photo #7239469

The distinction between land and water is not obvious in this NASA satellite image photographed over the floodplains of the Mania River on the west coast of Madagascar. The colours are confusing because sediment browns the river floodwaters as they flow out into the Mozambique Channel, while deeper waters look the same green as the coastal forests. Helpfully, however, Cumulus clouds are around to make sense of things – at least, for those who know how to read them.Cumulus are low individual solid-looking mounds of cloud that drift along in the breeze on a sunny day. And they can reveal whether the surface below is water or land because they form more readily over whichever is hotter. That’s because Cumulus develop on air currents called thermals that rise as the surface warms the air in contact with it. As the thermals rise, the air can cool enough for some of the invisible water vapour they carry to condense into visible droplets of cloud. In the tropical sunshine, the land heats up more rapidly than the water, and so vigorous thermals develop over the land. With the humidity of Madagascar’s rainy season, Cumulus soon form on the thermals over the land, helpfully distinguishing it from the cloud-free waters to help any confused satellite observers.