Desert Locust Migrations and Synoptic Meteorology in the Gulf of Aden Area

1. The meteorological data relating to a number of swarm movements recorded in the Gulf of Aden area between 1941 and 1947 have been examined, using the corresponding synoptic charts. 2. The trajectory of the air in which a flying swarm is observed, considered in relation to recent determinations of the air-speed of Schistocerca, is shown to provide evidence of the origin and destination of the swarm, affording in favourable circumstances a good indication of its probable track, and making possible some deductions as to the course or orientation of the locusts. 3. Trajectories relating to swarms arriving on the coast of British Somaliland during the winter indicate the migration of swarms of the Monsoon generation across the Gulf of Aden from the Hadhramaut. 4. Records of swarms reaching Aden and the adjacent coast in summer give trajectories indicating the migration of swarms of the Long Rains generation from western British Somaliland, also across the Gulf. 5. It appears probable that locusts of the phases transiens and solitaria may similarly migrate across the Gulf. 6. Swarms landing on the coasts of the Somali peninsula during the summer are shown to be probably mainly of local origin, swept out to sea by the monsoon and subsequently brought in again by the sea-breeze. An alternative possibility in some of these cases is that of transportation across the Gulf by the north-easterlies which overlie the south-west monsoon and are probably continuous with the surface north-westerlies of Arabia at this season. 7. Evidence of the significance of convection currents in relation to flying locusts is provided by the frequent association of swarm movements with rising sand, dust-storms and similar manifestations of intense convective activity near the ground, and by an aircraft report of locusts at 7000 ft. at an air temperature indicating unimpeded convection currents from the surface to at least this height.