Most of Verus's climate guides exist to warn runners about heat. San Francisco is the opposite kind of guide entirely: across eleven years of dawn records, not a single month qualifies as a genuinely warm morning by Verus's own definition. The problem here has never been heat. It's fog — thick, reliable, and specific enough to have its own name, its own social media following, and a seasonal pattern that runs backwards from what every other Californian city experiences.
This guide uses Verus's own eleven-year climate archive, sampled at 6am local time — roughly when most organised San Francisco races and club runs actually start — to show what a San Francisco morning is really like, month by month, and why the city's warmest stretch of the year isn't the one most visitors expect.
Karl the Fog: what it is and why it happens
San Francisco's summer fog is reliable enough to have earned a nickname — Karl — that originated from a parody Twitter account created in 2010 and has since become a genuine piece of local identity, with hundreds of thousands of social-media followers tracking its daily movements across the city. The mechanism behind it is straightforward: the cold California Current keeps the Pacific Ocean just offshore cold year-round, while California's inland Central Valley heats up hard in summer. That temperature difference creates a pressure gradient, and cool, moisture-laden marine air gets pulled inland through the Golden Gate — the literal gap in the coastal hills the bridge is named for — condensing into the fog that rolls over the city most summer mornings and evenings.
Fog isn't evenly distributed across the city, either, which matters for anyone picking a running route. The western, ocean-facing neighbourhoods — Sunset, Richmond, Ingleside, Mount Davidson Manor, Stonestown — see it most, with some averaging around 12 hours of fog cover a day in peak season. The northeastern side of the city, sheltered by hills — Mission, SoMa, Nob Hill, North Beach — gets far less, often only 6-7 hours. If a foggy, cool run isn't what you're after, running east rather than west is a genuinely effective way to find sun.
What a San Francisco morning actually looks like
These are averages across 2015-2025 from the Verus climate archive, at 6am local time.
| Month | 6am temp / dew point | Avg high / low | Rain days |
|---|---|---|---|
| January | 8.8°C / 6.6°C | 13.8°C / 7.8°C | 9.5 |
| February | 9.0°C / 6.5°C | 14.8°C / 8.0°C | 8.5 |
| March | 9.8°C / 7.4°C | 15.0°C / 9.1°C | 9.8 |
| April | 10.6°C / 8.2°C | 16.1°C / 10.0°C | 4.5 |
| May | 11.5°C / 9.4°C | 17.2°C / 11.3°C | 2.2 |
| June | 12.9°C / 11.1°C | 19.1°C / 12.6°C | 0.3 |
| July | 13.6°C / 12.1°C | 19.1°C / 13.4°C | 0.2 |
| August | 14.5°C / 13.1°C | 20.1°C / 14.2°C | 0.2 |
| September | 14.9°C / 13.1°C | 21.2°C / 14.4°C | 1.1 |
| October | 13.7°C / 11.0°C | 20.6°C / 13.0°C | 2.9 |
| November | 10.8°C / 8.3°C | 16.4°C / 9.9°C | 6.9 |
| December | 8.9°C / 6.6°C | 13.5°C / 7.9°C | 9.8 |
The number that never moves
In eleven years of dawn records, zero months in San Francisco register a genuinely warm morning by the threshold Verus uses across its other climate guides. Compare that to Jeddah, where August dawns average 29.6°C, or Kingston, Jamaica, where summer dawns sit above 24°C dew point. San Francisco is, by a wide margin, the mildest climate this site has measured for early-morning running — the entire 6am dew-point range across all twelve months is 6.5°C to 13.1°C.
Why September, not July or August, is the warmest month
Look at the table again: July's average high is 19.1°C, identical to June's own high-point plateau — but September jumps to 21.2°C, a full 2.1°C warmer, and stays elevated into October at 20.6°C. This is the single most counterintuitive fact about San Francisco's climate, and the one most visitors get backwards when they plan a trip around "California summer."
The cause is the same fog mechanism described above, running in reverse by season. Through peak summer, the Central Valley's heat is at its strongest and the marine layer it draws in is at its thickest and most persistent — locals sometimes call the August stretch "Fogust." By September and October, the North Pacific High that drives this pattern retreats westward, the temperature differential eases, and the marine layer collapses far more often, leaving the warm, clear days San Francisco is known for locally as its own kind of Indian summer. If you're planning a trip specifically to avoid fog and cold, September and October — not July — are the months to target.
The city's rainfall follows the opposite, more expected pattern to what a runner from a temperate climate might assume: June through August average just 0.2-0.3 rain days a month, properly dry — but also cold and grey, not the warm dry summer the numbers alone might suggest. The wet season runs November through March, peaking around 9.5-9.8 rain days in December and January.
What this actually means for your training
- Layer for wind and damp cold, not heat, even in July and August — a warm-looking forecast at midday can still mean a genuinely cold, fog-bound 6am start.
- If you want a warm, clear morning, aim for September or October rather than the conventional "summer" months — this is one of the few cities where that's true.
- Pick your neighbourhood deliberately if fog (or sun) matters to you: the western half of the city runs far foggier than the sheltered northeast.
- Dew point here never gets high enough to meaningfully limit evaporative cooling the way it does in Verus's hot-climate guides — heat management genuinely isn't the concern it is elsewhere.
- The real weather risk in San Francisco running is wind and cold-related, not heat-related — a heat-adjusted pace calculator has little to tell you here, unlike almost anywhere else Verus covers.
San Francisco Bay's water temperature follows almost exactly the same seasonal lag as the air above it — coldest in spring, warmest in autumn rather than midsummer — which matters directly if you're training for open water. That's covered in full in Verus's guide to San Francisco Bay swimming and triathlon culture.