Thick, soupy fog used to cover parts of Los Angeles a few dozen times a year and interfere with traffic at LAX. Not so much anymore, and a Cal State L.A. professor says it's because we are warmer. Not globally warmer, but locally: the paved and built-up city is retaining more heat.

In 1950, Los Angeles International Airport recorded more than 80 days of fog so thick that it would have made it hard to see more than a few blocks ahead. By 2001, the number of days with any recordable periods of dense fog had fallen to less than half that.

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LaDochy picked through 50 years of weather records from LAX and from Long Beach's municipal airport. He concluded that the dense fog of Los Angeles "is slowly disappearing and may become a relic of the past."

He suspected that the growth of the city may have something to do with that. All that glass, steel and concrete traps heat during the day and then releases it slowly at night.

LaDochy has been known to arm his students with thermometers and send them on long, nighttime drives across the region. They always find that downtown Los Angeles is a few degrees warmer than its less built-up suburbs.

LaDochy guessed that, as Los Angeles grew, it trapped more heat and eventually kept overnight temperatures from slipping below the dew point, where fog forms. And sure enough, when he looked at the numbers, he found a close relationship between the rise in temperatures downtown and the decline in foggy days.

Our less-dirty air also has something to do with it, since all those industrial exhaust particles that used to form our smog gave the water droplets something to hang onto. For those who are new to L.A. and think smog is bad now, you should have seen it when it was really bad.