I have an area of my world that is essentially a large crater. It is a huge barren land surrounded by mountains. The ground is stone, then below are underground rivers, and below those are pools of lava. The lava causes the water to boil and steam, forcing the moisture through cracks in the stoney ground above. Since the area is surrounded by mountains, the wind currents go over the area and the fog never dissipates.

Would the climate behave this way?

In the land is a creature who has become adapted to the heat and moisture. They are intelligent like humans. I can think of two ways they would be able to see in this climate. One, the creatures have infra-red vision and have learned to ignore the heat of the air, so other creatures or structures would appear because they have different heats. This would also mean that if a person were to enter the area and change their body temperature to that of the air, they would be invisible? Two, the creatures have some sort of tunnel vision which allows them to pierce the fog, caricaturing blind spots everywhere except the very center of where they are looking.

Are either of these ideas realistic?

Are there other ways to penetrate fog without some kind of technology? Magic is acceptable, tho preferably not used unless it has to do with genes


The lava is significantly far underneath the ground. Hot steam rises through cracks and are cooled high in the mountain air, then the fog produced descends and fills the crater. Assuming that's realistic.

I was assuming that there was a good chunk of stone between the lava and rivers, but apparently the lava would melt it or cool. Could this create a balance that would both heat the water to form steam and also allow enough stone to be left for the water not to fall directly into the lava?

I was imagining moss and small squishy/shrubbery type plants. Possibly some kind of foliage from Avatar, that is well adapt to damp conditions.

  • $\begingroup$ What do you have in your toolbox? Is there advanced technology (possible only in the past) or magic? Or is it basically like Earth? $\endgroup$
    – Rob Watts
    Commented Mar 13, 2015 at 22:34
  • $\begingroup$ Its fantasy, so there's magic. The planet is basically earth, but its medieval times $\endgroup$
    – Towell
    Commented Mar 13, 2015 at 22:38
  • $\begingroup$ The hot air from the steam would still be warm, and continue to rise with its condensate cloud, out of the crater. $\endgroup$
    – Oldcat
    Commented Mar 13, 2015 at 23:28

4 Answers 4


Without Magic:

If the air in the crater is rather cool, then I believe Twelfth's answer is correct. If the crater is full of hot air (I assume this because you mentioned the creatures "have learned to ignore the heat of the air") I don't believe that this will work. First of all, what you're describing sounds like an inversion, but those occur when their air is colder than the air above it. If you have a hot environment the air is going to rise and mix with the air above it. I believe the convective forces that emerge would cause the crater to be rather windy, with lots of updrafts and downdrafts. That would most likely clear out any water vapor that would form.

Also, fog would have trouble forming in such an environment. According to Wikipedia,

Fog forms when the difference between air temperature and dew point is generally less than 2.5 °C or 4 °F.

The dew point is also affected by temperature and pressure - higher temperature and lower pressure (which you'd have here) increases the dew point, making it harder to have fog. If the humidity is high enough you could still have fog, but that introduces its own problems - an environment that is very hot and very humid would be unbearable for everything but extremophiles, and those are limited to micro-organisms. If a person matched their body temperature to that of the air to be invisible to IR vision, they'd be dead.

As Twelfth mentioned, infrared vision wouldn't work very well through fog/steam, even in the hot environment scenario.

Unfortunately tunnel vision wouldn't work either. In heavy fog you can't see things because the light bouncing off of the objects gets scattered before it can reach you. Nothing passive will allow you to "pierce the fog" because there's simply no organized light that reaches you.

One other thing that might not work for your purposes would be to change the composition of air in the crater. If you had heavier elements in the air it would be more dense, making an inversion more likely and also reducing the dew point and making fog possible without killing people from the heat. Unfortunately the denser air would also push out oxygen (and probably be toxic anyway), so people who visit would have to bring oxygen with them. This also makes it less plausible for any non-microscopic creatures to naturally develop to live there as it would be a large evolutionary leap to handle air like that.

