Everyone in San Francisco has been waiting for the Indian summer to kick in and this week’s high temps have made good on the promise. Everyone’s heard of an Indian summer before and likely explained it to every under-dressed friend visiting during the summer months as they stare in disbelief at the blanket of July fog. We all more or less agree that it has something to do with autumn being warm but it turns out the real definition is much more complex. According to the Old Farmer’s Almanac, an Indian summer has specific stipulations that San Francisco’s autumnal warm spells don’t always meet: hazy sky, high barometric pressure, and warm days with cool nights. Furthermore, it must come after a cold spell and fall between November 11th and 20th.
Just to be sure we’re on the same page, it’s not just local opinion that September and October are the warmest months in San Francisco — it’s scientifically legit. Even if people don’t go by the strict OFA definition, most people in the US seem to agree that an Indian summer has to follow the first frost. Not having the kind of weather that would generate a killing frost, we’re pretty much disqualified anyway.
Beyond the technical requirements, the origins of the phrase are much less defined. No one is even really sure what kind of Indian the name refers to. Going by Albert Matthews’ account in the Monthly Weather Review of February 1902, you could assume the phrase came from of any number of Native American tribes who believed the warm autumn winds came from a benevolent god, offering a brief respite from the oncoming winter.
There are however a couple convincing theories that place the origin further east. William R. Deedler of the National Weather service found evidence that ships sailing the Indian Ocean during that time of year had markings of “I.S.” (Indian Summer) on the hulls to indicate increased load levels in the calmer late autumn seas. Some of the earliest letters Albert Matthews found were from an Englishman named Horace Walpole complaining about the unseasonable heat of England in Autumn of 1778, making reference to the summers of India. Regardless of the origins of the name and technicalities of its usage, I’m sure most of us can agree that any time you can wear a t-shirt all day long, San Francisco is a much happier place.
