It may be a simple question and I've read the PHAK and searched online, but I can't seem to understand why adding moisture to a dry cold air mass would create fog. I understand when an air parcel is cooled to the dewpoint neared the surface, it will become saturated and fog appears. However, why would adding moisture create fog?

  • 3
    $\begingroup$ You may want to ask this on Earth Science.SE for a better reply. $\endgroup$
    – Ron Beyer
    Mar 2, 2017 at 4:38
  • $\begingroup$ I've re-closed this as a duplicate of our "How can you anticipate cloud formation" question because the concepts are fundamentally similar, but the answer below captures the gist of it. $\endgroup$
    – voretaq7
    Mar 3, 2017 at 18:46

1 Answer 1


You can either cool the air to reduce it's capacity to hold water, or you can add water to the air. Either way will eventually make the air saturated

You can either make the bucket smaller, or you can add water in the bucket. Either way you will eventually make the bucket spill ...

  • $\begingroup$ When it's completely full and saturated, so would it be at its dewpoint? $\endgroup$
    – nyorkr23
    Mar 2, 2017 at 7:28
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    $\begingroup$ Correct. When temperature = dewpoint relative humidity will be 100%. At that point the water vapor will start to condense creating fog (or clouds if you're above the ground level.) Add enough water and you'll raise the dewpoint to ambient temperature. Or you can lower ambient temperature to the dewpoint and create fog without adding water. $\endgroup$
    – Gerry
    Mar 2, 2017 at 16:18

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