Do Clouds Have Favourite Countries?

Look at a satellite map of Earth’s clouds averaged over many years and a pattern jumps out. Some places are wrapped in cloud almost all the time, while others hardly ever see one. It is tempting to imagine that clouds simply have favourites, drifting back again and again to the same coasts and hills like holidaymakers returning to a beloved seaside town. The truth is less romantic, but it is also rather wonderful, because the favourites are chosen by the way the whole planet breathes.

Clouds are everywhere. NASA’s Earth Observatory, drawing on more than a decade of observations from the MODIS instrument on the Aqua satellite, notes that about 67 percent of Earth’s surface is typically covered by clouds. But that cover is not spread evenly. When the measurements are averaged, three broad bands stand out: a narrow strip of frequent cloud near the equator, and two wider strips in the middle latitudes, one to the north and one to the south.

The equatorial band comes from a giant circulation in the tropics known as the Hadley cells. Near the equator, where winds from the two hemispheres meet, warm, moist air rises. As it climbs it cools, and the water vapour in it condenses into towering clouds. That air eventually sinks again around 30 degrees north and south. Where air sinks, clouds struggle to form, so between roughly 15 and 30 degrees clouds are rare and deserts are common. The Sahara is not unlucky; it simply sits under the part of the planet where the air comes down.

Oceans have their own preferences. Low clouds turn out to be more common over the sea than over land. Along some coasts, cold water from deep in the ocean rises to replace the water at the surface. That cold water chills the air just above it, and as the moist sea air cools, its water vapour condenses into droplets. The result is a low blanket of cloud that can sit offshore day after day, looking for all the world as if it has decided to stay.

Mountains are the other great cloud magnets. According to the Met Office, when the wind blows against a hill or mountain it is forced to rise, the air cools as it climbs, and the water vapour within it condenses to form cloud. These are called orographic clouds, from a Greek word for hill. They include banner clouds streaming from a single peak, and smooth, lens-shaped lenticular clouds that form in waves of air downstream of a mountain range. A mountain can therefore keep making its own cloud for as long as the wind keeps blowing.

So clouds do not have favourite countries, but they do have favourite conditions: rising air, cool water and high ground. Wherever those come together, clouds form again and again, and a place can earn a reputation for grey skies that has nothing to do with taste and everything to do with physics. The next time a cloud seems to hang over the same hill all afternoon, it is not loyalty. It is air being lifted, cooled and turned back into water, over and over, in exactly the same spot.