The Great Lakes Experience Their Deepest Freeze in 20 YearsS

For the first time since 1994, 88% of the surface of the Great Lakes has frozen solid.

The image above, like the image of the lakes we posted earlier this week, is a true-color image, which can make it difficult to discern ice from snow and clouds. In the image below, NASA's Aqua – an Earth-observing satellite that maps the planet's water – has photographed the lakes using infrared filters that make these features easier to tell apart. As Phil Plait explains over on Bad Astronomy:

The Great Lakes Experience Their Deepest Freeze in 20 YearsS

Different wavelengths (colors, if you will) of infrared light are absorbed and reflected differently by the features below, making it easier to tell them apart. In this case, water is deep blue, ice is pale blue (where it's thicker it looks brighter), snow is blue-green, and clouds are blue-green or white.

As you can see, a lot of water is covered, some of it in pretty substantial ice sheets. If they hadn't labeled Lake Erie, I would've missed it.

"Persistently low temperatures across the Great Lakes region are responsible for the increased areal coverage of the ice," said Nathan Kurtz, cryospheric scientist NASA's Goddard Space Flight Center. According to NASA Earth Observatory, cold air and winds remove heat from the fresh water until it reaches the freezing point, at which point ice begins to form on the lakes' surface. "Low temperatures are the dominant mechanism for thickening the ice," said Kurtz, "but secondary factors like clouds, snow, and wind also play a role."

[NASA Earth Observatory via Bad Astronomy]