At this time, Cheapflights has noticed price decreases for many domestic flights due to COVID-19. Travel is not necessarily recommended at this time, and flights may be prone to cancellation: please check with local authorities in both Newquay and San Francisco for travel alerts, and be sure to review the airline's cancellation policy and travel advisories before booking. For more information, please view our post on what to know about COVID-19 (coronavirus) and travel.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Newquay is roughly greater than that of San Francisco. You can assume this to be the case over the next 21 days.
Expect the weather to be around 25 °C on average in September. As the temperature peaks, try to get your plans done early, as flight price and availability will have an impact on your travels. The climate in San Francisco is generally hotter than in Newquay.