Traditionally one thinks of San Francisco as having quaint foggy mornings. Things change. Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of a cloud, the term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or the ocean, or from nearby moist ground or marshes). Fog is a common feature along the West Coast during the summer, but a University of Washington scientist has found that summertime coastal fog has declined since 1950 while coastal temperatures have increased slightly. Fog formation appears to be controlled by a high-pressure system normally present off the West Coast throughout the summer, said James Johnstone, a postdoctoral researcher with the Joint Institute for the Study of the Atmosphere and Ocean at the UW.
The foggiest place in the world is the Grand Banks off the island of Newfoundland, the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. Some of the foggiest land areas in the world include Argentina, Newfoundland and Labrador and Point Reyes, California, each with over 200 foggy days per year. Other notably foggy areas include coastal Chile (in the south), coastal Namibia, and the Severnaya Zemlya islands. Seattle, Washington, USA, has many foggy days per year.
The western coast fog decline could have negative effects on coastal forests that depend on cool and humid summers, but Johnstone, who presented his findings at the American Geophysical Union annual meeting in San Francisco, hasn't seen evidence of that yet.
In fact, climate models indicate that coastal fog should be increasing because of global warming, but he believes that is not happening because of the strong influence exerted by regional circulation patterns related to the Pacific Decadal Oscillation. That climate phenomenon, centered in the North Pacific, has wide-ranging effects that last for years or even decades rather than for just a year or two.
The Pacific Decadal Oscillation (PDO) is a pattern of Pacific climate variability that shifts phases on at least inter-decadal time scale, usually about 20 to 30 years. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs.
"You would eventually expect to see significant effects on the coastal forests if the fog continues to decline," he said.
Johnstone examined records from airports up and down the West Coast that have taken hourly readings on cloud height for the last 60 years. He looked closely at two stations in particular, Monterey on the central California coast and Arcata on the northern California coast, and found that their decline in fog and increase in temperature matched very closely despite being separated by about 300 miles. Both also reflected a great deal of variability.
Historically there have been stark temperature differences at times between the coast and areas just a short ways inland. But the differences have been shrinking in recent years, mostly because of rising coastal temperatures, he said. Cooler temperatures typically are located near sea level, and the warmer inland temperatures begin to show up at about 1,300 feet in elevation.
Johnstone found that the contrast between inland and coastal temperatures was much greater from 1900 to 1930 than during the last 60 years, indicating that summers on the coast were much foggier in the early 20th century.
But he notes that while coastal fog has generally declined, the data in general have shown consistent variability. For example, the Pacific Northwest, and Seattle specifically, had record fog frequency in the summer of 2010, and many places along the West Coast recorded their foggiest summer since 1991.
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