Photo: FAA Photo: Chris McGinnis Photo: Chris McGinnis Photo: Flightstats Photo: Flightstats Photo: Flightstats Photo: Michael Macor, The Chronicle Photo: Charlie Riedel, Associated Press Photo: Chris McGinnis Photo: Chris McGinnis Photo: Boarding1Now/Getty Images/iStockphoto Photo: Jeff Chiu, AP Photo: TOM RYAN, TOURISM BC Photo: Finn O'Hara, Getty Images Photo: Neale Clark/Getty Images/Robert Harding World Imagery Photo: Los Angeles World Airports Photo: Kathy Willens, AP Photo: Chris McGinnis Photo: Education Images, Education Images/UIG Via Getty Photo: Chris McGinnis Photo: Nick Ut / Associated Press Photo: Ted S. Warren, Associated Press Photo: Chris McGinnis Photo: David Becker, Associated Press Photo: Allen J. Schaben, TNS Photo: Handout Photo: Chris McGinnis

Flight delays at San Francisco International Airport regularly exceed two hours when it's rainy, windy or foggy, while airports in San Jose and Oakland report few if any delays. The problem is exacerbated at this time of year by a steady stream of winter storms blowing in off the Pacific.

Today (the busiest travel day of the year) is a perfect example of that. According to the Federal Aviation Administration flight delays website, the agency has a "traffic management program in effect" today which means planes bound for San Francisco this afternoon are being held at origin airports for four and a half hours on average due to wind and rain. Ouch! The FlightStats website ranks SFO delays as "excessive." Oakland and San Jose airport delays are "low" or "moderate" according to FlightStats.

As most frequent travelers know, delays mount at SFO almost every time a storm blows in off the Pacific. Just look at these soggy numbers from the U.S. Bureau of Transportation Statistics: SFO ranked 28th out of 30 major airports in on-time arrivals in 2018 (the most recent full year for which stats are available). It ranked 20th out of 30 in on-time departures in 2018, which is actually a dramatic improvement over the previous year when it ranked 28th.

Why does SFO seem to suffer so many more weather-related delays than other airports in the Bay Area?

Of course, the answer is complicated, but for the most part, the main reason is capacity. SFO's runways are too close together to allow simultaneous operations during wet weather. Due to environmental concerns (read about that here), there are no current plans to further separate the runways, which would require expansion into the Bay.

SFO runways are designed to handle up to 60 aircraft arrivals per hour in dry weather. That's because the airport operates two sets of parallel runways – one set for takeoffs, the other for landings. These pairs of parallel runways intersect at their midpoint forming a giant "X." (See figure here or in slideshow up top.) During dry weather, two streams of planes can land and take off from these parallel runways.

The problem is that planes are only allowed to take off and land simultaneously on these parallel runways during clear, dry weather.

Photo: SFO

When storms blow in, air traffic control changes up the formation in which planes land, from the dry weather "West Plan" (with aircraft arriving on runways 28L or 28R and departing on runways 01L or 01R) to the stormy weather "Southeast Plan" (when aircraft arrive on 19L & 19R and depart on 10L & 10R).

When air traffic controllers switch to the Southeast plan (see map), simultaneous operations on parallel runways used to be forbidden, which cuts the airport's capacity (about 60 landings per hour) in half during storms. In 2013, Closely-Spaced Parallel Runway (CSPR) procedures began at SFO, allowing staggered approaches to parallel runways. Prior to this, bad weather days reduced the arrival rate to as low at 25-27 landings per hour, but these enhancements now allow as many as 36 per hour in bad weather. That's still pretty slow.

If you live in San Francisco, you can tell when planes switch to the Southeast Plan because some arriving aircraft whistle and moan as they descend through the clouds over the city as they approach SFO. (To me, that's always a sign that the winter storm door has opened.)

Don't miss a shred of travel news! Sign up for our free bi-weekly email alerts

The main reason that airports in Oakland and San Jose don't face such on-time performance issues– even when it's raining— is because their volume is low. They rarely exceed their capacity for arrivals in good or bad weather. (For example, neither airport is included in the BTS's top 30 airports cited here.) You can check the FAA's delay map and notifications here.

Photo: SFO

The airport is depending on a technological fix to the problem. "Further enhancements will continue to improve this rate," said SFO's spokesperson Doug Yakel. "We're funding the installation of a Ground-Based Augmentation System, which broadcasts a hyper-accurate GPS signal that can support far more sophisticated approaches than currently possible. This could allow aircraft to use a touchdown point further down the runway, curved approaches, approaches further offshore, or at higher altitudes. Any of these would allow more efficient use of our parallel runways."

So until those enhancements come into play, and we still face 3+ hour delays on stormy days at SFO, what can travelers do to avoid this? Fly early during storm season. Before 9 a.m., arrival volume at SFO is below 30 per hour. But after 9 a.m., just over 30 aircraft are scheduled to land. The arrivals rate peaks between 12 noon and 2 p.m. when 40+ aircraft per hour are scheduled to land at SFO. The situation usually does not improve until later in the afternoon when arrival volume falls below 30 per hour.

Have you or would you consider switching your flying to Oakland or San Jose due to delays at SFO? Why or why not? Please leave your comments below.

Read all recent TravelSkills posts here

Chris McGinnis is SFGATE's senior travel correspondent. You can reach him via email or follow him on Twitter or Facebook. Don't miss a shred of important travel news by signing up for his FREE biweekly email updates!