With Magic:

Obviously anything can happen with a sufficient amount of handwavium, anything can be possible. So how do we get this environment with minimal effort?

One of the key points of inversions is that the air below becomes more dense than the air above. So what if the area is inherently more magical, and that causes the air to be more dense? This solves your problem in the same way that the non-magical denser-air solution does without killing people from asphyxiation. Additionally, if we already have a strong magical field there why not replace IR vision with vision that relies on the ambient magical field? It's easy to believe that fog wouldn't obstruct the magical field, and a person trying to match the ambient magical field would be less dead than someone who tries to match the ambient (hot) temperature. Also, the creature's tunnel vision could be explained by requiring the creature to be able to concentrate to see the field clearly - without concentration they can see the general shape of the field, allowing them to navigate without stumbling, but they can only see subtle changes in the field if they are concentrating in a particular direction.

I'm not sure how much more dense the air would need to be in order to make this work, but based on some rough calculations I did I think it would only need to be somewhere around a 30% increase in density. For being on a mountain top, that means you'd have around the same density as if you were at sea level. That could also give your characters an added reason to go into the crater - after hiking through the thinner air to get there, they find a place where they feel like they can really breathe again.

  • 1
    $\begingroup$ Rob - I think we have differing versions of his image here. I took this to be the lava significantly underneath the ground...hot steam rises through vents that cools in the high mountain air and the fog produced descends and fills a crater (vents don't even need to be in the crater). I didn't take the question to mean the fog is warm while it's in the crater. Maybe I'm off with my interpretation of his question. $\endgroup$
    – Twelfth
    Commented Mar 13, 2015 at 22:55
  • 1
    $\begingroup$ @Twelfth I agree that we are envisioning the scenario differently. I'm going to add a note to my answer that mentions I'm dealing with the hot-environment scenario. $\endgroup$
    – Rob Watts
    Commented Mar 13, 2015 at 23:05
  • $\begingroup$ Would some kind of gill-lungs help with oxygen intake in you're more-dense climate? $\endgroup$
    – Towell
    Commented Mar 13, 2015 at 23:11
  • $\begingroup$ @XosMel I totally forgot to talk about how dense I meant. The difference really doesn't have to be that much - sea-level density should do it. $\endgroup$
    – Rob Watts
    Commented Mar 14, 2015 at 3:32

The nature of the fog you have there sounds correct. Water turns to steam, rising to surface where it cools quickly and condenses to fog. A crater can insulate from winds as well, though there can be a circumstance or two where the fog gets cleared out if you need. If you want to go a bit further, are you aware of geysers such as those located in Yellowstone national park in the US?

I don't think infrared will function well here...fog holds heat and would likely give minimal visual penetration and it's also dependent on daylight or a source of the infrared rays to see in the first place. I'd recommend something closer to sonar, somewhat akin to how a bat or dolphin navigate. It's 'active', meaning the creature emits the sonar itself and it's not dependent on any other source.


Water hitting lava generates so much steam so fast you would have something like Krakatoa, and the top of your mountain would blow off.

Fog usually occurs when warm air laden with humidity is cooled by bodies of water, which lowers its capacity to hold vapor and forces it to condense. If you had a cool lake and a source of humid, warm air a bowl could trap it for some time.

  • $\begingroup$ There is a good chunk of stone between the lava and rivers. The rivers are heated through conduction from the stone walls. $\endgroup$
    – Towell
    Commented Mar 13, 2015 at 23:25
  • $\begingroup$ lava is molten stone, it will melt through those walls eventually, or cool itself. $\endgroup$
    – Oldcat
    Commented Mar 13, 2015 at 23:26

In terms of the fog creation, a nearby sea could supply the moisture and a jet-stream like wind could carry it over. If the air in your crater were at the right temp, you would get dense fog.

A creature in the area could be able to see UV light, which would pass through fog much more easily than the light that is visible to us.


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