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My country is on the coast and spans deep into the continent. The technology level is before the invention of explosives. 10 years ago with naked eyes, anyone could see further than 10 kilometers away (if he/she was standing on a tall enough building). But today, the furthest anyone can see is 1 km tops.

What could possibly caused this? (non superstitious, mind you).

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    $\begingroup$ 1KM top? stand in the centre of a big crater. $\endgroup$ – user6760 Feb 28 at 8:53
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    $\begingroup$ Is it 1km in all directions or just one direction? Why are explosives relevant? $\endgroup$ – chasly from UK Feb 28 at 14:08
  • $\begingroup$ What about physical obstacles like trees? But then again, since it is inhabited, people might just chop them down. $\endgroup$ – Lord of the Larks Feb 28 at 14:20
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    $\begingroup$ Maybe you should fork a more specific question "Under what climate conditions might natural fog be a permanent occurrence on a continental scale?" $\endgroup$ – dhinson919 Feb 28 at 14:29
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    $\begingroup$ Do people care that they can't see far? Will they take action to improve visibility? What do you mean by "can't 'see"? Things are blurred beyond that distance? It is dark beyond a certain point? There is smoke? $\endgroup$ – chasly from UK Feb 28 at 14:58
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Air pollution can heavily limit visibility.

Look at Beijing: in days where the smog is heavy visibility is very limited.

Beijing with and without smog

Since you are setting your world in medieval time, have a lot of coal and wood to be burned. The resulting pollution of the atmosphere will do what you are asking for.

Climate can also help: fog, lack of winds, thermal inversion can also contribute to enhance the phenomena.

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To add to the other answer which mentions air pollution, here's a possible cause of the pollution:

Volcanic ash.

You mentioned that your country is on the coast and that it spans into the continent. As shown by the following diagram, volcanoes occur in mostly coastal regions:

World volcano map

This means your country could have a string of volcanoes along its coast. If the general wind patterns push air into the continent from the direction of the coast, then a prolonged eruption would cause a constant influx of volcanic ash into the air of your country. This would reduce visibility.

The extent to which it does this is up to you - look at the Toba eruption, which may have caused a global cooling period of up to 1000 years. Alternatively, the volcanoes could simply be slow burning. Rather than a huge, dramatic explosion, they could constantly release ash into the air, which is dragged inland by the wind. This is all up to you of course and depends on what you want to do with the story. Hope this helps.

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    $\begingroup$ Inland fires can generate a lot of ash too, although usually these do not last for months. But neither do volcanoes. $\endgroup$ – Willk Feb 28 at 15:14
  • $\begingroup$ @Willk I didn't think of that. I agree about the timescale though. That's why it would have to be either a supervolcano or a group of smaller ones producing the smog $\endgroup$ – user43712 Feb 28 at 15:23
  • $\begingroup$ I had this idea in mind but It's already been used. Good one though, this creates an opportunity to expand on plot. It could lead to further disaster. such as smoke inhalation on a large scale. failing crops. And eventually a volcano erupting obviously. $\endgroup$ – Elias Rowan Albatross Feb 28 at 17:05
  • $\begingroup$ @Willk: Re "usually these do not last for months", I'm guessing you don't live just to the east of California. $\endgroup$ – jamesqf Feb 28 at 18:34
  • $\begingroup$ Coal seam fires can last for decades (or even millennia), but I don't think that they typically put out enough ash to smog up a coast. I'm not sure that there's enough fuel in any one place for a long standing coal fire that intense. $\endgroup$ – Arcanist Lupus Feb 28 at 23:19
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VIRUS

@Roger thank you for the inspiration.

The world hasn't changed but the people in it has. A virus / bacteria / [something normally occurring] has undergone a trivial change for itself but in humans this change has caused our eyesight to degrade rabidly.

Once infected by the virus / bacteria at a young age the eyes of the victim start to warp in shape and this causes the limited view range.

Human and/or animal eyes now have an effective range of 1km and as it stands there is no cure or vaccine.

Could be fun if all animals had this happen to them vs only humans. Over time there would be a genetic push away from eyesight.

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  • $\begingroup$ Trachoma and river blindness are two real world infectious causes of eyesight loss / blindness. $\endgroup$ – Willk Feb 28 at 16:53
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    $\begingroup$ As far as eye optics are concerned, if we want to blur everything beyond 1 km, the range as small as 10 meters would have to be severely affected. $\endgroup$ – Alexander Feb 28 at 19:13
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Since your country is a coastal one, you could make sea fog a possible factor. Some kind of climatic changes over time (a combination of change in wind patterns and drop in temperatures) could possibly turn your country into a region where sea fogs occur frequently - both during day and night. And strong winds could push the fog inland so that visibility is highly reduced on towns lying by the coast.

https://eos.org/features/coastal-fog-climate-change-environment

Your country seems to resemble Australia quite a bit. :P

Sydney's Bondi Beach Fog on a beach at Australia

https://www.theaustralian.com.au/video/id-5348771529001-5980291757001/Sea-Fog-Blankets-Sydney-s-Bondi-Beach

Newport beach, Australia - 2004 Newport beach, Australia - 2004

https://www.telegraph.co.uk/news/earth/earthpicturegalleries/6501752/Australian-Weather-Calendar-2010.html?image=9

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  • $\begingroup$ I was going to say fog, but not after they said "spans deep into the continent". It then becomes a question of what the geography would need to be for this to happen. I'd assume California gets foggy once in a while, but Colorado does not? $\endgroup$ – Mazura Mar 1 at 1:07
  • $\begingroup$ @Mazura Oh okay, I guess I got carried away by the 'coastal' part and missed out on that point. $\endgroup$ – Lord of the Larks Mar 1 at 5:26
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Tragically, sudden onset vitamin A deficiency has swept the population. The high priest blames certain rascally-rabbits for decimating the carrot crop.

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Fuel

As towns grow, they burn more fuel. All of that fuel goes up in smoke since the tech is such that there is no way to move the fuel use far away (like with electricity) and there are no scrubbers or the like.

People burn what they can find for cookfires and campfires:

  • Wood
  • Other plant material
  • Meat (while they won't use meat for fuel, they use the fires they build to cook meat and other foods and they put out their own smoke)
  • Dried dung (a very common source of fuel, especially in places with lots of livestock but without a lot of wood)
  • Coal (way back when, coal deposits were on the surface or easily accessed; now you have to dig very deep to get what's left)
  • Peat (in communities with peat bogs, this is a primary source of fuel)
  • Oil (food grade, mostly used in lamps)
  • Bones (a prehistoric source of fuel)

Also: Large-scale fires (not the cookfires and campfires discussed above). These can be caused by nature or they can be deliberately set. Many cultures used fire to prepare fields for planting and also to manage forests/orchards. If there is a war going on, fires may be more common.

As others have pointed out here, all that smoke obscures vision like nothing else.

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