Steam Fog over the Great Lakes


Steam Fog over the Great Lakes

A swirling mass of Arctic air moved south into the continental United States in early January 2014.

On January 3, the air mass began breaking off from the polar vortex, a semi-permanent low-pressure system with a center around Canada's Baffin Island. The frigid air was pushed south into the Great Lakes region by the jet stream, bringing abnormally cold temperatures to many parts of Canada and the central and eastern United States.

When the cold air passed over the relatively warm waters of Lake Michigan and Lake Superior, the contrast in temperatures created a visual spectacle. As cold, dry air moved over the lakes, it mixed with warmer, moister air rising off the lake surfaces, transforming the water vapor into fog--a phenomenon known as steam fog.

On January 6, 2014, the Moderate Resolution Imaging Spectroradiometer MODIS on NASA's Terra satellite captured this natural-color image (top) of fog forming over the lakes and streaming southeast with the wind. A false-color image (bottom) produced with data from the same instrument helps illustrate the difference between snow (bright orange), water clouds (white), and mixed clouds (peach). Water clouds are formed entirely by liquid water drops; mixed clouds contain both water droplets and ice crystals.

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