Spattering of Rain Is Most This Month Since 1983 : Weather: Showers cool inland valleys up to 15 degrees and clean the air of pollutants, dispelling the need for a second August smog advisory.

The remnants of a tropical storm spattered Ventura County with a fraction of an inch of rain on Monday in the first measurable precipitation during August since 1983.

The damp weather, carried north from the Pacific off Mexico by the dissipating tropical storm Hilda, had been expected to bring more than an inch of rain to the county’s foothills and mountainous areas, according to the National Weather Service.

But the storm spent most of its moisture at sea, and by late Monday only a 20% chance of more rainfall or mountain thundershowers remained, said Meteorologist Ted Mackechnie.

“These systems are always tricky to predict,” Mackechnie said. “The problem is that we don’t have any sensors out over the ocean.”

The light rain--0.04 of an inch in the mountains and 0.01 at the County Government Center in Ventura--offers no help in replenishing ground water or easing pressures of the fifth year of drought, said Ventura County hydrologist Dolores Taylor.

“It might wash the air and make the Air Pollution Control District folks a little happier, but that’s about all,” she said.

The afternoon showers cooled temperatures by as much as 15 degrees to the mid-70s in inland valleys and cleaned the air of pollutants, dispelling the need for a second smog advisory this month.

Air quality officials issued an advisory Saturday for Simi Valley because ozone, a harmful gas formed when pollutants react together in sunlight, reached a concentration greater than 15 parts per million in the air.

Air pollution officials had warned Saturday that more advisories were expected this week. But the cooler air mass that began moving in on Sunday changed the weather and pollution picture, said Philip Moyal, a meteorologist with the air pollution district.

“It replaced our stagnant mass of polluted air with clean, fresh air,” Moyal said. However, by the end of the week, a strong inversion layer and accompanying marine fog were expected to move back over the county, trapping pollution-causing emissions and setting up conditions for more smog advisories next week, Moyal said.

“The thick inversion layer at about 1,500 feet acts like a lid over us and doesn’t give pollutants much of a chance to disperse,” Moyal said.

Trace amounts of rain also fell at Los Angeles International Airport, Santa Monica, Long Beach, Riverside and Big Bear, according to National Weather Service measurements. Thunder and lightning storms struck the eastern Mojave Desert area.

The National Weather Service issued flash flood warnings for various mountain and desert areas early Monday but canceled them when rain and storms did not materialize, spokeswoman Pat Rowe said. “The moist air was drying out, and we had a Santa Ana condition,” she said.

Along with heavy clouds and unseasonably warm nights in the Los Angeles area, Hilda has brought higher levels of humidity, National Weather Service meteorologist Steve Burback said. “Humidity levels are now in the 50% range during the afternoons and quite a bit higher at night, 85% at least,” he said.

With the clouds shrouding Southern California, the meteor light show expected to streak through the northeastern sky is being blocked from view. The annual August event, visible when the Earth intersects debris left in space from a comet known as Swift-Tuttle, should last another two nights, said Griffith Observatory guide Morgan Harmon.

Los Angeles residents could find the meteors hard to spot. “With city lights which bleach the sky, clouds and condensation, it’s not possible to see anything,” Harmon said. Residents of rural Ventura County may find a better vantage point if the cloud cover clears.

Hilda was expected to have little effect on the rainfall totals in Ventura County, where the countywide average was at 95% of normal for this time of year.

County hydrographer William Minger said rainfall at the County Government Center in Ventura was at 108% of normal for this time of year with 15.69 inches of rain. Normal for the full rain year, which runs from October through September, is 16.09 at the County Government Center. Other areas of the county, ranging from Simi Valley to Matilija Canyon, have had from 90% to 99% of normal rainfall, Minger said.

Times staff writer Penelope McMillan contributed to this report.