File:Low clouds in the Connecticut and Hudson River Valleys (MODIS 2015-12-13).jpg

English: It is not unusual for the mountains of New England and New York to wear of coat of white snow by early December. But on December 6, 2015, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA�s Terra satellite captured a true-color image of a less typical scene: snow-free, green mountains punctuated by valleys filled with dense, white, low clouds (fog). The fog was most remarkably dense in the Hudson and Connecticut River Valleys, but also filled valleys in New York�s Adirondack Mountains and in northeastern Pennsylvania.

By Saturday, December 5, a high-pressure system had set up over New York and Pennsylvania, bringing mild temperatures to the northeastern United States. Daytime high temperatures in the Hudson Valley, typically near 40˚F (4.5˚C) at this time of year were registering about 10˚F or more above that average over most of the region, although nighttime lows dropped below freezing in many areas. The mild temperatures help set up the conditions for the fog formation.

The fog formed on the night of December 5 as the air near ground cooled and stabilized. As the cool air reached saturation, fog formed. As the air over the initial layer of fog cooled overnight, the fog continued to extend upwards. In this way, thick fog banks can eventually form, and such large fog banks can be remarkably stable and persist for days in areas that are sheltered from the wind � such as in deep valleys.

The National Weather Service issued a Dense Fog Advisory on December 6 and lasting until December 8 that warned of visibility less than one-quarter mile and hazardous driving conditions. In areas where temperatures dropped below freezing, black ice on the roadways was also a hazard.

Where the fog was especially thick it prevented the ground from warming, and it was reported that several areas remained both fog-bound and cold (in the 30s) on December 6-7, although the surrounding, fog-free, countryside continued to enjoy mild temperatures at or above 50˚F (10˚C). Reports on the Hudson Valley Weather Facebook Page described freezing fog that was beautiful, difficult driving conditions, and a fog horn sounding steadily, every twenty minutes, on the Hudson River.