thumbnail Uncommon clouds are gathering
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Uncommon clouds are gathering © NASA Earth Observatory image by Joshua Stevens, using Landsat data from the US Geological Survey
A satellite view of the Mania River in Madagascar allows us to see a curious cloud pattern. It"s common for cool, moist marine air to rise and form dense clouds over bodies of water, then for the clouds to evaporate as they drift over warmer, drier land. The opposite is happening here: Puffs of clouds are forming over land, but not over water. That"s because Madagascar"s tropical rainforests are warm and wet enough that evaporating moisture rises as the day heats up. When it rises high enough, the moisture encounters cooler air, which condenses the water into clouds. Generally speaking, clouds will form where the air is rising, which in this case is only over the land. Above the river, the air is cooler and descending, so no clouds are forming there.