[Slight spoilers for the recent movie The Mist]
Godzillas in the Mist?
By Wil McCarthy
When a light ray strikes water, it bends. Stick a pencil in a glass of water and you can see this effect clearly: the pencil appears to kink sharply at the water's surface. This occurs because the water, being denser than air, has a higher index of refraction (technically speaking, because the speed of light is lower in a denser medium), so it acts as a prism. So what happens when the air is full of water? Nothing much, if it's in the form of vapor. As the amount of water in the air rises, though, a property called relative humidity (the amount of water in the air divided by the amount the air can actually hold) rises with it, and as this value approaches 100 percent, the water begins to condense into tiny droplets.
When this occurs in the sky, we call it a cloud. When it happens at ground level, we call it fog. Or "mist," although the strict meteorological definition of that word implies a very thin fog, with visibility between 1 and 2 kilometers. (Any thinner than that and it's simply called a "haze.") Still, most people use these terms interchangeably, so what the heck.
Thick fog — the kind we call "pea soup" — happens when a layer of humid air is trapped below a heavier layer of denser, colder air. This is known as a temperature inversion, and it doesn't usually happen unless the weather conditions are changing rapidly. And the sharper the temperature mismatch, the thicker and denser the fog will be. The clouds are literally squashed against the ground. Anyway, because it contains millions of tiny prisms, fog (or mist) can bend a light ray millions of times, scattering it in every possible direction. The result—even in full daylight — is a diffuse gray illumination that obscures details, baffles human senses and has fascinated writers and mythologists since the beginning of time.
( More, concerning survival in a parallel universe... )