> 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.”