I have a canyon, with similar size and shape to the Grand Canyon. The canyon is carved by a large river, similar to the Colorado river.

I want this canyon to be filled with fog so thick that, if you are standing at the top, you can't see the bottom.

I would prefer the Canyon be

  • Not too cold at the bottom
  • Livable, if not nessecarily comfortable, for human life (ignoring growth of crops or anything like, just that someone wouldn't die if they went down there)
  • Completely, or if this is not possible, very dark at the bottom of the canyon, even at noon

Is this possible? If so, what do I need to do to create it?

  • $\begingroup$ if it is that humid the canyon will quickly fill with greenery. $\endgroup$
    – John
    Jan 11, 2022 at 22:04
  • $\begingroup$ @John Hopefully. That's actually the goal. $\endgroup$ Jan 11, 2022 at 22:35

2 Answers 2


Valleys can totally be full of fog.

fog http://www.personal.psu.edu/users/m/d/mdw5005/meteo481.html

Grand canyon size is perfect. A river runs thru a desert. It comes a long way and it is shallow; the water is warm. As the river goes along, the valley gets deeper and deeper because the land is rising - the river has been there a long time and has cut a deep channel.

Farther along the course of the river, the highlands now on either side of the river have cold air. It rolls down into the valley and hits the humidity coming off the warm river. Fog is born.

If you want to fantastic it up, have a geothermal spring contribute its boiling water. That will make some steam. Such rivers exist!

boiling river https://www.sciencefriday.com/articles/amazon-boiling-river-microbes/

  • $\begingroup$ Great! One question though, does this match the "dark at the bottom?" This is mostly the key. $\endgroup$ Jan 11, 2022 at 23:56
  • $\begingroup$ The Wollemi Pine canyon floor gets only 10% of the light at the surface. It is a lot narrower than the Grand Canyon. Also in addition to the canyon walls, it is full of big trees. wollemipine.com/news/The_Jurassic_Tree_And_The_Lost_Valley.php $\endgroup$
    – Willk
    Jan 12, 2022 at 0:03
  • 1
    $\begingroup$ @Firedestroyer not so much, but almost. I have been in such a canyon - albeit not much of a one, in the Pantanal - and you can't see a meter from your face, you can't see the path, it's all a light-greyishness. It's even difficult to hear. We were warned never, ever, whatever the reason, to let go of the guide rope. If you made it one or two hundred meters thick instead of maybe fifteen or twenty, and added some clouds above, then it would be quite dark. $\endgroup$
    – LSerni
    Jan 12, 2022 at 0:05
  • $\begingroup$ @LSerni Thanks for the first-hand knowledge. Unless some better answers come, I'll be accepting this one. $\endgroup$ Jan 12, 2022 at 0:15
  • $\begingroup$ @LSerni If it was completely unacceptable to let go of the guide rope why was it even possible to do so? Why weren't you hooked to it? $\endgroup$ Jan 13, 2022 at 4:48

To get large amounts of fog/cloud you need large flow of warm moist air to be then be cooled in the target area. Easiest is with preexisting volcanism to supply the energy.

Scenario one: Older yet still active volcano.

A large volcano near a large body of water, on the down wind side. This will ensure valleys will have rivers due to high rain fall. Then the volcano needs to be still active/have and there to be lava near surface. If molten rock is near surface close to one or more rivers near the base of the volcano then there would be a source of warm moist air, along with cooler air in the valley bottom.

This implies basaltic lava and newer igneous terrain which tend to not form large canyon systems. This would scenario would tend to have smaller steeper valleys. But more common then scenario two.

Scenario two: Broad hotspot.

A hotspot such as Yellowstone or Hawaii. A river flows through the region which is heated up and provides warm moist air, upon mixing with the cooler surrounding air, clouds form. this is similar to scenario one but assumes a broader area and more flexible terrain. A hotspot can move under sedimentary rocks where grand canyon like valleys can be carved.

This could allow closest to requested features of terrain but would be least likely to have contentious broadly covering cloud. It would be less continuous due to more opportunities to have broad mixing of air. This would imply shifting locations of cloud/fog.

Scenario Three: Mad scientist secret hideout.

To hide secrets, a nuclear power plant is installed in a valley. A system of piping is installed to release steam throughout the valley thus ensuring permanent thick clouds. The planned nature of this allows for much more consistent and thicker cloud cover.

This would have the highest consistent density of cloud by far thus providing the most shade. However this is an deliberately engineered outcome, which might not fit desired plot.


There are other scenarios that are possible. But to get reliable thick cloud/fog a reliable energy source in close proximity to water is required.


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