Friday, Nov. 1, 2002 | 11:05 a.m.
A weather condition, common for this time of year, is trapping cooler air in Las Vegas, causing a hazy smog to linger over the valley, the National Weather Service said.
However, winds of 15 to 20 mph out of the northeast are blowing out cooler air that is trapped under a mass of lingering warm air -- a phenomenon known in weather circles as an "inversion" -- preventing unhealthful air quality days from developing.
"This is an atmospheric condition that helps create stagnant air," said Brian Fuis, a weather service spokesman. "Along with the cooler air, pollutants also can be trapped. This condition is common in the fall and winter."
An inversion occurs when there are light winds and clear skies at night and the warm air from the daytime rises and forms a layer. Because colder air is denser, it is, in effect, trapped below the lighter warmer air, Fuis said.
Fuis said Thursday's inversion was worse than today's because winds were lighter Thursday.
To break a daytime inversion, winds have to pick up or the ground temperature has to rise above that of the lingering warm air. Both are expected to happen today.
According to data from a weather balloon at Desert Rock in Mercury, ground temperatures must rise above 54 degrees there and 59 in the Las Vegas area today to break the inversion. A high of about 70 degrees was expected today.
Despite the inversion, the readings from the 22 pollution stations of the Clark County Department of Air Quality Management the last two days have been good.
Early today a spot check of all stations showed good air quality -- all under a reading of 50.
A reading of 101-150 is considered unhealthy for sensitive groups, including seniors and those with respiratory problems.
On Thursday, the East Charleston station had a reading of 58 for an eight-hour average for "particulate matter-10," which is dust. Readings of 51-100 are considered moderate.
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