December 19, 2021 - Snow and Fog in France and Spain
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A cool autumn morning brought large banks of fog to southern France and
northern Spain in mid-December 2021. The Moderate Resolution Imaging
Spectroradiometer (MODIS) on board NASA’s Aqua satellite captured a
true-color image of the spectacular scene on December 15.
The snow-covered heights of the Pyrenees Mountains courses along the
border of France (north) and Spain (south), flanked by massive banks of
low cloud (fog). Both fog and snow are highly reflective, so both
appear white in a true-color image. Because of the similar reflectivity
and color, sometimes it’s difficult to separate the two just by
looking, but there are a few characteristics that may be helpful. Snow
tends to be very bright white with a generally smooth appearance, while
fog leans towards a duller white color and usually looks to be finely
textured. Topography is helpful as well: fog spreads across low plains,
forms over rivers, and creeps into the lower valleys of hills and
mountains. Snow can fall anywhere, of course, but bright white covering
the high elevations is almost always snow.
The reason that fog lays low over the land is a result of how it is
formed. Radiative fog develops over land when the humidity is high and
the sky is initially clear, especially in the long nights of autumn.
The ground, which is warmed by the sun’s ray (radiant energy) during
the day, quickly loses heat in cool temperatures after the sun sets.
This loss of radiant energy from the ground cools the adjacent air to
below its dew point, so droplets of air form in the air right above the
ground. On calm nights with little wind, this cooling effect can extend
far above the surface, creating a layer of thick fog.
Image Facts
Satellite: Aqua
Date Acquired: 12/15/2021
Resolutions: 1km (428.2 KB), 500m (1.1 MB), 250m (859.4 KB)
Bands Used: 1,4,3
Image Credit: MODIS Land Rapid Response Team, NASA GSFC
https://modis.gsfc.nasa.gov/gallery/individual.php?db_date=2021-12-19
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