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Images: Ice, Wind & Fury

Piteraqs pose hazards to the inhabitants of Tasiilaq, a town in southeastern Greenland. During piteraqs, freezing winds sweep down off the Greenland ice cap and can reach hurricane intensity. (Fiamma Straneo, Woods Hole Oceanographic Institution)
Tasiilaq is the seventh-largest town in Greenland and the most populous community on the eastern coast, with more than 2,000 inhabitants. It is located in a place where many different factors combine to create piteraqs, including the ice sheet, variable weather patterns that can create suitable atmospheric conditions, and topographic features such as mountains and fjords. (Google Earth; Image Landsat; Image U.S. Geological Survey; Data SIO, NOAA, U.S. Navy, NGA, GEBCO; Image IBCAO)
The trigger for piteraqs seems to be low-pressure systems, or cyclones, that frequently occur to the east and southeast of Greenland. When a reservoir of cold air has built up over the ice sheet, cyclonic winds from the northwest can push it down, sending it downhill in a jolt.  (Eric S. Taylor, WHOI Graphic Services)
When the cold air rushes downhill, it creates a mountain wave along the boundary between the colder, denser air masses and the warmer, more buoyant air masses. The lower layer of colder air is squeezed into a smaller volume and then accelerates out of its confines, rushing  downward along the steep slopes. It breaks, like a big wave of water that collapses and crashes onto the shore, creating a lot of turbulence. (Eric S. Taylor, WHOI Graphic Services)
A second driving force for piteraqs is gravity, which accelerates the speed of anything falling downhill, even a mass of cold air. This increases the strength of the winds during piteraqs. Wind speeds pick up, increasing the strength of the piteraq. (Eric S. Taylor, WHOI Graphic Services)
Other aspects of topography play a role in driving piteraqs. Tasiilaq is located inside a valley, which funnels the flow of cold air into a smaller and smaller space, increasing its velocity over the ice sheet and toward the fjord. By the time the air reaches the fjord, it shoots out at top speed. (Eric S. Taylor, WHOI Graphic Services)
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