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I'm working on investigating the effect fog has on drag. I have assumed an air density of $1.225 \frac{\text{kg}}{\text{m}^3}$ for dry air, but I don't know what value for density I could assume that would be typical of fog.

I can't even reason out whether or not fog is more dense or less dense than dry air: I know that air density is lower at higher humidity since water vapor is less dense than air, but it seems to me that fog should have a higher density than air, since you can observe fog being more dense closer to the ground, and collecting in valleys.

What air density is typical of fog?

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This is an instrument that measures fog density and has an experimental plot, figure 9 .

Once you have the relative humidity at the fog appearance at a temperature and pressure , one can use known equations to get the density. This link gives a calculator.

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Anyone who has ever seen fog pour over a mountain range can tell you that it is significantly more dense than either moist or dry air. It does, after all, settle to the bottom of valleys....

See https://www.dailymail.co.uk/video/news/video-1031660/Incredible-wave-fog-rolls-Canadian-mountain-range.html for a nice example.

I have no clue where to get a quantitative answer, but qualitatively it seem denser.

I came here looking for for an explanation for a significant loss of range in my EV when driving through dense fog. It is again, an empirical qualitative result, but it does seem like I needed to charge a lot more than when I do this same reasonably routine drive in dry air.

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