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I am a writer. In my fictional world, I have an area that is supposedly covered with fairly thick fog during most of the night and nearly all of the morning, basically every day (it can leave a bit before noon if necessary). My question is this: What factors would I need to achieve this effect?

Factors: It is a fantasy world, so the factors need to be natural. The area is within a city, and is very densely inhabited. The area is enclosed by large trees, and large roots run all over the ground. The fog does not need to reach any higher than 7 feet, but it does need to cover nearly the whole area (a generous approximation would be 288 ft by 228 ft - about 65,000 square feet). The walls of the area (giant roots/tree trunks) vary in distance from each other a lot.

Topography: There is a massive forest a few hundred yards away from the city on three sides. On one side stretches a generally flat plain. The city itself is on a fairly flat piece of land. Rain water is collected and can be found in large standing pools throughout the city (don't think fountains. Think small swimming pools.). I can somewhat alter the topography if needed.

Let me know if you need more details on the surroundings.

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    $\begingroup$ Have you done any research on what fog is and how it forms? I suggest you at least start with the Wikipedia article: en.wikipedia.org/wiki/Fog $\endgroup$ – Nick2253 Jan 20 '15 at 22:02
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    $\begingroup$ River or anything like that? The best set up for heavy fog tends to be a cool air rolling over top of a warmer ground (rivers tend to work well for this). The cold air mixing with warmer humid air causes the thick fog. I'm not sure if standing pools of water would be able to provide the humidity needed for heavy fog. As an alternative, a cold airmass can hit the nearby forest (forest having a river/stream that provides the humidity...or just warm wet ground really) and this fog is blown in from the forest. $\endgroup$ – Twelfth Jan 20 '15 at 23:37
  • $\begingroup$ @Nick2253 Yes, I did do some research, but I wasn't sure where all of the variables for fog would come from (What causes the air masses, how topography can change things, etc.) $\endgroup$ – Thomas Reinstate Monica Myron Jan 21 '15 at 0:06
  • $\begingroup$ @Twelfth So if I had a relatively warm stream running nearby in the forest, what might cause a cold air mass to hit it with regularity every night? $\endgroup$ – Thomas Reinstate Monica Myron Jan 21 '15 at 0:09
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    $\begingroup$ Visit San Francisco. $\endgroup$ – James Feb 20 '15 at 20:26
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Fog is created most frequently by a warmer air (relatively speaking here...warmer air just holds more water/humidity) meeting a much drier and cooler mass of air. The quickly cooling warm humid air becomes fog...it's the same effect as when you see your breathe on a cold/dry evening.

A nearby stream could provide your water source...to be fully honest, a forest floor can work just as readily if it's holding the moisture needed.

A cool evening air can be provided by a few sources...from my experience, air coming from nearby mountains can easily provide this. Arctic air can also be pushed/pulled into the region by prevailing weather systems as well. A cloud free evening can also work, but not as well...the cooling effect doesn't provide the abrupt change that creates your really dense fog at ground level. There are a few solutions here...Easiest solution in my opinion is to have cooler air in hills or mountains and have that air flow into your city.

Remember you do need cloud cover...sunlight tends to heat up and 'burn off fog' and the prevailing winds can't be too strong or it simply blows the fog away.

Incidentally..you could go for the opposite cause here.

A warm air mass slowly creeping into a much colder air mass also creates fog as well. So your city would be cold (around 2 degrees celcuis). Your river remains a bit warmer and warms the air above it and supplies it with humidity. This warm air creeps into the cold air within your city and fogs it over.

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  • $\begingroup$ So how does this sound? My city is fairly warm, and a nearby stream provides the moisture, as well as lots of plants within the city itself. Nearby hills send in cool air in the evenings, which creates fog overnight. The fog burns off in the morning save for in the 'area,' which is heavily shaded by leaves, thus keeping the sun out. Would that work to consistently create fairly thick fog every morning? $\endgroup$ – Thomas Reinstate Monica Myron Jan 21 '15 at 1:24
  • $\begingroup$ I don't think this is going to work reliably, given your terrain. The slightest breeze will disperse the fog from the area. About the only thing I can think of that would work halfway reliably for a small area is a hot spring or springs in an otherwise cool & humid area. $\endgroup$ – jamesqf Jan 21 '15 at 4:14
  • $\begingroup$ The current topography actually has the area enclosed by trees and gigantic roots taller than houses (it's an elven city). It is also sunken, so it is in a sort of depression. Would that be able to keep the fog in? $\endgroup$ – Thomas Reinstate Monica Myron Jan 21 '15 at 5:17
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    $\begingroup$ That sounds like it should work...keep winds low as it removes the fog pretty quickly. A depression (city built on lower land, almost a valley) would help contain the fog, and quite likely lessen the impact of wind. $\endgroup$ – Twelfth Jan 21 '15 at 17:56
  • $\begingroup$ @Twelfth Thanks! Sounds great! One last question: To achieve the cooling air from the hills, is there any minimum height that they need to be? $\endgroup$ – Thomas Reinstate Monica Myron Jan 21 '15 at 18:18
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As someome mentioned in earthscience.stackexchange.com, San Francisco is a great example.

I recommend having a look at this book:

Weather of the San Francisco Bay Region, Harold Gilliam, UC Press, 1962, 2002.

On the coast of Northern California, the wind is predominately from the NW and pushes the surface layer of the waters near the coast towards the equator. Because of Coriolis, that top layer of water will veer to the right (offshore), and is necessarily replaced by cool water from the deep ocean (This is called upwelling and is a necessary ingredient for the persistent fog). Therefore the sea surface temperature of the coastal ocean is relatively low. When the marine layer interacts with the surface water it cools to its dewpoint and if there are particles for the droplets to attach to then you get fog.

I am pretty sure the mouth of the golden gate as well as the large hills flanking the city is an extra factor. The fact is, what you described is the typical San Francisco day. The book will probably help much more than I can.

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  • $\begingroup$ Thanks. Unfortunately, my city is very far from any coast. $\endgroup$ – Thomas Reinstate Monica Myron Jan 21 '15 at 18:17
  • $\begingroup$ You might want to rethink that, @ThomasMyron. You need a large body of water to create the fog. $\endgroup$ – Cyn says make Monica whole Dec 19 '18 at 2:47
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Although you're asking about a fog, a good explanation for this in an urban area would be a smog like the old London Pea-Soup fogs of the nineteenth and early twentieth centuries. If you have sufficient population burning soft coal this is a very likely consequence. Alternately as you're in a fantasy setting you could have a type of wood that burns to release a very thick smoke but is also the best ( or cheapest, depending on the district ) source of heat available to your citizens. In that circumstance the smoke could even be relatively harmless or even hold other beneficial properties such as residents in that part of the city being relatively free from skin parasites. Alternately from domestic burning, there could be an industrial or craft process going on in the district - or nearby - that releases a lot of particulate smog.

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  • $\begingroup$ That is a good conversion from industrial to fantasy I had not thought of. I kind of feel like I want the natural clean feel of plain fog though (granted, I've never been around smog, so I don't know how it feels... which might be another reason for me to stick with fog). $\endgroup$ – Thomas Reinstate Monica Myron Jan 22 '15 at 18:36
  • $\begingroup$ As I understand it the number of smog-related fatalities in the early 20th Century ( and indeed still going on in China ) would indicate that it doesn't feel that clean... $\endgroup$ – glenatron Jan 22 '15 at 23:09

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