full text of the classic FAA guide
Water exists in three states—solid, liquid, and gaseous. Water vapor is an invisible gas. Condensation or sublimation of water vapor creates many common aviation weather hazards. You may anticipate:
Fog when temperature-dew point spread is 5° F or less and decreasing.
Lifting or clearing of low clouds and fog when temperature-dew point spread is increasing.
Frost on a clear night when temperature-dew point spread is 5° F or less, is decreasing, and dew point is colder than 32° F.
More cloudiness, fog, and precipitation when wind blows from water than when it blows from land.
Cloudiness, fog, and precipitation over higher terrain when moist winds are blowing uphill.
Showers to the lee of a lake when air is cold and the lake is warm. Expect fog to the lee of the lake when the air is warm and the lake is cold.
Clouds to be at least 4,000 feet thick when significant precipitation is reported. The heavier the precipitation, the thicker the clouds are likely to be.
Icing on your aircraft when flying through liquid clouds or precipitation with temperature freezing or colder.