A paddock cools from evening into night. The amount of water vapour in the air (dots) stays the same. A gauge compares the air temperature with the dew point, and a bracket shows the gap between them, the temperature-dew point spread. As the air cools the spread narrows. In moist air the temperature falls to the dew point: relative humidity reaches 100%, vapour condenses and fog and dew form. In the morning the air warms, the spread opens and the fog clears. In dry air the dew point is much lower; the air cools by the same amount but the spread stays large and no fog forms. No values are shown.
