Popcorn clouds dot the landscape over the Brazilian cities of Rio de Janeiro and Sao Goncalo in this photograph taken by an astronaut looking down from the International Space Station (ISS). Rio de Janeiro is home to more than 6.5 million people.

Clouds had formed over rural areas and densely populated cities but are noticeably absent above Guanabara Bay, the coastal lagoons, and the ocean due to the mechanisms of cloud formation. Heat from the Sun warms the land surfaces in the area, which then warms the air directly above it. That warm air, and all of its cloud-making water vapor, then rises and condenses into clouds.

Bodies of water, on the other hand, do not change temperature as rapidly; the water remains cooler even during full Sun exposure. The water does not heat up enough to significantly warm the air above it, preventing air from rising to make clouds. This, and many additional climate processes, can be traced to the different average heat capacity of water and land.

The cloudless window over Guanabara Bay allows for a view of its largest island, Governador Island. A sharp boundary separates the telltale shapes the runways of Rio de Janeiro-Galeao International Airport's and the densely populated eastern half of the island. The famed beaches of the area, such as Copacabana, line the Atlantic shore.