Recipe for a gloomy day in Hawai'i: Take a blanket of vog. Add dark rain clouds. Don't stir with wind.

The result was a sky over O'ahu that looked a lot worse than it actually was yesterday.

Although the unusual weather conditions turned a clear morning into a dreary afternoon, there was little danger either from the rain or the vog, officials said.

"Visually it looks bad, but there aren't a lot of dangerous particles in the air," said Lisa Young of the Health Department's Clean Air Branch. "Our monitors weren't picking up much at all. There's just no wind to blow the visual pollution away."

While the vog lingered on, an unstable air mass developed over O'ahu, dumping heavy afternoon rain showers over Central and Windward O'ahu and cutting available sunlight even more.

"When the rain showers boiled up over Central O'ahu and started fanning out, the overcast sky got even soupier," said Chris Brenchly, a forecaster for the National Weather Service.

The rain showers were heavy enough that the weather service issued a flash flood advisory for several hours yesterday afternoon. At the Olomana Fire Station, 1.2 inches fell in about a half hour; a Ka'a'awa rain gauge recorded about 1 inch of rain in the same period.

Police said there were no reports of flooding problems.

By late afternoon, the rain had subsided and a weak sun was forcing its way through the gloom, offering hope for a brighter weekend.

"We should get our trade winds back," starting today, Brenchly said. "And visibility should gradually improve over the weekend. We've probably seen the worst of it for awhile."

On the Big Island, volcanic conditions cleared enough yesterday to allow Hawai'i Volcanoes National Park to be reopened after being closed for two days.

A change in weather conditions led to acceptable levels of sulfur dioxide around the park, a park spokesman said.

Park Superintendent Cindy Orlando had ordered the closure of the park Wednesday as a lack of wind caused the gas from Kilauea to linger over the summit area, forcing the evacuation of around 2,000 people from the park.

Scientists at the Hawaiian Volcano Observatory yesterday said sulfur dioxide emissions continued to emerge from the Halema'uma'u vent at a rate of five times background levels, but were drifting away from populated areas of the park.

An ash plume from Kilauea was rising straight up about 1,500 feet before being dispersed northward.