Considering some of the answers given in a previous question, I would like to ask what climatic phenomenon would be necessary to generate a mega-fog, thick enough to at least reduce the intensity of sunlight on a hypothetical planet.

In the previous thread it was indicated that perhaps one convenient way to achieve this end would be the continuous generation of volcanic ash, but it would be unoriginal in relation to sagas such as Mistborn, by Brandon Sanderson, or Tolkien's Mordor itself.

What I'm looking for is a mega-fog. A fog originated naturally, but exaggerated. What causes would this mega-fog have? What effects would it have on the ecosystem?

  • $\begingroup$ well, just have your atmosphere be composed of something blocking the visual spectrum. Our atmosphere blocks a significant amount of sunlight from reaching the surface. The reason we see the visual spectrum is because it is a big part of what the atmosphere lets through - that is not much though. Or are you asking about a planet that is much earth-like and sustain earth-like life? If yes, you should specify that in your question as it drastically lowers the amount of possible answers. $\endgroup$ Sep 22, 2018 at 21:37
  • $\begingroup$ As ArtificialSoul state, we see in the visual spectrum with optimal performance at "daylight" intensities because that is what works on Earth's surface. Any creatures in a different environment (eg deep oceanic) "see" with what works there - they do not perceive it as "dark" because by their standards it is not. So your viewpoint characters will not perceive their world as "dark" unless they are visitors from Earth or somewhere with equivalent light levels. $\endgroup$ Sep 23, 2018 at 0:35
  • $\begingroup$ My planet is earth-like. It should sustain earth-like life. $\endgroup$
    – Jano Moore
    Sep 23, 2018 at 1:46

3 Answers 3



In Victorian era London so much coal was burned and so many industrial pollutants were in the air that the infamous "Pea Soup" began. The pollution reacted with the climate to produce a dense, foul smelling, toxic fog that would periodically blanket London. The worst instance of Pea Soup actually killed 12,000 people over the course of a few weeks in 1952. The main cause for this was twofold, particulate pollution in the form of very fine soot allowed London's natural fog to form much thicker and more easily. The cause for the Pea Soup's lethality was sulfur dioxide from the burning of cheap low grade coal.

The ecological effects of this phenomenon were very wide ranging. One of which was that it blotted out sunlight and made many species of plants unable to grow. it even effected the evolution of some local species of moth. The trees which they took shelter on had white bark, so they had evolved to be white, but after decades of industrial pollution the trees bark and become black, and the moths had evolved over generations to be black as well. Animals and people alike suffered from severe respiratory illnesses, sometimes even resulting in death. In one historical account a housewife's chief complaint with the toxic fog was that it made laundry harder to dry because it made everything damp and blocked the sunlight. Interestingly enough, people of the era didn't rarely seemed to make much connection between the foul smog and the thousands of respiratory deaths every year and primarily bemoaned the smell and the difficulty it caused with everyday tasks. Point being is that looking back on it today we an see that the millions of coal burning ovens, factories, and steam engines had rendered London so polluted to the point that we probably wouldn't consider it safely habitable by today's standards.

Possible Natural Cause

Now, I know that the whole "careless industry and greed of man poisoned the planet" thing is kind of an overdone cliche now days, but hear me out. What if it was a naturally occurring pollutant? You've got a planet that has lots of smallish landmasses covered in swamps, essentially many continents the size of the UK or new Zealand in an ocean much larger than ours. Over time these island-swamps have built up massive low grade coal beds, and after millions of more years have risen a wee bit to become forests and grass lands. The same tectonic activity that has pushed these many micro-continents upwards out of the vast ocean also has forced these massive coal beds to the surface. Lighting strikes have ignited them and the entire planet is essentially belching sulfur dioxide and soot from gargantuan natural coal fires. The proximity to the humid ocean air means that the soot and humidity combine to form a constant murky miasma of foul smelling fog that blankets large portions of the planet.

  • 1
    $\begingroup$ Isn't volcanic ash a naturally-occuring polutant, which the OP excluded? It seems Jano is specifically looking for water vapor, if possible. $\endgroup$
    – JBH
    Sep 22, 2018 at 22:22
  • $\begingroup$ It's not volcanic ash, its coal soot that is allowing the water vapor to condense more effectively. $\endgroup$
    – TCAT117
    Sep 22, 2018 at 22:26
  • $\begingroup$ Is there a practical difference between coal soot and volcanic ash for the purpose described? $\endgroup$
    – JBH
    Sep 22, 2018 at 22:53
  • 1
    $\begingroup$ Volcanic ash actually isn't actually ash, its powdered stone. This means its heavy, and sinks down to the ground fairly rapidly. Coal soot is finer and lighter. It hangs around a lot longer, it also attracts all the little water droplets in the air into denser larger drops, which makes heavy fog. $\endgroup$
    – TCAT117
    Sep 22, 2018 at 23:09
  • $\begingroup$ Thanks to everyone. I'm looking for something like water vapor, if possible. But it's only because I want to understand the ecological issues of that kind of fog in an earth-like ecosystem. After that, I will extrapolate and exaggerate them a lot. $\endgroup$
    – Jano Moore
    Sep 23, 2018 at 1:46

Fog is cloud that forms at the surface. The only distinction between cloud and fog is altitude. Cloud / fog forms when the temperature of the air mass falls below it's dew point, the temperature at which the air mass becomes saturated with water vapour and the excess water condenses out. Eg a warm moist air mass moves over a cold land (or water) surface (advection fog) as in Labrador or the Grand Banks. There are other mechanisms for this to happen. Any decent Meteorology text or google site should cover them. If we are talking about water fog you need a mechanism to:

  • evaporate water into the air

  • cool the air mass below it's dew point

    If you want to do this on a global scale that mechanism has to operate continuously.

Dew point can be raised (slightly) by adding fine particulates as condensation nuclei - cf London pea-souper. It helps if the particulates are hydrophilic (soot, also salt). Volvanic ash can be but this is dependent on the properties of the magma.

Just spit-balling here but if your planet had extensive sub-sea hot-spots causing massive evaporation over otherwise cold ocean / land surfaces (cloud cover would produce a high albedo and reduce insolation) you could probably handwave it from there. I don't think this would be stable over geological time, it would probably lead to a snowball earth scenario but much would depend on the precise land / sea distribution and continent size.

The only mechanism that I can think of for night to always be longer than day is some sort of double star (Nightfall in reverse?) or possibly an almost tidally locked moon.

For any planet with axial tilt the day/night cycle will always be seasonal.

You could pull a Game of Thrones and just not explain the seasonal cycles but imply [insert mcguffin here].


You seriously need to wait for our Sun to be a Red Giant, and Neptune to become livable, then your foggy, dark planet is possible.

But the plants? How could they thrive in a planet where its full of water? Aside from marine plants (e.g Water Hyacinth) You can use spaceship from earth, land it (well, make it float) on Neptune, and make it somewhat of an island where people can live, and where trees (or plants) can grow.

This is one idea where you can achive a global fog where the fog will last of eternity, why so?

First of, Neptune is a Gas Giant yes, but in the course of millions of years with heat nearing it (Sun becoming Red Giant Star). It could (a hypothesis) become a livable planet, well without land that is.

Add to it that the heat source's temperature is less than our current Sun (from 5,778 K to 5,000 K or lower). Neptune could produce global fogs that will not disspate immediately(maybe for all eternity) because of abundance of water and cooler surface temps. But I think this planet will be rainy most of the time.


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