Fog is essentially a cloud that touches the ground.

Fog and clouds are both categorized as atmospheric aerosols consisting of liquid water droplets or ice crystals. The primary difference is altitude. Fog is defined by the National Weather Service as a cloud that reduces horizontal visibility to less than 1 kilometer or 0.62 miles. It forms when the difference between air temperature and dew point is less than 2.5 degrees Celsius or 4.5 degrees Fahrenheit.Radiation fog is the most common type, occurring on clear, calm nights when the ground loses heat through radiation. As the ground cools, it chills the adjacent air layer. If the air reaches 100% humidity, water vapor condenses around microscopic particles like dust or salt, known as cloud condensation nuclei. This process is governed by the laws of thermodynamics and psychrometrics.Historically, the study of fog visibility became critical during World War II for aviation safety. In 1943, the British developed the Fog Investigation and Dispersal Operation (FIDO) to clear runways using petrol burners. Modern meteorologists use transmissometers to measure the extinction coefficient of light through fog. These sensors help airports determine if Category III automated landing systems are necessary for incoming flights.Fog plays a vital role in various ecosystems, such as the redwood forests of California. These trees can absorb up to 40% of their water intake directly from fog needles through foliar uptake. Without this ground-level cloud formation, many coastal species would not survive the dry summer months. This demonstrates that fog is not just a weather hazard but a critical biological resource.