When glaciers grind against underlying bedrock, they produce a silty powder with grains finer than sand. Geologists call it “glacial flour” or “rock flour.” This iron- and feldspar-rich substance often finds its ways into rivers and lakes, coloring the water brown, grey, or aqua. When rivers or lake levels are low, the flour accumulates on drying riverbanks and deltas, leaving raw material for winds to lift into the air and create plumes of dust. That’s what was happening on October 23, 2012, when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite acquired this image of northerly winds blowing a plume of dust from the Copper River delta out over the Gulf of Alaska. Large dust storms like this are most common in Alaska in the fall, when river levels are at their lowest. Satellites have observed similar events in recent years. This dust storm was first noticeable in MODIS imagery on October 21 and continued through October 25. Though the plume extended for more than a hundred miles, it did not appear thick in comparison to some produced during previous events. A two-week event in 2006, for instance, which researchers described in detail in a paper in Geophysical Research Letters, produced large plumes that lofted between 30 and 80 kilotons of glacial flour into the atmosphere. Since 2011, as part of a project to improve understanding of arctic dust storms and validate satellite observations, a team of NASA-funded scientists have been monitoring filtered air samples from a site on Middleton Island, a small island in the Gulf of Alaska. Scientists monitor Arctic dust for a number of reasons. The storms can reduce visibility enough to disrupt air travel, and they can also pose health hazards to people on the ground when they occur inland. The dust is also a key source of iron for phytoplankton, whose growth is limited by the availability of minerals and nutrients. Finally, there is the possibility that dust events are becoming more frequent and severe due to ongoing recession of glaciers in coastal Alaska.