Observer Comments

16:06 Fri Oct 07, 2022

A Sea of Clouds and Other Optical Phenomena
One of the main reasons why people head up to Mount Washington is to see the breathtaking views. The maximum visibility from the summit on a perfectly clear day is 130 miles. On the clearest of days, you might even spot a sliver of the Atlantic Ocean, southeast of the summit.
Since the mountain is typically in the clouds at least 60% of the year, clearing is always special to witness. Undercast is one of my favorite weather phenomena.
 Undercast blankets the Northern Presidentials on Sept. 7.
Undercast, or what I like to call a “sea of clouds,” is when clouds are under the summit instead of over it. It is possible to observe an undercast when a temperature inversion occurs. Temperatures normally decrease as you increase with height, but the temperature distribution of air is flipped with an inversion. This means that the inversion layer acts sort of like a lid by trapping the colder, moist air at the ground while the warmer air lies on top. You can observe an undercast between storm systems when conditions are just right.
For example, conditions may be favorable for undercast when leftover moisture from a departing low pressure system becomes trapped in the lower atmosphere by high pressure building into the area. This was the case as seen from the summit on Sept. 21, when a low pressure system that dumped a lot of rain finally departed the region, and weak high pressure moved in behind the exiting low.
 Undercast and a glory are seen from the summit on Sept. 21.
An optical effect known as a glory is also possible to observe during undercast. A glory appears as a faint multicolored halo, at times simultaneously with the viewer’s shadow. It might seem mysterious to notice your own shadow encircled by the glory, but your shadow actually has nothing to do with producing this optical phenomenon.
Glories are located directly opposite of the sun and are produced by the deflection of sunlight by billions of small water droplets in clouds that are below the observer. In other words, a viewer can witness a glory when positioned between the sun and a cloud, which is why glories can happen concurrently with shadows.
Glories are quite common, frequently seen from an airplane window. Mount Washington is a great location to witness a glory when weather conditions are right; all you need are clouds below the summit that align with the sun and the viewer’s eye.
The shadow of weather observer coinciding with a glory.
If there is still fog lingering at the summit, you might also spot a fogbow amid a sea of undercast. Fogbow formation is similar to the occurrence of rainbows. Sunlight is reflected by small water droplets in the fog rather than by raindrops. Since the water droplets in fog are so small, the sunlight that shines through creates weaker colors and results in fogbows appearing almost colorless.
It is difficult to capture photos of fogbows on Mount Washington because they come and go so quickly, but be on the lookout for this optical phenomenon when the fog is thinning and the sun is shining bright at the summit.
If you get the opportunity to see undercast or one of these extraordinary optical phenomena at the summit, make sure to appreciate their visual beauty and the science behind Mount Washington’s weather.

Alexis George, Weather Observer & Meteorologist


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