Fog is made up of tiny water droplets suspended in the air near the ground. Each droplet is microscopic, typically around 10 to 20 micrometers in diameter (roughly one-fifth the width of a human hair), and they float because they’re light enough to be held aloft by gentle air currents. Despite how thick fog can look, the actual amount of liquid water in it is remarkably small. A dense fog might contain only about half a gram of water per cubic meter of air, meaning the vast majority of what you’re walking through is still just air.
Water Droplets and the Particles Inside Them
Water vapor alone can’t turn into fog. Each droplet needs a tiny solid particle at its center to form around, called a condensation nucleus. Millions of these particles float in the atmosphere at any given time: sea salt, clay dust, wildfire smoke, and pollution particles from vehicles and industry. When the air becomes saturated with moisture, water vapor latches onto these specks and condenses into liquid droplets. Salt crystals and clay particles are especially effective at triggering this process because of how water molecules bond to their surfaces.
This means fog droplets aren’t pure water. They carry trace amounts of whatever particle they formed around, plus anything they pick up while floating in the air. In cities, fog droplets can absorb pollutants like sulfate, nitrate, and organic compounds from vehicle exhaust. Research in Strasbourg, France found that fog water in urban areas contained concentrations of major ions and organic pollutants more than 50 times higher than rainwater. Smaller droplets tend to be even more concentrated, with lower pH and higher levels of dissolved metals compared to larger ones.
How Air Becomes Saturated
Fog forms when the air at ground level cools to its dew point, the temperature at which it can no longer hold all its water vapor. Three basic processes can push air to that tipping point: cooling, the addition of moisture, or the mixing of two air masses with different temperatures and humidity levels. Once the gap between air temperature and dew point shrinks to near zero, fog becomes very likely, provided the wind stays light and skies are clear enough for the ground to keep cooling.
On a clear night, the ground radiates heat into the atmosphere faster than it receives it back, and the air directly above the surface cools at a rate of roughly 2 degrees Celsius per hour. That radiative cooling, combined with light turbulent mixing, can push the lowest layers of the atmosphere past saturation. This is how radiation fog forms in valleys and open fields on calm, clear nights. It’s the most common type of fog in many regions.
Advection fog works differently. It forms when warm, moist air moves horizontally over a cooler surface. The cool surface chills the air from below, dropping it to its dew point. This is the classic fog that rolls in off the ocean along coastlines like San Francisco’s, where warm Pacific air slides over cold coastal water. A third mechanism, evaporation fog, occurs when cool air sits over warmer water. Moisture evaporates into the cool air faster than it can absorb it, and the excess condenses into fog just above the water’s surface.
Fog vs. Mist vs. Clouds
Fog and clouds are physically the same thing: suspended water droplets formed by condensation. The only difference is location. Clouds form at altitude, while fog touches the ground. The World Meteorological Organization draws a formal line between fog and mist based on how far you can see through it. If horizontal visibility drops below 1 kilometer (about 0.6 miles), it’s fog. If you can still see at least 1 kilometer, it’s mist. The droplets in mist are generally more sparse, giving it that hazy, thin quality compared to fog’s denser blanket.
When Fog Turns to Ice
In cold climates, fog droplets don’t automatically freeze at 0°C. Water in droplets this small can remain liquid well below freezing, a state called supercooling. Fog stays composed of liquid water droplets until temperatures plunge to around minus 35 to minus 40°C. Below that threshold, the droplets freeze spontaneously into suspended ice crystals, creating what meteorologists call ice fog. In polluted air, the particles inside each droplet can trigger freezing at slightly warmer temperatures, around minus 35°C, because the pollutants give ice crystals a surface to form on.
Ice fog is common in subarctic cities like Fairbanks, Alaska, where winter temperatures regularly drop low enough and vehicle exhaust pumps both moisture and particles into the frigid air. It looks similar to regular fog but has a distinctive glittering quality as sunlight refracts through the tiny ice crystals.
Why Fog Looks White
Fog appears white for the same reason clouds do. The water droplets are large enough relative to the wavelengths of visible light that they scatter all colors roughly equally. When all wavelengths scatter together, the result looks white. At night or in dim conditions, fog simply looks gray because there’s less light to scatter. Fog that forms in heavily polluted areas can sometimes take on a yellowish or brownish tint from the particles and chemicals dissolved in the droplets, though clean-air regulations in many countries have made that less common than it was decades ago.
What Makes Fog Thicken or Disappear
Fog thickens when conditions keep pushing more moisture past the saturation point. Continued radiative cooling overnight, a steady flow of moist air over a cold surface, or calm winds that prevent the fog layer from mixing with drier air above all help fog persist and deepen. The densest fogs typically happen in the hours just before dawn, when the ground has been cooling all night.
Fog lifts when the process reverses. Morning sunlight warms the ground, which warms the air, which raises its capacity to hold water vapor. The droplets evaporate, visibility improves, and the fog either dissipates entirely or lifts off the surface to become a low cloud layer. Wind can also break up fog by mixing drier air into the fog layer, though too much wind prevents fog from forming in the first place by disrupting the stable, calm conditions it needs.