When it’s not quite clear outside, but there aren’t fluffy, white clouds floating around either, then there’s some kind of presence hanging low in the sky that prevents you from seeing too far away. It’s either fog, mist or haze. Here’s how to tell them apart.
What is mist?
If there’s precipitation that falls at a temperature much colder than it itself, that water rapidly cools. Those droplets of moisture turn from invisible to visible. That creates a low-lying, amorphous blob of a cloud, which is what we call fog. It’s also what we call mist. Yes, fog and mist are pretty much exactly the same thing, the difference being the visibility factor. In the official parlance of airlines (which have a lot riding on visibility in the sky), fog describes conditions where pilots can’t see more than 1,000 meters (or 3,280 feet) away. In civilian, scientific circles, that visibility wall drops to 200 meters (which is 650 feet). If it’s “foggy” outside but you can see farther than 650 feet away, well, then it’s not foggy — it’s “misty.”
What is fog?
Fog is kind of an umbrella term for a number of meteorological phenomenon that involve temperature-changing precipitation. Valley fog is just that — fog that tops a valley, and which can stay around for a while because it has a hard time dissipating, being surrounded on all sides by hills and mountains. If fog descends and leaves tiny ice crystals on rooftops, that’s freezing fog. There’s even vog, a portmanteau of “volcano” and “fog.” It looks a lot like “normal” fog, but it’s formed by a volcano venting sulfur dioxide into the air, which then gets trapped low in the atmosphere.
What is haze?
Then there’s haze, which could also be confused with fog (or mist). But while those things are the result of airborne water droplets, haze is made of pollution particles. The wind carries air pollution away, where they gather and hang in a mass together. Then sunlight reflects off of that cloud of mostly inorganic material, resulting in visibility-affecting haze.