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📘 What is fog?

Picture yourself on a woodland path at dawn: nearby branches remain sharp, but the trail fades into a pale wall and each breath feels cool and damp. You are inside a cloud whose base reaches the ground; meteorologists call it fog when suspe

4
lessons
~20 min
to learn
Adults
level
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What you’ll learn

  1. A cloud at eye levelIdentify fog as suspended liquid droplets or ice particles at the surface and explain why those particles reduce visibility.Fog is a surface-level cloud whose microscopic particles scatter light; visibility thresholds distinguish fog operationally from mist.
  2. Cooling air into fogConnect saturation and dew point to radiation fog, valley drainage, inversions, and the conditions that favour nighttime formation.Near-surface air can reach saturation as the ground cools it, while sheltered valleys organize and trap the cold moist layer.
  3. Fog that travels and climbsDistinguish advection, upslope, steam, and precipitation fog by the movement, cooling, or moisture addition that produces saturation.Fog may arrive with air crossing a cold surface, form as air rises over terrain, or appear when evaporation adds moisture to cold air.
  4. Fog can freeze, move, and vanishDifferentiate freezing fog from ice fog and explain why heating, drying, mixing, and continued advection change fog persistence.Fog droplets can remain supercooled, deposit rime, evaporate, lift, or be renewed, making local formation and clearing difficult to forecast.

Questions this course answers

What is the visible material in ordinary fog?

Water vapour is invisible; fog becomes visible when vapour condenses into tiny suspended droplets.

Put the main steps of radiation-fog formation in order.

Radiation fog is a cooling-from-below sequence that ends with saturation and suspended droplets.

Match each fog-forming route to its defining process.

Each name emphasizes a different route to the shared endpoint of saturation near the surface.

How does freezing fog differ from ice fog?

The distinction is the phase of the suspended particles, not simply whether the air temperature is below freezing.

Why can sunshine clear one fog bank while another persists or moves inland?

Fog clearing depends on the process sustaining saturation, as well as heating, drying, wind, terrain, and new moisture arriving.

Grounded in trusted sources

  • World Meteorological Organization, Aviation Hazards — Low Visibility and Low Cloud, https://community.wmo.int/site/knowledge-hub/programmes-and-initiatives/aviation/aviation-hazards-low-visibility-and-low-cloud
  • Met Office, What is fog?, https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/fog
  • NOAA National Weather Service, Radiation Fog, https://www.weather.gov/safety/fog-radiation
  • NOAA National Weather Service Glossary, Fog and fog types, https://forecast.weather.gov/glossary.php?word=fog
  • NOAA Center for Satellite Applications and Research, Visibility, https://www.star.nesdis.noaa.gov/portfolio/detail_Visibility.php
  • Federal Aviation Administration, Aeronautical Information Manual, Chapter 7 Section 1 — Meteorology, https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap7_section_1.html

Every Wunder lesson is built from real, reputable sources — never invented.

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