Today’s forecast: cloudy with a chance of maps. The NASA Earth Observatory has condensed more than a decade of data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to create a single map of Earth’s average cloud cover for the last 13 years.
Around two-thirds of the Earth’s surface is covered by clouds at any given point, especially concentrated over the oceans. Clouds are so prevalent in the atmosphere that when NASA does manage to capture satellite images of Earth without clouds, it’s news.
MODIS cloud observations from July 2002 to April 2015 show Earth has some perpetually sunny regions. (Dark blue indicates fewer clouds on average, while white indicates frequent clouds.) Notably, the deserts of northern Africa and Saudi Arabia show up as clear dark spots in an otherwise opaque Blue Marble. You can also pick out certain mountain ranges based on the phenomenon of rain shadows: Mountains form a kind of wind break, protecting the region on the far side. As a result, one side of the mountain range forms clouds and remains lush and green, while the other side dries out. This pattern contributes to the dry weather west of the Sierra Nevadas in California’s Death Valley and west of the Andes Mountains in Chile’s Atacama Desert.
Cloud cover from January 2015 to April 2015. Image Credit: NASA Earth Observatory
By contrast, Europe and the upper half of South America seem to be perpetually hazy. Skies are more likely to be cloudy near the equator, where tropical weather patterns create thunderstorms. Clouds are also more likely on the western edge of continents, because of the way ocean water circulates due to Earth’s rotation on its axis: water at the surface of the ocean gets pushed west, away from the western edge of continents, and cool water from the bottom of the ocean rises to replace it.
See more cloud maps from MODIS here.
[h/t: Scientific American]