Cloud PointField Guide

Surface fog — evaporation

Steam Fog

When the sea appears to smoke

Written by Laurence WayneEntry No. 27Uncommon

Steam Fog: When the sea appears to smoke

Steam fog forms when very cold air drifts across much warmer open water. Vapour evaporating from the warm surface meets the cold air, instantly saturates it and condenses into rising wisps and columns that look like smoke. Unlike radiation or advection fog, its moisture comes straight off the water — which is why over the sea it is known as sea smoke.

Worth knowing

  • Steam fog is driven by temperature contrast, not extreme cold: the air only needs to be around 10°C or more colder than the water for the small amount of vapour it picks up to tip it past saturation. Strong steam fog can even spin up little 'steam devils' over the water.
Note

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Test yourselfThree levels

Choose a difficulty

What does steam fog look like?

  1. aFog that appears to rise like smoke off water
  2. bA green glow on the horizon
  3. cA coloured ring around the moon
  4. dHail falling from a clear sky
Show answer

AnswerFog that appears to rise like smoke off water

Vapour condensing in cold air just above warmer water rises in wisps and columns, resembling smoke — hence the nickname 'sea smoke'.

Steam fog forms when…

  1. aWarm air flows over a cold sea
  2. bVery cold air flows over warmer water
  3. cRain falls through dry air
  4. dThe ground cools overnight under clear skies
Show answer

AnswerVery cold air flows over warmer water

Cold air moving over warmer water lets evaporated vapour saturate the air and condense almost instantly into rising fog.

Steam fog stays shallow — rarely more than a few hundred metres deep. What caps it?

  1. aThe sheer weight of the water vapour
  2. bA temperature inversion: just above the surface the air warms with height, halting the rising fog
  3. cThe curvature of the Earth
  4. dSunlight continuously burning off the top
Show answer

AnswerA temperature inversion: just above the surface the air warms with height, halting the rising fog

The cold air above the warm water sits under a strong low-level inversion; the rising fog reaches that warmer layer and can climb no further, so it stays shallow.

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