> Unlike the SFO → SEA charts, both charts are relatively flat over the years. However, when looking at seasonality, SFO → JFK dips in the summer and spikes during winter, while JFK → SFO does the complete opposite: dips during the winter and spikes during the summer, which is similar to the SFO → SEA route. I don’t have any guesses what would cause that behavior.
This one seems easy:
SF “summer” is actually foggy winter, whereas the winter months are mostly pretty mild. (Admittedly less so at SFO than in the city but it’s often still foggy.)
Airports tend to experience delays in wintery (wind and low visibility) conditions.
Thus the seasonal delay is from arriving at the airport experiencing “winter conditions.”
Could be, though with improved short-range forecasting and more sophisticated planning, bad weather at the destination increasingly manifests itself as departure delays.
Interestingly, and a tie between the another comment and this. The jet streams, thus winds aloft, I believe are stronger in the winter and more southerly.
SFO is even worse during the fog -- if there is insufficient visibility, they have to close one of the two runways because they're too close to use both using instruments.
It’s actually a bit more wild than that! SFO is always too close to land both runways at once via IFR alone. The second tail plane is required to land while maintaining separation visually, which can’t be done in bad weather. Also, since wakes are what they are, the second plane has to be the heavier one. Which is how you get extremely large planes landing at a major international airport under requirements in part for VFR.
This one seems easy:
SF “summer” is actually foggy winter, whereas the winter months are mostly pretty mild. (Admittedly less so at SFO than in the city but it’s often still foggy.)
Airports tend to experience delays in wintery (wind and low visibility) conditions.
Thus the seasonal delay is from arriving at the airport experiencing “winter conditions.”