The trajectory of an air mass during the forecast period can be another important factor in fog formation.
Warm air moving over a colder surface is a primary fog producer.
This can happen when a station is in the warm sector, following a warm frontal passage. Cooling of the air mass takes place, allowing condensation and widespread fog or low stratus to form. To determine the probabilities of condensation behind a warm front, compare the temperature ahead of the front with the dewpoint behind the front. If the temperature ahead of the warm front is lower than the dewpoint behind the front, the air mass behind the front will cool to a temperature ahead of the front, causing condensation and the formation of fog or low temperature near the stratus. Over water areas, warm air passing over cold water may cause enough cooling to allow condensation and the production of low clouds.