The fog rolled in last night. I like the fog, but it has become an increasingly rare occurrence in recent years. I've read that this is due to the heat island effect. The heat island effect simple states that the sun will warm a parking lot or other low-shade urban area more effectively than a grassy meadow. The slight rise in overnight temperatures due to residual heat can tip the balance away from fog-forming conditions. Urbanization is also blamed for altering normal wind patterns and therefore contributing to changes in fog frequency. However, that's not the whole story. It turns out that there's also correlation of the decrease in coastal fog over the last 22 years to the decrease in airborne particulates (because we've improved our particulate emissions the airborne water vapor has fewer particles to condense upon) and to periodic, long-term, ocean water temperatures.

Many native plants are adapted to take advantage of the coastal fog and drip the condensate from their leaves into the root line. My memory is vague on this, but I recall reading an article a while back that claimed some large fraction of the soil moisture in redwood forests came from condensed fog. (The figure I seem to recall was near 50%).

My electrician, an old timer with a long history in the area, commented that he used to have to open the door on his car and navigate by the center stripe on the road in the early morning hours. That used to be fairly common a couple decades ago, but he hadn't seen fog like that recently.

Driving to work this AM shortly after sunrise, I noticed that the fog was denser over the grassy fields of nearby schools, suggesting a very local observation of the heat island effect.

The abstract below from "Trends in fog frequencies in the Los Angeles Basin" by M.R. Witiwa and Steve LaDochy is informative.

Data from throughout the Los Angeles area were examined to determine the horizontal distribution of dense fog (visibility < 400 m) in the region and trends over time. The relationship between the occurrence of dense fog to the phase of two atmosphere–ocean cycles: the Pacific Decadal Oscillation (PDO) measured by the PDO Index and the Southern Oscillation measured by the Southern Oscillation Index (SOI) was investigated. In addition, the influence of the urban heat island and the amount of suspended particulate matter were assessed. For the three stations that had 22 or more years of data, we examined trends and the relation to atmosphere–ocean cycles. Results show a decrease in the occurrence of very low visibilities (< 400 m) at the stations in close proximity to the Pacific Ocean, Los Angeles International Airport (LAX) and Long Beach International Airport (LGB). Occurrence of the frequency of low visibilities at these two locations was also highly correlated with the phase of the PDO. Only a weak, non-statistically significant relationship was seen with the SOI. At Burbank, a reporting station about 30 km from the ocean, no trends were noted, and there was no evidence of a correlation with either the PDO Index or the SOI. In the Los Angeles Central Business District (CBD) when comparing dense fog occurrence in the early 1960s to a similar period in the early 2000s we saw a decrease in dense fog from a mean of 10 h per year to a mean of 3 h per year. Also contributing to the decrease was decreasing particulate pollution and increased urban warming. A downward trend in particulate concentrations coupled with an upward trend in urban temperatures were associated with a decrease in dense fog occurrence at both LAX and LGB [Long Beach Airport]. These trends were evident for the period 1966–1997, but appear to have ended by the late 1990s.

1 comment:

  1. Fog in LA! Who knew.

    If it's fog you want, come to San Francisco. Even with 40ish square miles of more or less continuous development there isn't enough heat island effect to offset the mighty Pacific.