[![[SF-microcliamtes.png]]](https://www.sfchronicle.com/bayarea/article/heat-wave-map-18388216.php) Regardless of when you visit you should expect to wear layers. Even when the day is warm we rarely have warm nights. And often just by crossing the city you will enter a new [microclimate](https://en.wikipedia.org/wiki/Microclimate#Americas). Yep, that's a real thing. The difference in temperature between the fog belt and the Mission can be as much as 30°F (17°C). Always leave the house in a sweater with something much lighter underneath for when the mid-day temps turn up.