the mysterious inefficacy of weather

I like to think of myself as a data-respecting guy–by which I mean that I try to follow the data wherever it leads, and work hard to suppress my intuitions in cases where those intuitions are convincingly refuted by the empirical evidence. Over the years, I’ve managed to argue myself into believing many things that I would have once found ludicrous–for instance, that parents have very little influence on their children’s personalities, or that in many fields, the judgments of acclaimed experts with decades of training are only marginally better than those of people selected at random, and often considerably worse than simple actuarial models. I believe these things not because I want to or like to, but because I think a dispassionate reading of the available evidence suggests that that’s just how the world works, whether I like it or not.

Still, for all of my efforts, there are times when I find myself unable to set aside my intuitions in the face of what would otherwise be pretty compelling evidence. A case in point is the putative relationship between weather and mood. I think most people–including me–take it as a self-evident fact that weather exerts a strong effect on mood. Climate is one of the first things people bring up when discussing places they’ve lived or visited. When I visit other cities and talk to people about what Austin, Texas (my current home) is like, my description usually amounts to something like it’s an amazing place to live so long as you don’t mind the heat. When people talk about Seattle, they bitch about the rain and the clouds; when people rave about living in California, they’re often thinking in no small part about the constant sunshine that pervades most of the state. When someone comments on the absurdly high rate of death metal bands in Finland, our first reaction is to chuckle and think well, what the hell else is there to do that far up north in the winter?–a reaction promptly followed by a twinge of guilt, because Seasonal Affective Disorder is no laughing matter.

And yet… and yet, the empirical evidence linking variations in the weather to variations in human mood is surprisingly scant. There are a few published reports of very large effects of weather on mood going back several decades, but these are invariably from very small samples–and we know that big correlations tend to occur in little studies. By contrast, large-scale studies with hundreds or thousands of subjects have found very little evidence of a relationship between mood and weather–and the effects identified are not necessarily consistent across studies.

For example, Denissen and colleagues (2008) fit a series of multilevel models of the relationship between objective weather parameters and self-reported mood in 1,233 German subjects, and found only very small associations between weather variables and negative (but not positive) affect. [Klimstra et al (2011)] found similarly negligible main effects in another sample of ~500 subjects. The state of the empirical literature on weather and mood was nicely summed up by Denissen et al in their Discussion:

As indicated by the relatively small regression weights, weather fluctuations accounted for very little variance in people’s day-to-day mood. This result may be unexpected given the existence of commonly held conceptions that weather exerts a strong influence on mood (Watson, 2000), though it replicates findings by Watson (2000) and Keller et al. (2005), who also failed to report main effects. –Dennisen et al (2008)

With the advent of social media and that whole Big Data thing, we can now conduct analyses on a scale that makes the Denissen or Klimstra studies look almost like case studies. In particular, the availability of hundreds of millions of tweets and facebook posts, coupled with comprehensive weather records from every part of the planet, means that we can now investigate the effects of almost every kind of weather pattern (cloud cover, temperature, humidity, barometric pressure, etc.) on many different indices of mood. And yet, here again, the evidence is not very kind to our intuitive notion of a strong association between weather and mood.

For example, in a study of 10 million facebook users in 100 US cities, Coviello et al (2014) found that the incidence of positive posts decreased by approximately 1%, and that of negative posts increased by 1%, on days when rain fell compared to days without rain. While that finding is certainly informative (and served as a starting point for other much more impressive analyses of network contagion), it’s not a terribly impressive demonstration of weather’s supposedly robust impact on mood. I mean, a 1% increase in rain-induced negative affect is probably not what’s really keeping anyone from moving to Seattle. Yet if anyone’s managed to detect a much bigger effect of weather on mood in a large-sample study, I’m not aware of it.

I’ve also had the pleasure of experiencing the mysterious absence of weather effects firsthand: as a graduate student, I once spent nearly two weeks trying to find effects of weather on mood in a large dataset (thousands of users from over twenty cities worldwide) culled from LiveJournal, taking advantage of users’ ability to indicate their mood in a status field via an emoticon (a feat of modern technology that’s now become nearly universal thanks to the introduction of those 4-byte UTF-8 emoji monstrosities 🙀👻🍧😻). I stratified my data eleventy different ways; I tried kneading it into infinity-hundred pleasant geometric shapes; I sang to it in the shower and brought it ice cream in bed. But nothing worked. And I’m pretty sure it wasn’t that my analysis pipeline was fundamentally broken, because I did manage (as a sanity check) to successfully establish that LiveJournal users are more likely to report feeling “cold” when the temperature outside is lower (❄️😢). So it’s not like physical conditions have no effect on people’s internal states. It’s just that the obvious weather variables (temperature, rain, humidity, etc.) don’t seem to shift our mood very much, despite our persistent convictions.

