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A combination of a warming climate climate and increased urbanization (heat island effect) has caused a 25 to 50 percent decrease in low cloud cover in the greater Los Angeles area since the 1970s, according to a new peer-reviewed study led by Columbia University's Lamont-Doherty Earth Observatory.
Low, marine-layer clouds along the southern California Coast from satellite.
The cloud decrease is driven mainly by urban sprawl, which increases near-surface temperatures, but that overall warming climate is contributing, too. Increasing heat drives away clouds, which admits more sunlight, which heats the ground further, leading to drier vegetation and higher fire risk, said lead author of the study and bioclimatologist Park Williams.
Low stratus clouds typically form over coastal Southern California during the early morning. As temperatures rise during the day, the clouds typically dissipate later in the morning and into the early afternoon.
These low cloud bases have risen an average of 150 to 300 feet since the 1970s. The higher bases have allowed the clouds to burn off an average of two hours earlier than what they did 40 or so years ago.
The research team compared hourly cloud observations at many California airports since the 1970s and compared that information with the U.S. Wildland Fire Assessment System data base. The team determined that periods of less cloud cover during the summer correlated neatly with lower vegetation, moisture and thus more danger of fire, according to the report
However, the team did not find an increased trend in total area burned in the summer due to decreasing cloud cover. The likely reason for this is that newer technology better firefighting techniques in recent years have been able to better contain fires compared to the earlier years.
This study was recently published in the Geophysical Research Letters.
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A combination of a warming climate climate and increased urbanization (heat island effect) has caused a 25 to 50 percent decrease in low cloud cover in the greater Los Angeles area since the 1970s.