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!