What’s the Deal with “Indian Summer”?

Wow, was that a nice weekend or what? I don’t know how it was everywhere else, but this past weekend in Northern Utah was gorgeous- sunny and 70’s.

Family Reproduction Update

But first, let’s talk about me. I’m finally an uncle. I’ve been mentally ready to be an uncle for probably a couple of decades*, but my siblings refused to reproduce until 3 hours ago, when my sister-in-law delivered a 9 lb 15 oz baby** after 21 hours of labor, including 4 ½ hours of pushing. Women are super-crazy-tough.

*Because let’s face it- every man is ready to be an uncle long before he’s ready to be a father.

**With a 15” circumference head. My family is notable for our extraordinarily large heads. No, it doesn’t mean we’re all that smart; it just means we always have a hell of a time shopping for hats.

Mom & baby are fine; I can’t wait to meet my niece.

The Post

Saturday afternoon OCRick and I mtn-biked up in Park City, riding for a couple hours on a series of lower-elevation trails that I haven’t ridden much in recent years*. The trees were mainly bare and that East-coast smell of rotting leaves was all around, but the air was warm, the sky sunny and the trails tacky. The combination of warm air and bare trees seemed rather incongruous, and the strange look and feel of the forest lent to the feeling that we were “getting away” with something- enjoying weather and riding in a place and time we normally shouldn’t be able to.

*Trails like around Masonic Hill and lower DV. I used to ride these trails all the time early and late season, but have been passing them up in recent years as all the stuff around Pinebrook and Jeremy has opened up closer to home.

Tangent: For a native New-Englander in living in the West, I’m convinced that the scent of rotting leaves is one of the strongest Flashback-Smells there is. Even though I smell it fairly often now in the relatively leafy forests of the Wasatch, the smell instantly takes me back a couple of decades and reminds me of times and places long gone. Someday I want to do a post all about smell and memory.

The sunlight seemed different, somehow soft and gold-tinged, which struck me as wonderful but odd. Usually after the leaves fall the mid-day sunlight in the forest seems harsh and an unfiltered, almost too bright. But Saturday’s light seemed strangely easier on the eyes*.

*And the skin. Almost 3 hours riding in direct sunlight, and I forgot sunscreen. No burn, no tan.

After the ride we drove back down to the valley, and stopped by OCRick’s place, up on the East Bench, for a beer. As we sat out on his deck overlooking the valley and the late afternoon sun, I noticed the haze over the valley; not a brown, polluted haze, but a whitish haze, almost like a mild inversion, but without the cold. Later at home, and throughout Sunday, I noticed a sudden explosion of insect life brought on by the warmth- Box Elder and Stink Bugs everywhere. All in all, it was a classic Indian Summer weekend.

Say what? “Indian Summer?” What the heck does that mean?

All About Indian Summer

Whenever it’s warm in the Fall, say >70F, and the leaves are down, or mostly or even partly down, people call it “Indian Summer.” But what does that mean, and what does warm Fall weather have to do with Indians? The question lead me to a search, which lead to another, which lead to… this post.

The broadest, most common use of the term seems to be what I just described above. But it seems that the term used to have a bit more specificity. It didn’t just mean a warm, clear Fall day, but also one characterized by low or no winds and a distinct haze, usually described as bluish or whitish. Traditionally, Indian Summer occurs only after the first “killing frost*.”

*Which has certainly happened already in Park City, though I don’t believe we’ve had one down in the valley yet.

The term appears to have originated as early as the late 1600’s in New England, and depending on the source, it may or may not be a phenomenon specific to that region of the country*.

*Europe has a similar condition known as “Old Wives’ Summer.” I don’t know the origin of the term, nor whether “Old” in this case specifies “Ex”, in which case I imagine the story to be quite interesting.

So the questions aren’t just what the meaning of the term is, but whether it’s real, what causes it, does it still occur, and does it, could it, might it occur here in Utah?

There are several ideas about, though no real consensus regarding, the origin of the term. Here are 3 possibilities:

1- In New England and the Northeast, Indians used to set deliberate fires* around this time of year. On warm, still days the smoke would linger.

*No, no, they weren’t pyros or anything. Indians regularly used fire to manage the composition/vegetation of forests and grasslands to optimize conditions for game and/or grown or gathered foodstuffs.

2- A brief warming period following the first frost would often present the last opportunity for Indian raids on early Euromerican settlements before Winter set in*.

*This one’s my favorite-even though it doesn’t sound all that plausible- just because of the drama and action associated with it. Check out that old engraving to the right- those guys mean business!

3- It’s analogous to the term “Indian giver”, in that the “summer” is given but quickly taken away.*

*This one’s just mean. Like the Indians didn’t get a bad enough deal from us already…

So short answer- nobody knows. OK, let’s try the second question…

Certainly there are warm periods in October and November, but is there any more to it than that? The general meteorological explanation for such warm periods is that tropical air masses get pushed Northward by South winds, and then remain in place and stagnate for several days, allowing haze to build up. But the haze was even more consistently emphasized in early descriptions, before automobiles or industrial air pollution. Certainly wood-burning caused regular air-pollution around settlements, but even in the mid-1800’s, observers noted a lack of any real correlation between significant wood-burning and Indian Summer haze. Furthermore, as the 19^th century progressed, observers noted the diminishing frequency of Indian Summers, during a period when population and industrialization were increasing dramatically in the Northeast.

