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WinterIn winter, much of the Northern Hemisphere is covered in snow. The warm waters of the Gulf Stream keep the eastern U.S. and western Europe warmer and relatively free of snow.
Each of the four seasonal images is culled together from thousands of pictures taken with NASA's Moderate-resolution Imaging Spectroradiometer, or MODIS, satellite. The images were compiled at NASA's Goddard Space Flight Center in Maryland.
As spring comes, snow in the Northern Hemisphere recedes, and forests in the eastern part of South America get heavier rains, turning most of the continent to a rich green color. Even when much of the Southern Hemisphere is lush, Australia remains dry through spring and most of the year.
Snowlines in Asia, Alaska and western Canada have almost completely receded in summer. The band of green on the African continent moves north, though the Sahara Desert remains dry--as it does throughout the year.