"Fog variability in a climate context has been an open question of interest for at least a couple of decades now, and one for which good data has been thought to be lacking," Jim Johnstone of the University of California, Berkeley told environmentalresearchweb. "I analyzed an underutilized archive of cloud height measurements from coastal airports to learn how fog has changed over time."

Johnstone and colleague Todd Dawson used hourly cloud ceiling height data for 1951 to 2008 from Arcata and Monterey airports. Arcata is towards the northern edge of the range for coast redwood (Sequoia sempervirens (D. Don) Endl.) while Monterey is near the tree's southern limit.

The scientists found a strong relationship between fog frequency and the difference between coastal and inland temperatures over the past 60 years. "Foggy summers tend to be associated with cooler conditions on the immediate coastline, and warmer conditions inland of the first line of coastal mountains along the entire US Pacific coast," said Johnstone.

This temperature contrast and northern California fog have varied "in lockstep" since the mid-20th century. "Early in the 20th century the temperature contrast was stronger than now, suggesting that a substantial reduction in fog frequency has occurred," said Johnstone. "Similar evidence is given by the coastal ocean temperatures, which also vary closely with fog frequency and the coast-inland temperature contrast."

In this way the team infers that fog was 33% more frequent in the redwood region along the Pacific Coast between 1901 and 1925 than from 1951 until 2008.

Coast redwoods are not particularly good at regulating their water use and are sensitive to ambient humidity. In summer they appear to rely on fog as a source of water, and also to create cooler, more humid conditions.

"We might expect that redwood health and new tree recruitment would be affected, particularly at the warmer, drier margins of the distribution where the trees are most susceptible to fog reductions," said Johnstone.

What's more, the fog data correlated with interannual and decadal changes in climate factors such as the California Current and the Pacific Decadal Oscillation.

"It is remarkable that coastal fog, which often appears to be a rather ephemeral entity, varies from year to year in a very systematic fashion with large-scale circulation patterns encompassing the entire coast of North America and the broader North Pacific," said Johnstone.

Now the team plans to look at redwood tree-rings to "indicate the magnitude of the recent changes in a longer-term context". An earlier analysis showed that fog variability was revealed by changes in stable isotope ratios in the trees. "Because summer fog and winter rain originate from different meteorological processes, the hydrogen and oxygen atoms in each source have different masses (isotopic ratios), which can be interpreted to assess the relative importance of each source," explained Johnstone.

The researchers reported their work in PNAS.