Saturday, February 27, 2010

How much does it REALLY rain in Seattle?

For twelve months now (March 2009 through February 2010) I've been keeping track of weather conditions for my bike commute. I have been telling everyone for years that I really don't get rained on that often, even in the winter - and now I have the stats to prove it.

First some basic definitions. I decided to code every commute on the following scale:

DRY - Totally dry commute
MISTY - Encountered some mist or very light rain, but not enough to get wet.
LIGHT RAIN - Encountered light to moderate rain, enough to have to hang up my bike clothes to dry, but not a soaking rain.
HEAVY RAIN - Got as wet as you can get - had to wring out my socks after arrival.
UNRIDEABLE - Weather looked so icky I wimped out and took alternate transportation
N/A - Didn't bike, but for reasons unrelated to the weather.

There were a total of 261 work days last year for a total of 522 potential commutes. 192 of those fell into the "N/A" category, meaning I was either on vacation, worked from home, or commuted some other way for reasons unrelated to the weather.

Here's a chart showing my commuting statistics by month:

The first thing that should be obvious is that most of the chart is green - 76% of my commutes were completely dry, and another 9% had only a little misty rain. Even in the rainiest months over half of my commutes were dry. How can that be, when Seattle gets rain 138 days/year? The answer is that on rainy days it usually doesn't rain ALL day. Often I get lucky and stay dry even on days that get a lot of rain.

Here's another chart that illustrates what I mean:

The column on the left shows commuting statistics on days that had measurable rainfall. 67% of those commutes were DRY or MISTY, meaning I only got wet one-third of the time. On days that had no measurable rainfall, of course, nearly every commute was dry. The one day that I wimped out was because of cold, not precipitation. (We had that stretch of weather in the teens in December).

This still doesn't tell the whole story. Suppose I look outside my window in the morning and see that it isn't raining, and furthermore I check the radar and can see that it won't rain for at least another hour, so I'm guaranteed a dry morning commute. What is the probability that my evening commute will also be dry?

According to my year's worth of statistics, the answer is 91%. There were 139 morning commutes coded as DRY or MISTY; 126 of them were followed by evening commutes coded as DRY or MISTY. On only 13 occasions was a dry morning commute followed by a rainy evening commute.

Is Seattle cloudy a lot? Yes. Does a light, misty rain fall frequently? Also yes. But the statistics prove that you won't get very wet very often. So get out there and ride!