The lower troposphere in the earth's equatorial region is conditionally unstable, and convection is developed spontaneously if clouds are generated. This leads to a paradox, because clouds are generated when convection is enforced. Because large-scale vortices do not exist near the equator, the sea-land breeze circulation along the coastline with a diurnal cycle is the most appropriate motion as such forced convection. (Over the oceans far from any coastlines, so-called conditional instability of the second kind (CISK) may induce intra-seasonal variations with super cloud clusters along the equator, or tropical cycles in subtropics apart from the equator, but they are transient.) Thus the most active convective clouds and the largest rainfall on the earth appear over the Indonesian maritime continent with the longest coastlines.