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System improves fog forecasts for S.F. airport

Researchers at MIT's Lincoln Laboratory expect that their new method to predict the clearing times of San Francisco fog will reduce delays for air travelers.

At the fall meeting of the American Geophysical Union in San Francisco, Lincoln Laboratory researcher F. Wesley Wilson presented information about an automated forecast guidance system being developed by a research team that includes scientists from Pennsylvania State University, San Jose State University and the University of Quebec at Montreal.

The system involves four forecast algorithms that make use of existing weather data and data from the special sensors that MIT has installed at San Francisco and San Carlos airports for this project. The equipment includes SODARS (sonic detection and ranging instruments) to measure the height of the inversion base; pyranometers to measure the intensity of the solar radiation at the surface; and time series of winds, temperature and humidity.

Wilson, who heads Lincoln Lab's Marine Stratus Project sponsored by the Federal Aviation Administration (FAA), believes this ensemble forecast, which combines the individiual forecasts through statistical and expert analysis to create a single, more accurate forecast, will make a significant difference for West Coast travelers.

"The goal is to give the human forecasters at the FAA's Central Weather Service Unit (CWSU) a sharp forecast they can work with," Wilson said. He emphasizes that this is "intended as an augmentation of the CWSU, not a replacement."

The Golden Gate city's ubiquitous morning fog isn't romantic to San Francisco International Airport travelers, who are subject to twice as many summer fog-related delays than travelers at other airports.

There are, on average, 70 mornings each summer when low marine stratus clouds cut peak arrivals from 55 per hour to 30 per hour. Compounding the problem are the airport's closely spaced, parallel runways, which force pilots to maintain visual separation during their final approach.

From May to October, moist air drawn off the ocean gets trapped over San Francisco Bay, where it cools and condenses at night and stays put until the sun gets hot enough to burn it off. Typically, the fog will burn off by 10:30am. Sometimes it's gone as early as 8am and sometimes it remains until 2pm or later. Dr. Wilson says the goal is to say that the stratus will clear at a specific time and provide an indication of the confidence in the prediction's accuracy.

The MIT team began giving information this past summer to human forecasters as an operational demonstration. They expect to have demonstrated the operational value of the system by the end of next summer.

The sooner controllers know when the fog is going to lift, the sooner they could get planes into the air and increase the rate at which planes are allowed to land. More accurate forecasts could translate to bringing the airport back to full capacity a full hour earlier than otherwise, which means adding 25 landing slots and saving $200,000 in operational costs for the airlines, in addition to saving inconvenience to passengers.

A version of this article appeared in MIT Tech Talk on January 10, 2001.

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