Needless to say, that project is currently languishing quite comfortably in the seventh level of file drawer hell (i.e., that bottom drawer that I locked then somehow lost the key to).

Anyway, the question I’ve been mulling over on and off for several years now–though, two-week data-mining binge aside, never for long enough to actually arrive at a satisfactory answer–is why empirical studies have been largely unable to detect an effect of weather on mood. Here are some of the potential answers I’ve come up with:

  • There really isn’t a strong effect of weather on mood, and the intuition that there is one stems from a perverse kind of cultural belief or confirmation bias that leads us all to behave in very strange, and often life-changing, ways–for example, to insist on moving to Miami instead of Seattle (which, climate aside, would be a crazy move, right?). This certainly allows for the possibility that there are weak effects on mood–which plenty of data already support–but then, that’s not so exciting, and doesn’t explain why so many people are so eager to move to Hawaii or California for the great weather.

  • Weather does exert a big effect on mood, but it does so in a highly idiosyncratic way that largely averages out across individuals. On this view, while most people’s mood might be sensitive to weather to some degree, the precise manifestation differs across individuals, so that some people would rather shoot themselves in the face than spend a week in an Edmonton winter, while others will swear up and down that it really is possible (no, literally!) to melt in the heat of a Texas summer. From a modeling standpoint, if the effects of weather on mood are reliable but extremely idiosyncratic, identifying consistent patterns could be a very difficult proposition, as it would potentially require us to model some pretty complex higher-order interactions. And the difficulty is further compounded by strong geographic selection biases: since people tend to move to places where they like the climate, the variance in mood attributable to weather changes is probably much smaller than it would be under random dispersal.

  • People’s mood is heavily influenced by the weather when they first spend time somewhere new, but then they get used to it. We habituate to almost everything else, so why not weather? Maybe people who live in California don’t really benefit from living in constant sunshine. Maybe they only enjoyed the sun for their first two weeks in California, and the problem is that now, whenever they travel somewhere else, the rain/snow/heat of other places makes them feel worse than their baseline (habituated) state. In other words, maybe Californians have been snorting sunshine for so long that they now need a hit of clarified sunbeams three times a day just to feel normal.

  • The relationship between objective weather variables and subjective emotional states is highly non-linear. Maybe we can’t consistently detect a relationship between high temperatures and anger because the perception of temperature is highly dependent on a range of other variables (e.g., 30 degrees celsius can feel quite pleasant on a cloudy day in a dry climate, but intolerable if it’s humid and the sun is out). This would make the modeling challenge more difficult, but certainly not insurmountable.

  • Our measures of mood are not very reliable, and since reliability limits validity, it’s no surprise if we can’t detect consistent effects of weather on mood. Personally I’m actually very skeptical about this one, since there’s plenty of evidence that self-reports of emotion are more than adequate in any number of other situations (e.g., it’s not at all hard to detect strong trait effects of personality on reported mood states). But it’s still not entirely crazy to suggest that maybe what we’re looking at is at least partly a measurement problem—especially once we start talking about algorithmically extracting sentiment from Twitter or Facebook posts, which is a notoriously difficult problem.

  • The effects of weather on mood are strong, but very transient, and we’re simply not very good at computing mental integrals over all of our moment-by-moment experiences. That is, we tend to overestimate the  impact of weather on our mood because we find it easy to remember instances when the weather affected our mood, and not so easy to track all of the other background factors that might influence our mood more deeply but less perceptibly. There are many heuristics and biases you could attribute this to (e.g., the peak-end rule, the availability heuristic, etc.), but the basic point is that, on this view, the belief that the weather robustly influences our mood is a kind of mnemonic illusion attributable to well-known bugs in (or, more charitably, features of) our cognitive architecture.

Anyway, as far as I can tell, none of the above explanations fully account for the available data. And, to be fair, there’s no reason to think any of them should: if I had to guess, I would put money on the true explanation being a convoluted mosaic of some or all of the above factors (plus others I haven’t considered, no doubt). But the proximal problem is that there just doesn’t seem to be much data to speak to the question one way or the other. And this annoys me more than I would like. I won’t go so far as to say I spend a lot of time thinking about the problem, because I don’t. But I think about it often enough that writing a 2,000-word blog post in the hopes that other folks will provide some compelling input seems like a very reasonable time investment.

And so, having read this far—which must mean you’re at least vaguely entertained, right?—it’s your turn to help me out. Please tell me: Why is it so damn hard to detect the effects of weather on mood? Make it rain comments! It will probably cheer me up. Slightly.