Your group is exploring the great outdoors...
How far can they see? Or, what is visible to them from where they are?
Say you're on top of a lone mountain on a clear day... How many miles?
Or what if you're in a CHAIN of mountains...
In the middle of a grassy plain...
I understand that in a forest or jungle your line of sight (usually 60ft) would be the norm.
Are there rules that govern this? Like how many miles into the next HEX can you see from where you are?
Ultradan
Your group is exploring the great outdoors...
How far can they see? Or, what is visible to them from where they are?
Say you're on top of a lone mountain on a clear day... How many miles?
Or what if you're in a CHAIN of mountains...
In the middle of a grassy plain...
I understand that in a forest or jungle your line of sight (usually 60ft) would be the norm.
Are there rules that govern this? Like how many miles into the next HEX can you see from where you are?
Ultradan
What you're asking has far, FAR too many variables to EVER be put into a single tabletop game.
At least one that's not named either FATAL or Rolemaster.
So, to help a little bit, I will note that the average distance a person with 20/20 vision on a flat plane (be it a plain, the ocean, a flat desert, etc.), can see with reasonable detail is 5km, or roughly 2.6 miles.
This is because the curvature of the earth only allows for so much distance.
If you get higher, you can see farther, assuming your sight is uninhibited by weather effects, by objects.
Luminosity is another factor, and the brighter, the easier it is to see.
So the answer is "infinitely, assuming the conditions are right"
In practice, however, you're limited by the speed of light.
Therefore, assuming an absolutely straight line, with a large and bright enough object, the human eye can see for a distance up to a number of lightyears equal to the age of the universe (in other words, the human eye can see approximately 13.5 billion lightyears away, because past that the light hasn't gotten to our eyes yet)
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So... the ruling is "whatever feels right," 'cause good stinkin' luck trying to calculate it all out on the fly.
Say you're on top of a lone mountain on a clear day... How many miles?
As a guideline, the maximum distance in mountain terrain at which a Perception check for detecting the nearby presence of others can succeed is 4d10 × 10 feet. Certain peaks and ridgelines afford much better vantage points, of course, and twisting valleys and canyons have much shorter spotting distances. Because there's little vegetation to obstruct line of sight, the specifics on your map are your best guide for the range at which an encounter could begin. As in hills terrain, a ridge or peak provides enough cover to hide from anyone below the high point.
It's easier to hear faraway sounds in the mountains. The DC of Perception checks that rely on sound increase by 1 per 20 feet between listener and source, not per 10 feet.
plains
In plains terrain, the maximum distance at which a Perception check for detecting the nearby presence of others can succeed is 6d6 × 40 feet, although the specifics of your map might restrict line of sight. Cover and concealment are not uncommon, so a good place of refuge is often nearby, if not right at hand.
forests
In a sparse forest, the maximum distance at which a Perception check for detecting the nearby presence of others can succeed is 3d6 × 10 feet. In a medium forest, this distance is 2d8 × 10 feet, and in a dense forest it is 2d6 × 10 feet.
Because any square with undergrowth provides concealment, it's usually easy for a creature to use the Stealth skill in the forest. Logs and massive trees provide cover, which also makes hiding possible.
The background noise in the forest makes Perception checks that rely on sound more difficult, increasing the DC of the check by 2 per 10 feet, not 1.
What you're asking has far, FAR too many variables to EVER be put into a single tabletop game.
But PF has rules for this.... sorta.
But PF has rules for this... sorta
The "sorta" is the kicker, though.
The PF rules are massively generalized and don't take bupkiss into account for things like atmospheric effects, temperature, etc.
You can use the rules presented if you want, sure, but they're a pretty god-awful presentation on how eyesight, hearing, etc. ACTUALLY work in conditions like mountains, deserts, etc.
Which is understandable - like I said, there are a LOT of factors that go into it that involve so many minute variables that a tabletop game (and even most COMPUTER games) can't handle it.
So, when in doubt, go with your gut.