Fog and mist are the same thing, tiny water droplets floating in the air near the ground, and one measurement separates them, which is visibility. When visibility drops below 1 kilometer, roughly 0.62 miles, it is called fog. When you can still see farther than that but the air is noticeably hazy with water droplets, it is called mist, and that is the whole official difference. It matters a great deal for aviation, shipping and driving, which is why meteorologists define it precisely. In everyday speech, though, most people use the two words loosely, and they are not really wrong to do so.
The Visibility Threshold
The 1-kilometer line between fog and mist is set by the World Meteorological Organization and used by weather services around the world. In the United States, the National Weather Service issues dense fog advisories when visibility drops to a quarter mile or less, a much stricter threshold. So not all fog triggers an advisory, only the densest fog that creates real safety hazards.
In practice, mist is when the air feels damp and visibility is reduced but you can still see across a city block or a valley. Fog is when you cannot see more than a few hundred meters and objects fade into gray. The Bay Area's summer marine layer is almost always fog by the technical definition when it rolls through the Golden Gate. In its thickest patches, visibility drops well below 1 kilometer. What people call "a little misty" on a San Francisco morning is often technically fog.
Droplet Size and Formation
Both fog and mist are made of liquid water droplets about 1 to 100 microns across. That is far larger than water vapor molecules but far smaller than raindrops, which are 1,000 to 5,000 microns across. The droplets are small enough that air currents keep them floating instead of letting them fall as rain. They form when the air cools to its dew point, the temperature at which it is full of water vapor and cannot hold any more. The extra water vapor then condenses onto tiny particles of dust, sea salt or pollen to form the droplets of fog or mist.
The same process that forms fog and mist also forms clouds. The only physical difference between a cloud and fog is location, because fog touches the ground and clouds do not. When you walk through fog, you are walking through a cloud at ground level.
Other Related Terms: Haze, Drizzle, and Brume
Haze is often confused with fog and mist, but it is a different thing. Haze comes from dry particles in the air, such as smoke, dust, pollution or sea salt crystals, rather than water droplets. It looks similar, with reduced visibility and a milky or brownish sky, but the cause and the health effects differ. Fog and mist are generally harmless to breathe, while smoky haze carries particles that can damage the lungs. Bay Area residents often need to tell the two apart, since a gray morning could be harmless marine layer or smoke from a distant wildfire.
Drizzle is different again, because it is precipitation, with drops large enough, 0.1 to 0.5 millimeters, to fall. Fog does not fall, but drizzle does. The Bay Area's marine layer often produces something that feels like drizzle, a wetness that seems to land on surfaces. Usually it is fog drip, made when floating fog droplets collect on clothing, hair, trees and other surfaces and run together. True drizzle falls from above and leaves droplets that build up on flat surfaces. In the Bay Area, rain itself mostly brings mist rather than fog, as our guide to precipitation fog explains.
Brume is the French word for mist, and it is why aviation weather reports use the code BR for mist. You may see it in airport and marine weather reports. For most Bay Area residents, simpler terms work fine. Call it fog when visibility drops below about two-thirds of a mile, and mist when the air is damp but you can still see well beyond that.
Deep Dive: How Do Tiny Droplets Cut Visibility So Sharply?
Fog blocks your view by scattering light, not by absorbing it. Every droplet sends light off in new directions, and over a few hundred feet that scattering washes out the contrast between an object and the background. How far you can see depends on how much water the fog holds and how that water is divided into droplets.
How Do Scientists Calculate Visibility?
In 1924, the German physicist Harald Koschmieder worked out a simple rule for how far a dark object stays visible against the sky. The distance equals about 3.9 divided by a value called the extinction coefficient, which measures how much light the air removes over each meter. The constant comes from assuming the eye can just detect a 2 percent difference in brightness.
For fog, the extinction coefficient depends mostly on the total surface area of the droplets. By our calculation, fog holding 0.2 grams of water per cubic meter in droplets about 20 micrometers across cuts visibility to roughly 200 feet. That is dense fog, below the quarter-mile threshold for a Dense Fog Advisory.
The same math shows how little water mist needs. By our calculation, with droplets of the same size, visibility of about 1.2 miles takes only about one thirtieth of that water. Mist and fog are the same kind of cloud, but fog simply packs in far more water.
Why Do Smaller Droplets Make Thicker Fog?
Splitting the same water into smaller droplets gives it more surface area, and more surface scatters more light. By our calculation, halving the size of every droplet while keeping the total water the same produces eight times as many droplets and doubles the extinction. Visibility then falls by half.
That is why forecasters cannot predict visibility from water content alone. In 2006, Ismail Gultepe of Environment Canada and colleagues published a visibility formula for weather models that uses both the liquid water content and the number of droplets. Aircraft measurements had shown that droplet numbers vary widely at the same water content.
The droplet count depends on the particles the droplets grow on. Clean ocean air offers fewer, larger nuclei, while polluted air offers many small ones. Fog drifting through a city can therefore turn denser even without gaining water, which is one reason visibility can drop suddenly near busy freeways.
