2012 Marine Layer Clouds

After a very gloomy summer in 2010, last year proved to be about normal as far as the amount of marine layer clouds. What is in store for 2012?

Of course, nobody can predict this exactly, but there are some indicators such as sea surface temperature (SST) that can give us some hints. Generally, cooler than normal SST off of the Southern California coast is associated with higher than normal amounts of marine layer clouds in San Diego and most of Southern California.

The water temperatures off the Western U.S. coast was a little cooler than normal in April 2012 as shown in the figure below. Relative to the long-term seasonal mean, the water temperature off our coast is a little warmer (~0.5C) this year compared to last year (2011). Also, the horizontal extent of the cold SST anomaly off the coast is smaller this April compared to last year.

The cool SST acts to increase the strength of the inversion layer making it more likely that marine layer clouds will form. Additionally, the cooler SSTs increase the coast-inland temperature contrast during the day which enhances the strength of the sea-breeze circulation. Near the surface, the sea-breeze is an onshore flow and can help transport marine layer clouds over the Southern California coastal land regions. However, too strong of a sea breeze can act to disrupt the temperature inversion which can lead to a decrease in marine layer clouds.

Closer to home here in San Diego, the SST at Scripps Pier was close to normal for December through February. However, from March to May, the water was about 2-3C cooler than normal. Towards the end of April, the water warmed up and was about 1-2C above the normal temperature for much of May. Since the beginning of June the water temperature has been close to normal. SST at any given coastal location usually exhibits some short term variability, which is evident in the "wiggles" in the red curve.

The table below shows the percentage of time that low clouds were present during daylight hours at San Diego Lindbergh Field for the months May through September. Included in the table are values for the present year and climatological ("normal") values. Values from 2009 and 2010 are also shown for comparison.

Values for 2012 will be updated as data becomes available.

Monthly Daytime Low Cloud Cover at San Diego Lindbergh Field for 2009-2012

MONTH CLIMATOLOGY 2009 2010 2011 2012
MAY 29% 45% 18% 25% 29%
JUN 34% 26% 47% 39% 44%
JUL 29% 19% 58% 24% 29%
AUG 24% 22% 19% 29% 22%
SEP 21% 11% 23% 32% 11%

The plot below shows the daytime low cloud fraction (1.0=100%) at San Diego Lindbergh for each individual day.

Present Year (2012) Low Cloudiness at San Diego Lindbergh
Low Clouds Only (Cloud Base < 2500 ft)
Red Line = Long-term Climatology

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