PHOENIX (AP) -- Cooler weather has created ideal conditions for the return of the seasonal brown pollution cloud over the Phoenix metropolitan area.

Visibility was rated as poor Thursday by the Arizona Department of Environmental Quality although pollution levels did not climb high enough to trigger a health watch or warning.

State air-quality officials issued a health watch Tuesday after a weekend cold front stirred up dust and other particulates and trapped them over the Phoenix area.

"We are going to see much more of this as we head into the winter months, especially in the mornings and evenings," said Holly Ward, spokeswoman for the Maricopa County Air Quality Department.

Pollutants such as dust, vehicle exhaust and particles from wood-burning fireplaces accumulate in the air and get trapped there by a temperature inversion.

An inversion occurs when a layer of cooler air develops close to the ground, beneath a warmer layer. The warm air acts like a lid and keeps the cool air and the pollutants in it from mixing and dispersing.

Such inversions are common in Phoenix during the winter.

Particulates pollute the air year-round but become more noticeable and more of a health risk during the winter months when inversions occur.

Maricopa County officials have devised a plan to reduce particulate pollution and have submitted it to federal regulators. If the region continues to exceed federal pollution limits, it could lose billions of dollars in transportation funding.

The particulate alerts, which differ from the ozone alerts issued during warmer months, usually become more frequent as temperatures fall, especially as more people use wood-burning fireplaces.

If pollution levels climb too high, the county will declare a no-burn day and ask residents not to build fires.

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Information from: The Arizona Republic, http://www.azcentral.com

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