San Francisco’s climate is genuinely unusual and genuinely microclimatic. The Sunset and the Richmond can sit under fog for weeks in July while the Mission is in full sun three miles away. The warmest, clearest months are typically September and October. June and July are frequently cold, grey and windy.
People arrive expecting California and get something closer to a temperate maritime climate, and for a subset of them that has real consequences for mood.
Why the standard seasonal picture doesn’t fit
Classic seasonal affective disorder is a winter pattern driven mainly by day length, and San Francisco’s day length variation is modest compared with northern latitudes.
What people report here is different: a summer pattern, driven by weeks of overcast rather than by darkness, and concentrated in the western half of the city. Light intensity under heavy marine layer is a fraction of what it’s in sun, even at midday, and cumulative low light appears to matter to some people independently of hours of daylight.
There’s also a mismatch effect that’s genuinely psychological. Everyone else is having summer. Your social media is full of it. You’re wearing a coat in July, and the gap between the expected experience and the actual one produces a specific dispiritedness.
What’s worth trying
Get to the light. The most effective and least discussed option, and it’s often a twenty-minute drive. The Mission, Bernal, Potrero and much of the East Bay are frequently in sun when the west side isn’t, and Marin almost always is. Deliberately going where the sun is, once or twice a week, does more than most interventions.
Morning light, outdoors. Even under overcast, outdoor light is far brighter than indoor light. Twenty to thirty minutes early is worth more than any amount of good lighting inside.
Light boxes. Well evidenced for seasonal patterns — a 10,000 lux box, twenty to thirty minutes in the morning. Worth a conversation with a clinician first if you’ve bipolar disorder or an eye condition.
Vitamin D. Worth testing rather than assuming, and easily done.
Consider your neighbourhood. Which sounds absurd until you’ve lived through three foggy summers. People do move east across the city for this, and report that it helped.
When it’s more than the weather
If the low mood persists into the sunny autumn, if it’s present most of the year, or if sleep, appetite and interest have gone — that isn’t a seasonal pattern and it warrants proper assessment.
Seasonal patterns are diagnosed on a consistent relationship between mood and time of year across multiple years, which means one bad July isn’t evidence of anything. Keeping a rough monthly note for a year is a more useful piece of information than any amount of speculation, and it’s the thing a clinician will ask for.