Learn all about undercasts with world-renowned undercast expert, upthemmountains*


Undercasts: they’re a frequent occurrence around high summits or on the West coast – last time I was there, I practically saw one every day. They don’t happen as often in the Northeast since there are no high summits and, well, the East is not the West.

While undercasts are rare in the Northeast, Fall and Winter are the best seasons to catch one. Last year, on the way to the MacIntyre range, we were bummed out by the cloudy weather we’d have for this scenic hike. But just before the first summit, we actually found ourselves over the clouds. It was the most intense undercast I’d ever seen. It might’ve actually been the first “real” undercast I’d ever seen, but still, it was intense. That was last November.

With the memory of that day still vivid, when this November came along, I went out thinking it was a sure bet. We’d just head to a high summit on a cloudy day and there’d be an undercast. Plain and simple. Bullet-proof right? Well, the plan failed as bad as the white out was thick. It turns out undercasts don’t just happen in November. There’s more to them, like science stuff. Here’s the definitive guide to undercasts**.


The term comes from aviation. Everyone who’s flown a few times has seen one at some point: when a plane flies above the cloud cover, it’s an undercast, or cloud inversion. Now convert that definition to a mountain activity such as hiking. It means that someone would be on a mountain or ridge completely above the cloud cover, like this:


Cloud inversions are the result of a few key elements
Temperature inversion. It usually gets colder as elevation increases, right? A temperature inversion simply means the opposite: it would be colder at ground level and get warmer as elevation increases. As the cool air is denser than the warm air, it remains trapped at ground level. It’s not that simple though.

The right conditions have to align for this cool air to stay trapped and transform into clouds. The ground is warmed by solar radiation during the day, which in turn, warms the air above it. The night before an undercast should be cool and have no wind, to allow the cold air to settle and the warm air to rise.

Then, the day of the undercast should be considerably colder than the previous day. This allows the moisture in the air to condense and can lead to the formation of clouds at low altitude.

Humidity. It’s not just a matter of warm and cold air. The cold air has to be moist, or have enough humidity. How much? Enough. In the vicinity of 100%. That’s all I got. The closer you are to the sea, the more likely you are to have moist air. This may be one of the reasons why the West sees a lot more undercasts, along with the fact that there are high mountains close to the sea to stop the moist air from flowing.

In the morning of the undercast, the sun will rise, its warm rays will hit the mass of cold humid air and it will produce some kind of fog, smog, steam or, with a little luck, thick clouds. The warmer and more stable air will trap the clouds at low altitude.

High Pressure (anticyclone). High pressure is the final ingredient. It’s not a key factor everywhere, but in the Northeast, it’s what helps trap the cold air mass underneath the warm air mass, which, as explained, is essential.

This cocktail of meteorological elements is more frequent in the Fall or Winter. That’s why odds of seeing a cloud inversion are so much better this time of year.

The best time and place to enjoy an undercast
Conditions are ripe for an undercast? Pick your time and spot to witness this spectacle. Undercasts are usually more intense in the morning, and light is better anyways, so plan for an early start.

They’re also known to be thicker or linger longer in rolling hills and sheltered valleys. A body of water such as a pond or lake can help as well. In the Adirondacks, the High Peaks Wilderness has the highest peaks of the region, many valleys and some water. The areas around the Dix Wilderness and the Great Range should be good too.

Choose a high mountain that has other high landmarks around. It gives a sense of perspective and adds drama to the scene. Compare the two shots below. They’re both taken from Algonquin. One includes other peaks that stick out like islands and the other only features part of a ridge. Imagine being on that ridge at that moment. It would’ve been nice, but there would’ve been nothing around to give the sense of perspective you see below. But then again, it’s all subjective isn’t it?

Here’s what you should be looking for in the weather forecast:

Day before the undercast
-Precipitations can help as it will add moisture to the air
-Cool and calm night
-Warmer day

Day of the undercast
-Considerably colder than the previous day
-High humidity
-High atmospheric pressure
-No precipitations (trace or drizzle is fine)

Well, that’s it. Good chasing!

*upthemmountains is not a world-renowned expert on anything and takes no responsibility for any hike that results in a total white out.
**This is not a definitive guide, just a summary of information compiled from a few meteorological websites and sources. Again, I take no responsibility for anything..!