I Was Just in the Valley – Why is it Foggy Up Here on the Mountain? (Or Vice Versa)

2017-08-08 15:18:14.000 – Julia Moreland, Summit Intern


One of the things Mount Washington is most notorious for is the seemingly endless fog that lingers on the summit. Many patrons will come from the valley, in beautiful, clear weather, and then arrive at the top of the mountain completely immersed in fog. With such a rapid change in weather, it seems almost impossible – so how is this fog developing so quickly, and why is it so constant?

Here on Mount Washington, we experience about 2/3 of the year in the fog, regardless of the weather in the valley. One of the biggest culprits are what is known as upslope fog. This occurs as a result of air being pushed up against the slope of the mountain, being forced up toward the summit, and in turn reaching saturation and condensing into clouds as it rises and the air cools. With strong winds blowing against the western slope of the mountain, we can experience days like the one pictured below. In this case, the synoptic scale setup in the region allows for clearing skies, but low level moisture (remaining in the valley) and the tight pressure gradient behind a low pressure system sends strong winds toward the mountains, inducing fog on just the tops of the mountains.

Summit fog can also come as a result of another lifting mechanism, such as a front. When a front approaches the mountains, air is again forced upward as it comes in contact with this large barrier. With the additional lifting that comes with a cold front – cold air cutting through the valley and forcing air to rise very rapidly with the help of the mountains – even more intense clouds can form on or above the summits. As this cold front traversed the White Mountains region, moisture in the valley was forced up dramatically, creating a beautiful display of cumulonimbus clouds, which can produce thunderstorms, precipitation, and lightning.

Sometime even the opposite might happen – you may see lots of cloud cover overhead while in the valley, but come up to clear, open skies. We can also see, from our perspective, some beautiful examples of valley fog, which is a form of radiation fog. This can form for a few reasons. Typically on calmer and clearer nights, as the radiation is released back into the atmosphere from the earth’s surface after being heated during the daytime, the air near the ground cools and all remaining cool air sinks, as it is denser than warm air. With this colder air now so close to the ground, it is now near or at its dew point, allowing fog to easily form without the air having to rise much further up in the atmosphere to saturate. This will result in beautiful pools of fog sitting at the bottom of the valleys, only able to evaporate once the sun comes out and again heats up the air near the ground.

Next time you come up to the summit, check out whether the valley or the summit is experiencing fog, and see if you can figure out why!


Julia Moreland, Summit Intern

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