One contributing factor is that the jetstream is stronger in winter months than summer months. Flights from BOS to SFO are up to 1 hour longer (heading into the jetstream) and 1 hour shorter from SFO to BOS (strong tailwinds).
In winter months, some airline's direct flights have to occasionally land and refuel mid-country[1]. I've experienced this first hand a couple times.
One contributing factor is that the jetstream is stronger in winter months than summer months. Flights from BOS to SFO are up to 1 hour longer (heading into the jetstream) and 1 hour shorter from SFO to BOS (strong tailwinds).
In winter months, some airline's direct flights have to occasionally land and refuel mid-country[1]. I've experienced this first hand a couple times.
[1] https://thepointsguy.com/2018/02/winds-forcing-virgin-americ...