Return of tradewinds is forecast to clear vog

Yes, the weather outside was frightful yesterday as a dirty haze of vog blanketed Oahu, but the National Weather Service predicts that north winds will blow in time to give Santa a tailwind for his weekend ride.

Lead forecaster Tim Craig said the haze resulted from the past few days of light, variable winds of less than 10 mph. He said the wind pattern is expected to begin changing by tonight.

"We have high pressure over the islands right now," Craig said yesterday. "Subsiding air will trap pollutants close to the earth's surface and prevent pollutants from being disbursed. So with light winds, the vog which normally passes south of the Big Island will tend to pool." He said that without tradewinds to push it southward, the vog from erupting Kilauea volcano spreads over the other islands. Vog is formed when sulfur dioxide emitted from the volcano reacts with sunlight, oxygen, dust particles and water in the air to form tiny droplets known as sulfate aerosols, sulfuric acid and other substances.

"It's not surprising there is vog around. It is certainly a big problem for those with respiratory problems in the first place," said Craig.

"A cold front will come through from the northwest; it will get to Kauai first and will probably stall over the Big Island," the forecaster said. "It will turn the winds around to the west and eventually to the north."

Sustained winds of 10 to 25 mph are predicted, with possible gusts up to 45 mph, which could test the stability of rooftop Christmas displays.

Craig said Windward showers are predicted, but "we don't expect any substantial rain with the cold front."

Nighttime temperatures will continue to be in the low 60s, with a low of 62 predicted for Saturday night, making it comfortable for Santa as he makes his rounds.