Thirteen years of cloud cover, in one beautiful map

Monday, May 11, 2015, 2:13 PM - Wondering where the cloudiest or most clear-skied places on Earth are? NASA has you covered.

What you're looking at in the image below is an image of the Earth collected by NASA's Aqua satellite, using its Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, representing average cloud cover (the darker the colour, the less cloudy it is).

While MODIS can take daily cloud coverage images, the picture below is an average of 13 years of cloud cover data, from July 2002 to April 2015 (click on it for a larger look).

It's easy to see where all the deserts are. Africa is dominated by darker hues, even beyond the legendary Sahara, as well as the Middle East and Australia. In the Americas, the U.S. southwestern states, including drought-stricken California, stand out with their darker blues, while in South America, the near-zero cloud cover of Chile's Atacama Desert is a stark contrast to the whiter hues of the rest of the continent.

Then there's the rain forests. While the map is about cloud cover, not precipitation, it's pretty easy to see why the jungles of South America, Central Africa and Indonesia are so lush and green.

Moving up to Canada, it's mostly uniform, but if you look carefully, you can see some regional variations.

On the East Coast, Newfoundland is barely detectable against the equally-cloudy Gulf of St. Lawrence. Westward, the lakes of Ontario and Manitoba are much more clearly defined, indicating a difference in the way cloud cover interacts with the land and the open water.

The Prairies, or at least the southern portions, seem noticeably less cloudy on average, while across the Rockies, we doubt anyone will be surprised by the bright white shade of the mountains and Lower Mainland.

As amazing as it looks, NASA had a couple of caveats when it released the image late last week.

"Note because the map is simply an average of all of the available cloud observations from Aqua, it does not illustrate daily or seasonal variations in the distribution of clouds," the agency says. "Nor does the map offer insight into the altitude of clouds or the presence or absence of multiple layers of clouds (though such datasets are available from MODIS and other NASA sensors). Instead it simply offers a top-down view that shows where MODIS sees clouds versus clear sky."

NASA also notes the reflectivity of the surface beneath the cloud can affect the MODIS instrument's efficacy, so different techniques are used to calculate the cloud cover depending on whether the landscape below is a desert, coast, forest or even ocean.

"For instance, the MODIS is better at detecting clouds over the dark surfaces of oceans and forests, than the bright surfaces of ice," NASA says. "Likewise, thin cirrus clouds are more difficult for the sensor to detect than optically thick cumulus clouds."

SOURCE: NASA