The Many Flavors of Fog

For the fog connoisseur, there is more than one flavor of pea soup.

The Federal Aviation Authority is interested in these different types to help them predict and manage flights during foggy weather in and around New York City.

Fog arises when air thick with water vapor cools, causing water droplets to form. Visibility can be greatly reduced - a situation especially dangerous for small planes, whose pilots are often not licensed to fly with instruments.

"Once you enter a fog bank, you may think you are flying level, but maybe you are not," said Warren Fellner of the FAA's Aviation Weather Research (AWR) program.

Studies in flight simulators have shown that pilots have about three minutes to get their bearings when their horizon becomes obscured. Fellner told LiveScience that poor visibility may have been a factor in last week's fatal crash in West Virginia of a plane owned by NASCAR racing team Hendrick Motorsports.

For large commercial planes, there is less of a safety issue because they are more accustomed to instrument landings. Airports, however, will often reduce their traffic during low visibility.

"Weather slows the number of planes that can go out or land," said Gloria Kulesa, head of the AWR program. "Traffic managers may place some planes in holding patterns, or redirect them to other airports."

This can consume a lot of fuel and cause schedule foul-ups all through the system.

"Managing a fog event gets complicated in a hurry, especially around New York City," said Robert Tardif of the National Center for Atmospheric Research. The AWR program is funding Tardif to look at the fog problem in this crowded region.

Tardif is using a 20-year old archive of observations taken every hour by personnel on the ground at 17 airports in the northeastern part of the country.

He has found there are between 50 and 300 hours per year of fog in the New York City area, not including low clouds, or "low ceilings" as they are called. The region is no San Francisco, Tardif admitted, but fog "still ranks pretty high as a factor in flight disruptions."

Moreover, Tardif found that there are more variations in the types of fog in the Northeast than on the West Coast. He categorizes fog in five ways:

• Precipitation fog: After a light rain, surface moisture evaporates, allowing fog drops to develop near the surface. This was the most common type of fog in the New York area.
• Sea fog: Forming over cold water and then propagating onto land, this type forms often at JFK airport, which is near the ocean.
• Radiation fog: When the ground is cool at night, especially in the fall, the air near the surface is cooled, saturating it with water. "This is what usually comes to mind for people when they think of fog," Tardif said.
• Cloud-base lowering fog: The second most common fog variety in the metro area happens when a low cloud ceiling - about 1000 feet above the surface, falls to the ground. "We are not sure about the conditions that lead to that," Tardif said.
• Morning evaporation fog: Dew evaporating at sunrise causes an influx of water vapor. As basic as it sounds, Tardif found this fog was rarely observed in the region he looked at.

With data on the frequency of these different kinds of fog, Tardif is now taking more specific weather measurements on the campus of Brookhaven National Laboratory on Long Island, NY.

On a 295-foot (90-meter) tower, Tardif and his colleagues have instruments to measure temperature, wind, and humidity at different elevations. They add in data from weather balloons and ocean buoys.

"The goal is to find the mechanisms that lead to the formation and evolution of fog," Tardif said in a telephone interview.

The FAA plans to use Tardif's observations to develop software that can automatically predict when a fog will burn off, allowing air traffic controllers to better decide when and where to send planes. Kulesa said her group recently put in place such a system in San Francisco, where summer fog often reduces the airport's capacity by half.

"We gave them up to a six-hour forecast capability," she said. "For instance, in the morning we might tell them there's a 50 percent chance of burnoff by noon."