What causes fog? Fog forms when air close to the ground becomes saturated with water vapor and some of that vapor condenses into tiny liquid droplets. This usually happens because the air cools to its dew point, gains extra moisture, or both. In simple terms, fog is a cloud that forms at ground level.
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What Causes Fog?
Fog is made of tiny water droplets suspended in air near the ground. According to the National Weather Service, fog forms when the air near the surface becomes saturated, meaning it is holding about as much water vapor as it can at that temperature.
Once the air reaches saturation, some water vapor changes from an invisible gas into tiny liquid droplets. Those droplets are small enough to remain floating in the air. When enough of them collect together, visibility drops and we see fog.
The U.S. Geological Survey describes fog as a cloud at or near the ground or ocean surface. The basic physics is the same as cloud formation higher in the atmosphere, except the condensation happens near the surface.
What Does the Dew Point Have to Do With Fog?
The dew point is the temperature at which air becomes saturated with the amount of water vapor it currently contains. If moist air cools to that temperature, condensation can begin.
That is why fog often develops as temperatures fall overnight. The air near the ground may cool until it reaches its dew point. Water vapor then condenses onto tiny particles in the air, forming the droplets that make up fog.
If the air is very dry, it has to cool much more before reaching saturation. If the air is already humid, only a small drop in temperature may be enough.
The Main Ways Fog Forms
There is more than one way to make near-surface air saturated. Meteorologists give different fog types names based on how that saturation happens.
Radiation Fog
Radiation fog often forms on clear, calm nights. The ground loses heat after sunset and cools the air directly above it. If that air cools to its dew point, fog can form close to the surface.
Advection Fog
Advection fog forms when moist air moves over a colder surface. The colder ground or water cools the air from below until it reaches saturation. Coastal areas often experience this type of fog.
Upslope Fog
Upslope fog can develop when moist air is forced up a hill or mountain slope. Air cools as it rises, and if it cools enough to reach saturation, droplets form.
Steam or Evaporation Fog
Steam fog can appear when cold air moves over relatively warm water. Water evaporates into the colder air just above the surface. If that added moisture brings the air to saturation, a shallow layer of fog forms.
Why Is Fog Common in the Morning?
If you are wondering what causes fog to be thickest around sunrise, one reason is that the ground has had all night to cool. The air immediately above it can become colder and more humid relative to its temperature.
Clear skies can make nighttime cooling stronger because heat escapes more easily from the ground. Light winds can help by mixing just enough moist air near the surface without dispersing the forming fog.
After sunrise, the ground begins warming again. The air temperature rises, and the relative humidity may drop. That is one reason morning fog often thins or disappears later in the day.
Why Does Fog Form Near Water?
Lakes, rivers, oceans, and wet ground can supply extra water vapor to the air. If moist air is cooled to saturation, fog can form. This is especially common where warm, moist air moves over colder water or where cold air passes over warmer water.
Coastal fog is a familiar example. The U.S. Geological Survey notes that along the Pacific coast, marine stratus and stratocumulus clouds can occur at the surface and become what we call fog.
Why Does Fog “Burn Off”?
People often say the Sun “burns off” fog, but the droplets are not literally burned. Sunlight warms the ground and the air near it. Warmer air can hold more water vapor before becoming saturated, so the tiny droplets evaporate back into invisible water vapor.
Wind can also mix drier air into the fog and help it disappear. In some cases, though, fog can last much of the day if cool, damp conditions continue.
Is Fog Different From a Cloud?
Fog and clouds are made of the same basic things: tiny water droplets, and sometimes ice crystals, suspended in air. The main difference is where they form. Fog is at or very near the ground, while most clouds are higher in the atmosphere.
For more detail, see What is the difference between fog and clouds? You can also read What are clouds? and How do clouds form?.
Frequently Asked Questions
Can fog happen when it is warm?
Yes. Fog does not require freezing temperatures. It requires air near the surface to become saturated. Warm, humid air can produce fog if it is cooled enough or gains enough moisture.
Is fog made of gas?
No. Water vapor is an invisible gas, but the fog you can see is mostly tiny liquid water droplets suspended in air.
Why is fog thicker in some places than others?
What causes fog to be thicker in one place than another? Fog thickness depends on moisture, temperature, wind, terrain, and how strongly the air is being cooled or mixed. Valleys, coastlines, and areas near water can be especially favorable under the right conditions.
The Short Answer
What causes fog? Fog forms when air near the ground becomes saturated and water vapor condenses into tiny droplets. Cooling to the dew point is one of the most common triggers, but added moisture can also help. Different weather setups create different types of fog, but the basic process is condensation near the surface.