Clear, calm skies are a pilot’s dream, but if the weather turns bad, modern airliners are equipped with radar and other instruments that can get them safely on the ground when visibility is poor.

That appears to be happening less often, however.

Scott Stevens, an atmospheric scientist at North Carolina State University, looked through 45 years of weather data from the nation’s 30 busiest airports and found that pilots are, on average, flying under “instrument flight rules" (IFR) less frequently. That’s good news for harried travelers—commercial flights are more likely to be on time in good weather because more aircraft can land and take off per hour.

In IFR conditions such as thick, low-lying fog, FAA regulations require that pilots leave more space between their airplane and others in the sky. Because fewer aircraft can approach and depart airports—San Francisco International, for instance, can accommodate at most 36 aircraft per hour in IFR conditions compared with 54 normally—flight delays are more common.

Stevens mined records from 1973–2017 and collected hourly measurements of weather conditions—including temperature, dew point, cloud ceiling, and visibility—at the 30 busiest airports in the United States. He then used these conditions to calculate the number of IFR hours each airport experienced per year. His motivation for the study was partly personal—he’s a private pilot. “I’m forced to think about the weather every time I go flying,” he said.

Stevens found that 28 of the 30 airports experienced fewer hours of IFR conditions, on average, each year. “I was surprised to find it so consistent,” he said. “It was the same almost anywhere.” This drop persisted at all times of day and throughout the year, the data showed.

More than half the airports saw at least a 25 percent decrease in their number of IFR hours between 1973 and 2017. Stevens noted the largest change at San Francisco International Airport: In the 1970s, SFO experienced roughly 1,500 IFR hours per year. In the last few years, that number has dropped to below 500.

Stevens presented his results this month at the 99th American Meteorological Society Annual Meeting in Phoenix, and will publish his study in the Journal of Applied Meteorology and Climatology.

The changes in IFR prevalence might be due to changes in urban air quality, he hypothesizes. Cleaner air around cities means fewer particles of pollution, which act as tiny seeds that promote the formation of clouds. “Fog requires particles in the air for the water to condense onto,” he said. Higher temperatures recorded in cities—the so-called urban heat island effect—may also play a role by boosting temperatures farther above the dew point and preventing the formation of fog and clouds, he said.