Trade Cumulus Cloud Groups: Some Observations Suggesting a Mechanism of Their Origin

On a recent field trip to the western Caribbean, temperature gradients in the surface layers of the trade-wind oceans were found and related to the presence of cumulus cloud groups. The cloud groups generally formed over and slightly downwind of warm spots which were several kilometers in extent along the wind and 0.1—0.3° C in amplitude. Clouds were not found in the absence of warm spots although warm spots were occasionally encountered without clouds.

Air circulations, similar to but weaker than those produced by heated land masses, are hypothesized to exist over oceanic warm spots and their magnitude is estimated from previous work on heated islands. Observed differences in low-level moisture structure between clear and cloudy areas may be interpreted in terms of such circulations and their evolution with time. Some suggestions are made concerning the life cycles of trade cumuli.