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The surface of the ground at night cools down because of outgoing long-wave radiation. In my understanding the cooling of the surface also causes cooling of the adjacent air. If the air contains enough water vapor, dropping of the temperature causes condensation of liquid water droplets and formation of fog.

In my region (Central Europe near 50°N) in winter, stratus clouds can be observed much more often than fog. Typically the base of the stratus cloud is 100 meters or higher above ground surface and the thickness of the stratus is several hundred meters. What puzzles me is the "empty" layer between the ground surface and the cloud base. Why does the stratus cloud often not reach all the way to the ground? What factors determine the stratus cloud base height?

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  • $\begingroup$ what is the elevation of your location? $\endgroup$
    – gansub
    Sep 29 '17 at 8:50
  • $\begingroup$ @gansub the elevation of my location is 300 meters above sea level (~ 1000 ft). the stratus base is typically 400 meters above sea level or higher. The location is inside a basin surrounded by mountain ranges 700 - 1200 above sea level on most sides ... $\endgroup$
    – jirikadlec2
    Sep 29 '17 at 8:56
  • $\begingroup$ so these are orographic influences - I mean the reason for your question. $\endgroup$
    – gansub
    Sep 29 '17 at 11:35
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I'd postulate that this is purely temperature-driven.

If the dew point temperature of your 'empty' layer is not reached then the water vapour will not condense to form fog.

Do you observe the clouds there during the day? Thin clouds such as stratus primarily transmit incoming solar radiation whilst trapping some of the outgoing long-wave radiation, thus insulating the air between the ground and the base of the cloud layer and preventing dew point temperature being reached.

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