Yes, fog is a cloud. It forms through the same physical process, contains the same tiny water droplets, and looks the same up close. The only difference is location: clouds form at various altitudes up to 12 miles above sea level, while fog forms right at ground level. When you walk through fog, you are literally walking through a cloud.
Why Fog Counts as a Cloud
Both fog and clouds form when water vapor condenses into tiny liquid droplets or ice crystals suspended in the air. The process requires two ingredients: moisture and microscopic particles of dust, salt, or smoke floating in the atmosphere. These particles, called condensation nuclei, give water molecules something to cling to. When air cools enough to become fully saturated, water attaches to those particles and forms droplets roughly 0.02 millimeters across or smaller. That’s true whether it happens a mile up or at street level.
Meteorologists classify fog as a type of stratus cloud, the flat, layered cloud variety that often blankets the sky in a uniform gray sheet. The only thing that separates fog from a stratus cloud overhead is altitude. If a stratus cloud descends low enough to touch the ground, it becomes fog. If fog lifts off the surface, it becomes a stratus cloud. People who live on mountaintops experience this regularly: what looks like a cloud from the valley below is fog when it rolls across the summit.
How Fog Forms at Ground Level
For fog to develop, the air temperature and the dew point (the temperature at which air becomes 100 percent saturated) need to nearly match. The National Weather Service identifies a gap of less than 5°F between the two as the threshold where fog becomes likely. That narrow spread can happen in two ways: the air cools down to meet the dew point, or enough moisture evaporates into the air to raise the dew point up to meet the temperature.
The most common type, radiation fog, forms overnight when the ground loses heat after sunset. The surface cools, and the thin layer of air sitting directly on it cools by contact. If that air is moist enough, it reaches saturation and visible droplets appear. This is the fog you see settled in valleys and low-lying fields on calm, clear mornings.
Advection fog forms when warm, moist air moves horizontally over a cooler surface. San Francisco’s famous fog rolls in this way: warm Pacific air drifts over the cold California Current offshore, chills rapidly, and pushes through the Golden Gate as a thick blanket. Upslope fog happens when moist air is pushed up a hillside or mountain slope, cooling as it rises until it reaches saturation. And steam fog appears when cool air passes over much warmer water, picking up moisture so quickly that condensation happens almost immediately, creating wisps that look like steam rising off a lake or river.
Fog vs. Mist: The Visibility Line
Fog and mist are both ground-level clouds made of water droplets, but the World Meteorological Organization draws a specific line between them based on how far you can see. If the droplets reduce horizontal visibility to less than 1 kilometer (about 0.6 miles), it’s fog. If visibility stays above 1 kilometer, it’s mist. The physical process is identical. The distinction is purely about density: fog is thick enough to meaningfully obstruct your view, while mist is a thinner version of the same thing.
This visibility threshold matters for aviation, shipping, and road safety. Dense fog can drop visibility to just a few hundred feet or less, creating hazardous conditions that mist typically does not.
Why Fog “Burns Off” in the Morning
The phrase “burning off” is misleading but descriptive. Nothing is actually burning. As the sun rises and warms the ground, the surface heats the air layer above it. That warming pushes the air temperature back above the dew point, and the tiny suspended droplets evaporate back into invisible water vapor. The fog appears to dissolve from the ground up as the lowest layers warm first.
This is why radiation fog is thickest just before dawn, when the ground has been cooling all night, and why it clears within a few hours of sunrise on most days. Thicker fog takes longer to clear because the sun’s energy first has to penetrate the fog itself, which reflects some sunlight back. In winter, when the sun is weaker and days are shorter, fog can persist all day.
Wind also plays a role. Light winds of a few miles per hour can actually help fog form by gently mixing moist air near the surface. But stronger winds break up the fog layer by mixing in drier air from above or by mechanically lifting the fog off the ground, at which point it simply becomes a low stratus cloud overhead.
What It Looks Like From Above
Satellite images make the fog-cloud connection obvious. Viewed from space, a fog bank looks exactly like a cloud layer because it is one. Weather satellites can’t distinguish between fog and low stratus clouds based on appearance alone. Forecasters use surface visibility reports from weather stations on the ground to determine whether a cloud layer visible on satellite is actually sitting on the surface as fog or hovering just above it.
If you’ve ever flown into an airport on a foggy morning, you’ve seen this firsthand. From above, the fog looks like a smooth white blanket of clouds. As the plane descends into it, the cabin windows go gray, and you’re suddenly inside a cloud, because that’s exactly what fog is.