How did you choose where to go to college? Academics? Sports? Maximum distance from your home town? And how did that college pick you? SAT scores? Leadership qualities? A rich alumnus in the family?

How about: the weather.

While the choice of a college is one of the most important decisions a young person will ever make, and one of the biggest purchases the parents will ever make for that young person, a number of recent studies have found that when it comes to matriculation, decisions can change (quite literally) with the direction of the wind. How could something so small affect a decision so big? Well, if you ve ever arrived independently at the conclusion that one ought not grocery shop hungry, you have an inkling of the mechanism at work.

Humans have to make decisions about the future. But we can only make these decisions now with the information at hand, with the limited intelligence in our heads, and with the feelings we re feeling. In what s called projection bias, we take what we re feeling now and project that we ll feel roughly the same in the future. Roaming the aisles of the grocery store like a half-starved T-Rex, a whole suckling pig may seem like a reasonable dinner for one. But, when we get it home, we realize after a few bites that our eyes were too big for four stomachs.

Of course, that s an example where you re likely to recognize the mistake you ve made (the ability to recognize our cognitive errors is one of the things that distinguishes us from the mighty T-Rex). Often, though, it s more subtle errors that creep into our thinking whether it s about money, love, a job, buying a house or letting someone into medical school.

Take the case of a med school admissions officer interviewing a candidate on a miserable rainy day in Toronto. Will that potential doctor get a fair shake? We d like to think so.

But that s not what Dr. Donald A. Redelmeier found when he and a colleague looked at admissions data for 2,926 candidates interviewed at the University of Toronto between 2004 and 2009. What he found, in short, was that candidates interviewed on rainy days scored 1% lower on average than those interviewed on sunny days. It might not sound like a lot, but Dr. Redelmeier found that when it came to ultimate admissions decisions, this was equivalent to a student having scored 10% lower on the MCAT. Whether the effect came from candidates performing worse because of the rain or admissions officers bad moods making them like candidates less in the rain, with 100 students a year at the school right on the bubble scoring within 1% of the admission threshold people s fates were sometimes decided by the rain.

A similar effect was identified by Uri Simonsohn, an assistant professor at the Wharton School at the University of Pennsylvania, in a paper published in the Journal of Behavioral Decision Making in 2006. Looking at 682 forms used in processing admissions at an unnamed university, Simonsohn found that admissions officers weighted the academic qualifications of prospective students as opposed to things like leadership or athletic skill more highly on cloudy days than on sunny days. In other words, nerds did better if their form was looked at when the sun was not shining.

Simonsohn s basic theory, borne out by research, is that cloudy days incline people to value indoor academic activities, like reading or studying, above outdoor activities like sports.

The same idea underlies another study of Simonsohn s, published last year in the Economic Journal, which found that cloud cover on the day of a college visit actually increased the chance that a student would enroll at least at the university in question, known for its rigorous academic life and lack of any other kind of life by around nine percentage points. (A recent college guide describes the unnamed school as such: sleep, friends, work, choose two. )

How pervasive are these effects of weather on mood and decision making? Pretty pervasive. In the early 1980s, researchers found that people asked about their general happiness would report much lower happiness based simply on the weather the day you asked them. Other studies have found that we tip better when it s sunny, that catalogue orders for winter clothing are more likely to be returned if they were made on a particularly cold day, even that investors react better to bad earnings news on a sunny day in New York City.

We have a hard time, it seems, sorting out the fleeting from the fundamental. For the Canadian med schools, Dr. Redelmeier recommends coarser measurements ( acceptable vs. unacceptable ) for candidate interviews a system acknowledging that humans are too prone to error to get much finer grained than that. For prospective college students, Simonsohn suggests making a list of things that might have influenced one s impressions of a school ( Tour guide was hot. ).

Both pieces of advice seem pretty sound. One of the other major findings of research in this area is that once you think about what might be biasing you, your brain tends to self-correct. Let s see a T-Rex do that.

Ryan Sager writes the blog Neuroworld at TrueSlant.com.