Radiation fog is called that because it forms through radiational cooling of the ground and the air just above it, not because of nuclear or dangerous radiation. When the surface loses heat by emitting infrared radiation on clear, calm nights, the nearby moist air cools to its dew point and condenses into fog.

Name meaning

  • The word “radiation” refers to the loss of heat by the ground as it radiates infrared energy to space on clear nights.
  • As the ground cools, it chills the thin layer of air in contact with it, and that cooling process is what gives radiation fog its name, rather than any link to ionizing or nuclear radiation.

How it forms

  • Conditions are typically: clear skies, light or calm winds, and enough moisture near the surface so the air can reach 100% relative humidity.
  • Once the air at ground level cools to its dew point, water vapor condenses into countless tiny droplets, creating a shallow, often dense fog layer that can thicken upward overnight.

When and where it appears

  • It most often forms overnight in fall and winter, when nights are longer and the ground can cool more efficiently.
  • Sheltered valleys, flat inland areas, and places near moist soils or water bodies are common hotspots, and in some regions it has local names (like “tule fog” in California’s Central Valley).

Why the name confuses people

  • The term can sound alarming because “radiation” is commonly associated with nuclear hazards, prompting jokes and misunderstandings in online discussions.
  • Meteorologists keep the name because it precisely describes the physical cooling mechanism, and it helps distinguish this type of fog from others like advection fog or upslope fog.

TL;DR: It is called radiation fog because it forms from radiational cooling of the surface and nearby air, not because it contains harmful radiation.