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📘 How does fog form?

You have seen a cloud without looking up: fog is a suspension of water droplets at the Earth's surface. The droplets appear when moist air reaches saturation and water vapor condenses.

5
lessons
~25 min
to learn
Adults
level
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What you’ll learn

  1. Fog begins at saturationExplain how cooling and saturation turn water vapor into visible fog.Fog is near-surface water droplets formed when moist air reaches saturation.
  2. Radiation fog cools from belowExplain how nighttime surface cooling produces radiation fog.Clear skies and light winds can let the ground cool nearby air to its dew point.
  3. Valleys collect cold airExplain how drainage and inversions help valley fog form.Dense cool air collects in valleys, where stable layering can limit mixing.
  4. Advection fog moves over a cold surfaceExplain how horizontal transport over a cold surface produces advection fog.Moving moist air is cooled from below until saturation and condensation occur.
  5. Use the ingredients to predict fogUse temperature, dew point, wind, moisture, terrain, and surface temperature as a fog checklist.A small dew-point gap is a clue, not a guarantee; local conditions decide the outcome.

Questions this course answers

What makes fog visible?

Fog is visible because condensed water forms suspended droplets that scatter light.

Why does dew point matter for fog?

The air must cool to its dew point, though moisture and local conditions still matter.

What starts radiation fog on a suitable night?

Radiation fog often forms overnight as the ground loses heat and cools the air above it.

Why can sunlight clear radiation fog?

This is common for radiation fog, though local conditions can change how quickly it clears.

What does an inversion do in valley fog?

A stable warm-over-cool arrangement can trap cool, moist air near the valley floor.

Why is cold-air drainage not enough by itself?

Terrain helps organize the air, but saturation is still required.

Grounded in trusted sources

  • National Weather Service, Radiation Fog, https://www.weather.gov/safety/fog-radiation
  • National Weather Service, Mountain/Valley Fog, https://www.weather.gov/safety/fog-mountain-valley
  • NOAA National Weather Service Glossary, https://forecast.weather.gov/glossary.php?word=fog
  • Met Office, What is fog?, https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/fog
  • Met Office, Dew, https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/temperature/dew
  • UCAR/NSF, Valley Fog Case Study, https://impacts.ucar.edu/en/publications/a-case-study-of-the-mechanisms-modulating-the-evolution-of-valley/
  • Wikimedia Commons MediaWiki API, https://commons.wikimedia.org/w/api.php

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

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