The nature of SE fog

Posted: Sunday, December 15, 2002

Fog comes on little cat feet. It sits looking over harbor and city on silent haunches and then moves on. That was Carl Sandburg's impression in his famous poem "Fog."

To lots of Juneauites this fall, though, the fog came in more like an unwelcome relative that refused to leave as unusually persistent fog disrupted many travel plans. So what exactly causes fog and why has it seemed such a problem this fall?

Simply stated, fog is a cloud that is based at the ground.

Cloud droplet sizes in fog are quite small, and a cubic meter of fog typically contains only about 0.1 gram of liquid water dispersed over 1 to 10 million droplets. These tiny cloud droplets reduce visibility, making fog hazardous to transportation whether by sea, by land or by air. Fog can occur through a number of different processes, so fog types are designated by the process that leads to their formation.

Upslope fog develops when moist air is cooled by being lifted over mountain slopes. This type of fog is commonly seen along the terrain in Southeast Alaska following rain showers and often forms in tendrils.

Radiation fog forms when outgoing long-wave radiation cools the air to its dew point temperature. It is most common during sunrise when nighttime cooling and relative humidity are at their maximums.

Advection fog is produced when warm air flows over a cold surface and cools from below until saturation is reached. This is common along the Pacific Coast during the summertime; cold water upwelling near the coast combined with seasonally warm air masses can lead to persistent episodes of advection fog sometimes called sea fog.

Steam fog develops in quite the opposite way when a cold air mass moves over a warm body of water. Evaporating water mixes with the adjacent air and raises its dew point to saturation. This is the type of fog we create with our own breath during cold winter days.

Precipitation-induced fog forms just as suggested by its name; falling precipitation adds moisture to the air and cools it as well until fog develops.

To get rid of the fog requires a change in the conditions that caused it to form. The shortest duration fog types are generally the radiation and steam varieties as daytime warming usually is enough to dissipate it. Precipitation-induced fog generally dissipates when the precipitation stops.

On the other hand, upslope and advection fogs, if they are extensive, may persist until there is a significant change in the wind direction or speed. An increase in mid-level clouds or a thick layer of high clouds also can stop the top of the fog layer from radiating and cooling and allow the fog to dissipate.

So why has the fog seemed so persistent over the Juneau area this fall?

Most of the blame would have to go to the warmer-and-wetter-than-normal weather we have been experiencing. Instead of transitioning to a colder and drier winter regime, typical fall weather has continued into December.

We also have had several multiple-day stretches of dry and, for most of the Panhandle, sunny weather. When heavy rains are followed by clear skies and light winds, radiation fog is sure to form. As the lower levels cool, temperatures at higher elevations can be 10 to 15 degrees warmer than at ground level. This strong temperature inversion traps moisture and pollutants in the lower layer, not allowing it to mix with the drier warmer air above.

The channels and tidal flats provide an abundant source of relatively warm water to evaporate and reinforce the fog. As we approach the solstice, the shorter days and low sun angle means there is less solar energy to warm the lower levels and break the inversion.

In Southeast Alaska, we typically see 20 to 35 days a year during which fog is a dominant weather feature. Our fog "climatology" is consistent with other coastal areas along the North Pacific, but it is not as intrusive as in areas along the Washington, Oregon and Northern California coasts where 60 to 80 days a year are foggy. So while fog is certainly not atypical for our area, it is unusual, thankfully, for it to persist over several days.

Paul Shannon is a meteorologist for the National Weather Service in Juneau. Aimee Devaris, former warning coordination meteorologist in Juneau, contributed portions of this article. Contact Juneau Audubon Society at ckent@alaska.net.



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