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On Earth there is a clear space between the ground and the cloud base. This phenomenon is not confined to Earth, it's the same on Venus, Mars and Titan (moon of Saturn).

Is there a law of physics which inhibits the formation of cloud at ground level?

Living in UK I know that ground fogs occur very occasionally, generally not lasting for more than a few hours, and for all we know it might be the same on the other planets, but it is not the normal state of affairs on the bodies mentioned above.

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In my recently updated answer to the question: What the humidity metric is hiding?

I cite a news article, originally dated 31 January 2020 : Oppressive, humid conditions for southern Australia as heatwave combines with tropical airmass.

In the news article, meteorologists are quoted:

Dew point is the indicator of the amount of moisture in the air ... it's the temperature that the air needs to cool to in order to form cloud.

The only time clouds form on the Earth's surface (in the form of fog) is when the temperature of the air in contact with the surface is same as the dew point, for the prevailing weather conditions. Generally, air temperature near the Earth's surface is higher than the dew point so no fog/clouds form.

Clouds form at altitude because the air temperature at that altitude is the same as the dew point temperature. Thus the reason for a gap between the surface and the base of clouds.

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    $\begingroup$ fine answer no problems. The only issue I have with this is this statement - "Generally, air temperature near the Earth's surface is higher than the dew point so no fog/clouds form.". What about the night when dew does form near the surface ? Why dont clouds form then near the surface? $\endgroup$
    – user1066
    Feb 8, 2020 at 15:59
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    $\begingroup$ My intention was that the explanation should cover the phenomenon on the other planets and Titan, not just Earth. It's difficult to imagine dew forming on Venus. $\endgroup$
    – Michael Walsby
    Feb 8, 2020 at 16:30
  • $\begingroup$ @gansub: While I can't support this with a citation, my casual observation suggests that dew (or frost) forms on surfaces because they are colder than the air. The surfaces might also have more nucleation sites: en.wikipedia.org/wiki/Nucleation $\endgroup$
    – jamesqf
    Feb 9, 2020 at 18:40
  • $\begingroup$ @jamesqf My question to Fred was why don't clouds form near the surface due to the presence of dew at night ? I don't think Fred or you have made that point clear $\endgroup$
    – user1066
    Feb 10, 2020 at 0:50
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    $\begingroup$ @gansub: Because the air isn't cold enough for moisture to condense? Like when you blow your hot, moist breath on say the bathroom mirror, and it condenses there to form "dew", but not in the air. But if you go out in cold enough weather, you can see the moisture in your breath condense into a fog. $\endgroup$
    – jamesqf
    Feb 11, 2020 at 4:05

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