BANGKOK, March 16 (Reuters) - The smoke haze choking northern Thailand thinned on Friday as efforts to stop dry season stubble burning, forest fires and wood-fired cooking started to yield results, an environment official said.

But hopes that rain arrive on Friday to wash away the smog had to be put on hold for a few more days.

"We see a clearer sky today," Apiwat Kunarak, director of the northern region's Environment Management Office, told Reuters by telephone from Chiang Mai, the tourism hub of the north.

"But our forecast of rain during March 16 to 18 has to be pushed back to March 18 to 20," Apiwat said.

The region has not seen rain since November in what has been an unusually long and dry cool season in Thailand.

Smoke from burning fields and forests in Thailand, Laos and Myanmar had poured into valleys in the hilly region as a cold front prevented it from escaping into the atmosphere.

But Apiwat said the latest satellite pictures showed efforts by fire fighters to put out forest fires and burning stubble had worked, improving the air quality in Chiang Mai.

The Air Quality Index had dropped to 114, a level considered to mildly affect the health of children and the elderly, from 247, a level at which the smoke affects everyone's health heavily, Apiwat said.

Chiang Mai officials sent fire trucks to hose down streets and spray into the air in hopes of bringing rain and urged its residents to do the same.

They hoped the collective effort would increase humidity as the seasonal scourge reached its densest in 15 years.

After a week of failure to provoke a deluge from the sky, rainmaking teams added water to their chemical mix of fertiliser and salt sprayed from planes to make rain in hopes that would be more effective, officials said.

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