Fog is a cloud that forms at or near Earth’s surface when air becomes saturated with water vapor. It matters because it can sharply reduce visibility on roads, near airports, and over water. Fog also shows how temperature, humidity, wind, and local landscapes work together in the lower atmosphere.
A cool field, a warm lake, or a sheltered valley can each create different fog conditions.
Key Facts
- Fog forms when air near the ground reaches its dew point and water vapor condenses into tiny liquid droplets.
- Relative humidity near 100% means the air is nearly saturated and fog formation is likely if cooling continues.
- Dew point is the temperature at which air becomes saturated: fog often begins when air temperature equals dew point.
- Radiation fog forms on clear, calm nights when the ground loses heat and cools the air above it.
- Advection fog forms when warm, moist air moves over a colder surface, such as cold ocean water or snow-covered ground.
- Visibility less than 1 km is commonly used to define fog, while greater visibility with suspended droplets is often called mist.
Vocabulary
- Fog
- Fog is a cloud of tiny water droplets or ice crystals that forms close to the ground and reduces visibility.
- Dew Point
- Dew point is the temperature at which air must cool for water vapor to begin condensing into liquid water.
- Relative Humidity
- Relative humidity is the percentage of water vapor in the air compared with the maximum amount the air can hold at that temperature.
- Condensation
- Condensation is the process in which water vapor changes into liquid droplets when air cools or becomes saturated.
- Radiation Fog
- Radiation fog is fog that forms when the ground cools overnight and chills the moist air directly above it.
Common Mistakes to Avoid
- Thinking fog is smoke or pollution, which is wrong because fog is mainly tiny water droplets or ice crystals suspended in air.
- Ignoring the dew point, which is wrong because fog usually forms when air temperature cools to the dew point, not just when the air feels cold.
- Assuming strong wind helps fog form, which is wrong because strong wind usually mixes the air and can disperse fog, while light wind often supports fog formation.
- Confusing fog with clouds high in the sky, which is wrong because fog is a cloud that touches or forms very close to Earth’s surface.
Practice Questions
- 1 At sunset, the air temperature is 12°C and the dew point is 8°C. By how many degrees Celsius must the air cool before fog can begin forming?
- 2 A weather station reports visibility of 0.6 km in a layer of tiny water droplets near the ground. Does this meet the common definition of fog with visibility less than 1 km?
- 3 A valley has clear skies, moist ground, and calm winds overnight. Explain why fog is more likely to form there than on a windy hilltop.