Just in Time for Labor Day: Summer

It isn’t just your imagination. It really has been cooler than usual. But that’s about to change.

Temperatures in Los Angeles will begin to climb today after an unusually cool and smog-free August, meteorologists said Tuesday.

The average high temperature downtown last month was about 80 degrees -- about 3 degrees lower than normal -- but today’s high could be closer to 90, with top readings near 105 in the San Fernando and San Gabriel valleys, the National Weather Service said.

William Patzert, an oceanographer and meteorologist at the Jet Propulsion Laboratory in Pasadena, said the rising temperatures, which should last through Labor Day, will be largely a result of a large ridge of high pressure building over Southern California, blocking the onshore flow of cool air from the ocean.

That high pressure wasn’t around much last month.

“What we had,” Patzert said, “was June gloom in August.”

The highest temperature in downtown Los Angeles last month was 88 degrees, compared with the record for the month of 106, set Aug. 19, 1885.

He said the cooler weather along the coast was caused, in part, by hotter than normal weather in the deserts.

Patzert said the ocean water between the Hawaiian Islands and Southern California was cooler than usual for much of the summer, keeping the air over the water cool too. At the same time, temperatures in the deserts routinely topped 100.

Cool air is denser than hot air, so air tends to flow from cooler areas toward warmer areas. With a temperature differential often approaching 40 degrees during August, the onshore flow of cool, damp air was especially persistent.

Many days, that dank air piled up against the coastal slopes above L.A. as fog and clouds. Midday sunshine burned away the overcast downtown, but along the coast, it often stayed cloudy and cool all day.

With less heat to generate the chemical reactions and inversion layers that produce smog, the Los Angeles Basin had less air pollution than normal during August, said Sam Atwood, a spokesman for the Air Quality Management District.

Patzert said the cool weather may have stemmed from a long-term weather cycle known as the Pacific Decadal Oscillation. This cycle, he said, is also largely responsible for the six-year drought that exacerbated last year’s disastrous fall wildfire season and shows no sign of ending.

“The Santa Ana season is starting,” he said. “And if we get a couple of weeks of Santa Anas, it’s really going to be grim.”

Scientists say a mild El Nino, which often brings excess rain to the Southland, is building in the southeast Pacific, but Patzert said the decadal oscillation could keep the region’s weather relatively dry for several years.