scale of about 1,000-2,000 km and a lifetime of five to seven days. In mid-latitudes a weak low (high) pressure system typically has a central MSL pressure of less (greater) than about 1,008 (1,016) mb. In low latitudes, wave disturbances are more usually identified in streamline patterns because of the weak pressure gradients and the semi-diurnal pressure oscillation, except in the case of tropical cyclones. In view of their importance to mariners, identification and tracking of storm systems over the tropical oceans began in the mid-nineteenth century. W.C. Redfield (1831), for example, traced the parabolic paths of hurricanes over the western North Atlantic-Gulf of Mexico and, about the same time, Henry Piddington (1842) documented the motion of tropical storms in the Bay of Bengal-Arabian Sea. He also introduced the term cyclone (from the Greek kyklon, meaning revolving). Its counterpart, anticyclone, was first used by Sir Francis Galton in 1863 (Khrgian, 1970).