MODIS true color image (bands 1, 4, 3) of Alaska, 25 December 2004.
While winter's snows cover the Alaska mainland, streamers of dust stretch out over the Gulf of Alaska. The dust is primarily made up of fine glacial silt, which is produced by glaciers grinding against the rock that makes up mountains. The silt builds up along riverbanks, in sandbars, and at the terminus of glaciers. When a strong enough wind blows across these deposits, the silt is lifted into the air, and shows up as a fine grey cloud over the deep blue of ocean water or bright white of snow. Sediment also creates clouds in the water along the shoreline — this silt is often from the same source as the wind-carried variety. The more concentrated the sediment is in the water, the more opaque and brown or tan it appears. As it disperses, the clouds become more blue-green, until they are indistinguishable from clear water.
This true-color Terra MODIS image was acquired on December 25, 2004. At bottom right is the northern end of the Alexander Archipelago (Juneau is just out of sight). At upper right, the Coast Range begins its long march south along the North American coast (eventually ending in southern Canada).
Text and image courtesy of NASA's MODIS Land Rapid Response Team.