During the 19^th century, an alternative explanation for the haze was proposed: decaying leaves and leaf litter released aerosols into the air, creating the haze. Aerosols are tiny particles, bigger than molecules, but still small enough to easily stay afloat in the air. Warm, still days would both hasten the decay of vegetation and allow the resultant haze to linger in the atmosphere. This idea was apparently proposed multiple times, independently. Zadock Thompson*, a Vermont naturalist, suggested it back in 1853, and Canadian G.W. Johnson** pitched basically the same idea again in 1899.

*Source: Autumn, by Peter Marchand, 2000

**Source: Science and industry, Vol. 4, 1899. Seriously, how impressive is it that I am referencing 100+ year old science journals? I keep reading this footnote over and over because it just looks so impressively science-like.

What was interesting about the idea is that it explained 2 characteristics of Indian Summer: that it traditionally follows the first killing frost, and its diminishing frequency over the late 19^th and early 20^th centuries. An early killing frost causes many leaves to drop before they’re fully dried, resulting in a large amount of sudden, moist leaf litter on the forest floor. And over the course of the 19^th century New England and much of the East in general, was heavily deforested. Peak deforestation- about 80%- occurred in the mid-1800’s*, after which time coal largely replaced firewood in cities and many New England farms were abandoned for better soil in the Midwest.

*Northeast forest cover has returned dramatically since then, but as biologist Peter Marchand notes in his book Autumn, the composition and character and character of modern-day New England forests is much different than in pre-settlement times, and so even if there is a link between forest cover and haze, it’s questionable whether we still do or will experience the same Indian Summers as a few hundred years ago.

Trees, Haze and Isoprene

It was an interesting idea, even if it was only conjectural. But in recent decades it seems that it may not have been that far from the truth, and that forests may indeed be involved in the production of haze. You might have caught a news item this past summer- spurred by a paper in the journal Science- that “trees cause smog.” The truth is a little more complicated than the headline: certain trees may- in concert with human-generated pollutants- contribute to a certain kind of haze, and the idea actually isn’t all that new. Many plants emit the hydrocarbon compound isoprene (C5H8.) It’s not exactly clear why some plants emit it, but since there appears to be a significant cost in its production, it’s assumed there’s some benefit to the plant. The likeliest benefit* seems to be thermotolerance.

*The next likeliest seems to be tolerance to ozone.

Leaves are thin and can change temperature quickly. We’ve talked previously about frost damage to leaves, but they can also be damaged by sudden increases in temperature. Studies have shown that leaves which produce isoprenes are better able to tolerate sudden repeated increases in temperature, such as can be caused by direct sunlight. Unsurprisingly, leaves at the top of isoprene-producing trees emit up to 4 times as much of the stuff as partially-shaded leaves lower down. Interestingly, isoprene production appears to have evolved independently multiple times, like C4 photosynthesis.

In recent years researchers have become interested in isoprene emissions because of their interactions with nitrogen oxides (NO and NO2) and the possible effect of such interactions on ozone*. But isoprene emissions have another effect: they foster the growth of aerosols in the atmosphere, which in turn can cause haze. Plants emit isoprenes mainly during the day, and much more so on warm days. And- and here’s the kicker- some researchers believe that a frost may result in a sudden spike of emissions. So maybe those 19^th century guys were on to something.

*More detail here.

Not all trees emit isoprenes, and not all to the same degree. In fact some of the biggest isoprene-emitters in North America appear to be oaks. All North American oaks are isoprene-emitters*.

*But not all oaks are. Many European Oaks emit other compounds- such as monoterpenes- instead.

Oaks, both Northern Red Oak, Quercus Rubra, and White Oak, Q. Alba, are all over the place in New England, and so a link between oak forests and Indian Summer haze would seem to make sense. And if that link is true, then it would offer a nice tie-in to the last question about Indian Summer: is it strictly a phenomenon of the Northeast, or might it occur here in Utah as well?

The foothills and lower reaches of the Wasatch (and the Oquirrhs- pic above left. I just love that shot. Is that pretty or what?)- on both sides- are carpeted with oak, and while our scrub oaks (Q. gambelii) may be nowhere near as tall and majestic as their cousins back East, I’d bet our sprawling clonal stands are some of the densest and most uniform stretches of oak anywhere. Higher up, our oak gives way to aspen, which- as it turns out- is another isoprene-emitter. If there really is a link between oak forests and haze, then Utah probably can claim true Indian Summers. I like the idea that Wasatch forests might be creating New England-style Indian Summers. Seems somehow like a link to home from this strange little island in the desert.