Why does SFO have so many delays?

SFO's delay problem comes down to physics and geography. The airport has two pairs of parallel runways, but the parallel runways in each pair are only 750 feet apart — too close for simultaneous independent approaches in poor visibility. When visibility drops below about 4 miles (which happens frequently due to Bay Area fog), SFO switches from two arrival streams to one, instantly cutting landing capacity by roughly 50%.

This makes SFO the most weather-sensitive major airport in the country, but not because of storms. Marine fog rolls in from the Pacific almost daily during summer months (June-August), typically arriving in the late morning and clearing by early afternoon. BTS data shows SFO has the highest weather-delay rate of any major airport during summer, which seems counterintuitive for a city with almost no rain.

The FAA regularly implements Ground Delay Programs for SFO, holding flights at their origin airports to manage the reduced arrival rate. A typical summer fog event delays arrivals by 45-90 minutes. The long-term fix would be to widen the runway spacing to allow simultaneous approaches in low visibility, but SFO is surrounded by the Bay on three sides, making expansion virtually impossible. For travelers: morning flights into SFO are almost always more reliable than afternoon arrivals during summer.