Observer Comments

15:55 Sun Sep 13, 2015

Above the Clouds
While on the summits of taller mountains one can find themselves above the clouds. This was the case on Friday at Mount Washington Observatory.
View of undercast from Mount Washington
 When one finds themselves above the clouds, what they are seeing is referred to as undercast rather than overcast because the clouds are under or below them instead of over or above. It is possible to have both overcast and undercast conditions occurring simultaneously.
View of undercast and overcast from Mount Washington
 A lot of people have never seen undercast before and for good reason: there has to be substantial elevation change to observe this phenomenon. When one is in an airplane above the clouds technically they are above the undercast. Tall mountainous terrain also provides the conditions for undercast to form. Undercast is typically composed of low level clouds and there has to be an inversion layer that traps the clouds in the valleys. To see undercast one would have to be at a point on the mountain above the inversion layer.
An inversion layer is a layer of the atmosphere in which the temperature increases with height, which is opposite of what typically happens. Inversion layers are stable and act as a cap on the atmosphere, trapping moist air below and forming a cloud layer. Inversions often form at elevations below the tallest mountain terrain, thus making it possible to observe undercast.

Andrew Henry, Summit Intern


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