Fog Facts

Fog is relatively common in the Pacific Northwest compared with much of the rest of the country. And we are entering the “fog season” now, so it’s pertinent to address a few questions about fog.

Q. How common is fog around here?

A. Fog statistics are kept at airports with long-term weather stations (in the Valley, Portland, Salem and Eugene have the best long-term records). At Eugene, the three months in which fog is most common are October, November, and December, with fog occurring on average 10.8, 9.8, and 9.7 percent of the time, respectively. Fog is much more common during late night and early morning hours than during other times of day, but at times it can persist throughout the day.

Q. Is there more than one type of fog?

Yes. For the most part, fog is distinguished by temperature or by the way it forms. In the first category are warm fog, cool fog, and ice fog. In the second we can identify Radiation fog, Upslope fog (or mountain fog), Frontal fog, Advection fog, Steam fog (also known as sea smoke), and Rain fog. In the Valley, the most common types are:

Radiation Fog – Radiation fog forms when cool air is above a warm, damp surface.  The cool air causes the moisture from the surface to condense.  This is the most common fog here in the fall and winter. It is more likely after rain when there is more moisture available.  Winds must be calm so that mixing of air does not occur.

Advection Fog – Advection fog occurs when warm, moist air passes over a cold surface such as the North Pacific Ocean. As the air cools, the water condenses forming fog. In summer, the cool, foggy air is often advected (blown by the wind) over the shoreline. On rare occasions it reached far inland.

Steam Fog – Steam fog occurs when air warmed by something such as a pond or lake rises and mixes with cold air aloft.  When the air is chilled the water condenses, forming fog. Usually steam fog is pretty thin and localized.

The dreaded “valley fog” we sometimes experience here is radiation fog. In the cool season, when high pressure moves over our area, we have generally light winds and mostly cloud-free skies. In such cases, colder air (which is more dense than warm air) flows to the lowest elevations (the bottoms of the valleys). If the temperature of the air is below the dew point, clouds will form – in this case, “stratus clouds,” or fog. The fog is often rather thin. It’s not uncommon, for example, for downtown Corvallis to be in the fog while the top of Witham Hill (only a few hundred feet higher) is above it.

Valley fog persists as long as winds stay calm. The cool air near the ground, and warmer air above, is known as an “inversion,” and these tend to remain as long as winds are light. The fog tends to reflect sunlight, keeping light from reaching the ground (thus keeping things rather dark, and also preventing the ground from warming up; if that happened, the warm air would rise and break through the fog). Sometimes these conditions can last a week or more. But it gets even worse in the Rogue Valley and California’s Central Valley, where fog is more common and more persistent. Fresno gets about twice as many foggy days in winter as we do here.

Finally, wind speeds pick up, and the warm air above mixes with the cold, foggy air. The temperature of the air near the ground rises above the dew point, and the fog breaks up. Skies clear, and the people rejoice (well, at least I do!). This often happens just before the arrival of a rain storm.

Just one more reason to pray for rain.

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