TROPICAL CLOUDINESS AND RAINFALL RELATED TO PRESSURE AND TIDAL VARIATIONS

A definitive statistical relationship is established between tropical cloudiness and rainfall and the semidiurnal solar (S2) atmospheric tide, as manifested in the semi‐diurnal surface pressure variation. Pressure and weather data are used from Batavia (seventy years) and Wake Island (twelve years). The S2 tidal effect is shown to enhance cloudiness and rain near sunrise and sunset, and to suppress them shortly after midday and midnight. The analysis is based on (a) the fact that the S2 amplitude varies by 15–20 per cent between months and by more than 100 per cent from day to day and (b) the amplitude of the S2 wave as computed from the pressure data at a station is closely related to the 5–6 hourly pressure changes during the periods around 4–5 a.m. to 10 a.m., from 10 a.m. to 4 p.m., etc. The crux of the analysis is the demonstration that days with large 5–6 hourly pressure changes during these periods have large cloudiness changes (in the sense described above) during these same periods relative to days with small 5–6 hourly pressure changes. Possible mechanistic connections between S2 pressure tendencies and cloud properties are examined. The varying convergence‐divergence field is suggested as the main link. It is shown how the concentration of active cloud updraughts in the Tropics can permit cloudiness to be extremely sensitive to small divergence fields. Finally, large‐scale simultaneous pressure changes over the Pacific Ocean area are shown and related to cloudiness changes. A need for re‐examination of the nature and origins of tropical disturbances is shown to exist, using the concept that possible small (terrestrial or extra‐terrestrial) triggers may set off significant changes in weather both locally and on the synoptic and global scales. Copyright © 1969 Royal Meteorological Society