If Ferreria forecasts a bad-air day, it sets into motion a string of actions: swim lessons and football practices are canceled, school recesses are spent inside stuffy cafeterias.

THE WATCHER: Shawn Ferreria, a pollution forecaster with the San Joaquin Valley Air Pollution Control District, explains the computer model he uses to help gauge the potential for bad air. (Mark Crosse / The Fresno Bee)

Ferreria pores over weather reports at his desk in the San Joaquin Valley Air Pollution Control District office on Gettysburg Avenue in central Fresno. He has a 4:30 p.m. deadline this Aug. 8 to release an air-quality forecast. Radio, television and newspapers will use his air-quality prognosis in their weather news.

Key numbers Ferreria looks at to make his forecast: tomorrow's expected high temperature – 98 degrees – and this morning's minimum temperature – 61 degrees; today's temperature at 2,500 feet – 76 degrees; wind speed – between 5 mph to 10 mph; and yesterday's eight-hour ozone average – 82 parts per billion.

From those and other numbers – and his intuition – Ferreria predicts an ozone level of 151 on the Air Quality Index, or AQI, which means unhealthy air.

The AQI is a measuring rod of the health danger of five air pollutants: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide and nitrogen dioxide. The higher the AQI number, the greater the pollution and the greater the health danger.

For example, an AQI of below 50 represents good air quality, while an AQI value over 100 means unhealthy air for sensitive groups. An AQI of 151-200 is unhealthy air for everyone; 201-300 is very unhealthy air and more than 300 is considered hazardous.

Forecasting smog levels is part hard science – meteorologists use weather charts, historical ozone readings and complex computer models. But they don't rely solely on empirical data. Experience and gut feelings play a role.

A meteorologist doesn't make a pollution forecast until one last check: a brisk walk around the block to look at the sky.

"That was one thing in college our professors told us, 'Don't get too stuck behind a computer terminal, because you've also got to stick your head out the window,' " says Ferreria, who joined the air district about a year ago.

Before that final visual check, forecasters study scientific data from a national weather balloon launched each day in Oakland. The balloon collects wind speeds and direction. It also records temperatures as it climbs, allowing a comparison of surface temperatures to those taken at several thousand feet.

A balloon sent up in Fresno measures wind speed and direction. A California Air Resources Board airplane is flown at the same height as the balloons to capture temperatures from different elevations.

Another tool at the forecaster's disposal is called an air profiler. It measures wind speed, direction and temperatures close to the ground, as well as in the atmosphere.

All of this information "gives you a snapshot at different levels in the atmosphere and how the atmosphere is behaving," Ferreria says.

For example, a cooler ground temperature, coupled with hotter temperatures higher in the air, spells pollution trouble in the Valley. Warmer air above the ground acts like a lid, trapping pollutants, such as smog or soot. Add stagnant air, lots of sunshine, car exhaust and, presto – smog.

Or, as Ferreria explains it: "You have what is called an inversion."

Besides the weather, forecasters look at ozone and particulate readings taken from air pollution monitors at various locations and compare them to historical pollution levels.

A forecaster can watch ozone levels climb hour by hour on what are known as "real time" air pollution monitors at 24 sites in the Valley. Currently, only five monitors provide hourly particulate readings, which adds more guesswork to the wintertime sooty-day forecasts.

So how often are air pollution forecasters wrong?

Over the last smog season, their track record was 81% accurate for days when the AQI was over 150; and 66% for days when the AQI was between 100 and 150.

On Aug. 8, Ferreria's experience and insight, as well as temperatures, wind speed and pollution readings, direct him to forecast unhealthy air for Aug. 9.

He couldn't know this forecast would not only be correct, but would be the first in a string of unhealthy air warnings he would issue in August.

From Aug. 9 through Aug. 17, a thick smog catapulted the Valley into health alerts that forced schools to keep students inside and everyone else to be happy they were indoors.