SPOKANE, Wash. — Every year as the fall season comes to a close and winter begins, fog becomes an ever-present theme of the morning in the Inland Northwest.
Fog is essentially condensed water-droplets, similar to clouds, except low to the ground and close to Earth's surface. When overnight temperature dip below the dewpoint ― the temperature that air can no longer hold moisture as water vapor ― the air converts water vapor into liquid water, that we see as fog.
In Spokane, we see plenty of moisture-rich soil, along with rainy fall seasons. These factors contribute to a higher presence of fog around the Northwest.
Temperature inversions are common during this time of the year too, which trap a layer of cold air below a layer of warmer air at a higher altitude. The warm air can act as a 'lid' and trap pollutants and fog in place. High pressure acts in a similar manner, contributing to air stagnation and a lack of vertical mobility. This essentially means that once the fog develops, it does not have anywhere to go so it will stay in place.
The afternoon sun can help encourage the burning out of fog, but as the winter approaches, less sun and a lower angle of the sun can make the impact somewhat futile. This November, Spokane has seen fog for 18 of its 27 mornings thus far. 11 of the 18 foggy mornings have led to dense fog, which limits visibility below a quarter of a mile.
Besides sometimes being just a general nuisance, fog can turn hazardous. Limited visibility contributes to a higher number of car accidents, and can also cause delays at airports during take-off and landing.
But, around the Northwest, foggy days are more likely than not. Spokane's strong dip in overnight temperatures, and the availability of plenty of fresh rain can help cause the fog we continue to see almost daily during this time of year.
Watch a time lapse of fog rolling over Louisiana:
