Whenever I see some fog, it usually is very opaque when I am far away, like cloud, but when I am it, it’s far more transparent, though obviously visibility is decreased
In: Physics
Fog is a cloud that is visiting the ground. Clouds are water floating in the sky that hasn’t formed droplets in order to make rain yet.
Water in any form refracts light; that is, when light hits some water, it redirects a bit. If this was a solid (or rather liquid) wall of water and you shone a light through it, you could measure a change in trajectory. The thicker the wall, the more drastic the redirection. But because the water is suspended in air in a vapor, rather than a uniform change in direction, it is a more random scattering as any given photon makes its way through air and hits water and adjusts course accordingly.
Your eyes see by absorbing light that reflects off of other objects. So combining everything: when you’re outside of the fog, you have more water being bent away from your eyes on that return trip. Since the light isn’t a constant angle when it bounces off of things inside the fog, it all scatters and mixes every which way and all the colors of light that hit the fog and reflect and refract within the fog mix together to that grey blob that fog is, unless you’re close enough to something so that the light reflecting off of it hasn’t scattered too much. Because all that scattering is effectively random, the light going in does scatter but it evens out enough that enough light hits whatever is in the fog to reflect off of it so that if you’re not having that light scatter too much on the trip off that object your brain can piece together what that light is bouncing off of.
The more fog between you and the object you want to see, the less you will see it.
If you look at fog from afar, you’re likely looking through a lot of fog (because there is just nothing else there but air). But if you’re inside, you can focus on objects that are close enough to still see them somewhat, and you’ll just ignore anything farther away.
Volume. Let’s say one cubic meter of fog reduces the amount of light going through it by 2%, so having one meter of fog between you and, say, a rabid bear would obscure your vision of it by a little, but not a lot. Now, if the density of the fog is consistent throughout the forest you’re in (forgot to mention, you’re in a temperate evergreen forest), then you just by moving your body, you change how much fog is between you and the bear, and how clearly you can see it. Closer to the bear = less fog and more visible light that reaches you. If you’re standing outside the cloud entirely, then whatever is on the other side is going to have to pass through the cloud entirely for you to see it. If there’s enough fog to almost completly block the light, then all you’re going to see is the fog.
Think of it like a pointillism painting. It looks like a beautiful painting from far away but up close you can see the individual dots and the spaces in between the dots.
From the outside, the fog boundary where it meets clear air is well lit, so the fog will reflect plenty of light into your eyes. When you’re inside the fog, most of the light has reflected or scattered out by the time it reaches the middle.
Your eyes distinguish contrast between objects with a biological equivalent to dynamic range – the darkest dark that can be seen in conjunction with the brightest bright. It’s a sliding scale. When it’s extremely bright, dark things can go missing, and it can be hard to tell the difference between 2 shades of ‘gray’ in the middle. When it’s very dark, everything with a little light stands out.
As such, once inside the fog, the darkness causes your irises to open up letting more light in, and the lower end of the range is where your sensitivity to subtle differences is higher. It’s the same as when you put on sunglasses and suddenly can see details a lot more clearly – you’re putting your eyes in the most sensitive range where they aren’t being starved for light nor blown out by too much.
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