No One Asked Me: Yesterday’s Weather

“I always dress for yesterday’s weather.”
-my brother

Okay so that’s not really a question.  In my defense though, my brother’s got a philosophy degree, which means most of what he says is an attempt to provoke a reaction, make a grand statement about life, explain his more questionable dating choices, or to get more attention, though not necessarily in that order.  Anyway, he posed this statement to me recently, then arched his eyebrow.  It’s possible I was supposed to take that as an opportunity to extrapolate some deeper meaning about his relationship with his ex-girlfriend, but instead I got curious.  If you really did always dress for yesterday’s weather, how often would this be okay?

It turns out this is one of those interesting stats questions that you can sort of come up with an answer for, but you have to make all sorts of assumptions to get there.   I did some poking around, and here are the parameters I figured I’d have to work with:

  1. You are perfectly rational.  Now this may not be a great assumption1, but it’s one we have to go with if we hope to get anywhere.  The problem with this is that people, especially those of us in northern climates, tend to start rebelling against winter every year. It’s a pretty well documented phenomena that some time around March/April people in northern climates just say “screw it” to the coat/gloves/scarf thing.  I don’t totally know how to take this in to account, but it’s something to keep in mind.
  2. You are like me.  It appears at least some types of cold/heat perception are pretty heritable, so when in doubt I assumed you’d act exactly like I do.  Hey, it worked in middle school.
  3. You modify clothes approximately every ten degrees (Fahrenheit).  This one was actually remarkably hard to find data about.  The problem is that apparently our bodies make lousy thermometers, and we have a remarkable spread of preferences.  The most consistent breakdowns I could find were actually on running or other outdoor sport sites, and they seem to support my “ever 10 degrees” hypothesis.  Apparently that’s where you can measure an impact on performance.
  4. You live in Boston. Yeah, you don’t.  Never have actually.  But I do, and the data’s actually stored for a while.
  5. Being stuck in the rain without an umbrella will bug you, but having an umbrella you don’t need won’t.  Umbrellas are like towels.  Always good to carry one.
  6. You don’t use an umbrella or other rain gear if it’s snowing.  Because snow’s not mean like that and you already have a jacket on and you’d look silly, that’s why.


Alright, with those out of the way, lets talk data.  I found a handy site called Weather Underground that actually keeps detailed archives of the weather.  From there I pulled all the data for Boston from Jan 1st, 2010 to June 22nd, 20152.  After that I measured a few things:

  1. How often the daily high temperature changed from one day to the next by more than 10 degrees in either direction
  2. How often the average daily temperature changed from one day to the next by more than 10 degrees in either direction
  3. How often a clear day was followed by a rainy day.

Basically if any of those three changes occurred, I assumed that you ended up dressed incorrectly.  It’s not perfect…the rain could have happened overnight for example, but it’ll get us in the ballpark.  I knocked off a few values because of fluctuations that fell in to either of the extremes (ie under 25 degrees or over 80 degrees).  Essentially if the day before was 85 and the next day was 96, I assumed you still dressed the same way.   At that point we normally resort to things like swimming or staying inside as opposed to clothing changes.  I did not account for changes in the daily low, as those usually happen at night, and the average picks up those changes. Based on all of this you ended up about 65.5% accurate.  Not bad!

Okay, so what went wrong on the other days?  Well, of the days you got wrong, here’s what tripped you up:

Temperature Changed: 49%

It Rained: 38%3

It Rained AND the Temperature Changed: 13%

Cool!   Now what if we wanted to know your luckiest month?  Well I have that too!

Month % of days you are properly dressed
August 75%
February 71%
July 70%
September 70%
October 68%
January 66%
November 66%
December 65%
June 63%
April 60%
May 60%
March 59%

So you’re actually headed in to a pretty good stretch here!  July’s almost here and August is really your month. At the very least you have some time to kill before March.  Use it wisely, and feel free to put this data on your LinkedIn/Facebook/ profile.  It’s sure to impress.

You’re welcome.


1. At least that’s what mom said when I mentioned it to her.
2. Hey, happy birthday!
3. Interestingly, that means if you took my advice and always carried an umbrella, your accuracy would go up to almost 78%.  Things to consider.