Mara Gray had a theory she wanted me to test:
"There's something I've been keeping track of that I'd like to pass on as a logical impossibility that seems to have some unexpected effectiveness," she wrote in email to me.
She had read in a gardening book that the weather often clears up around the full moon. She was hoping to track to see if there was really any truth to this but admitted she "slacked off this winter" and was asking me to go back through the past records to see if indeed this "logical impossibility" really did have any "unexpected effectiveness."
"It does seem that the full moon is often visible in our often cloudy skies," Gray wrote. "I've noticed the effect for two or three years and it happens often enough so that I'd schedule outdoor events on or close to a full moon myself."
Knowing full well that the moon phase would have no effect on weather patterns... but eager to see if she was on to perhaps unearthing a previously unknown Seattle weather anecdote (especially since our "always rains on the weekends" rule is taking a beating this spring), I took up the challenge.
I went back for the past 25 full moons and charted the rainfall for the day before, of, and after the full moon.
Survey says: Eh.
It rained on 12 of the 25 full moons over the past two years, and 13 of the day-after-full-moons. If you had to pick a day, maybe try the day *before* a full moon, which had rain on only 9 of the 25 dates.
But we all know it rains a lot here, so half of the dates rainy is probably right on par with climatology.
So to really check for an advantage I also went back and compared the three days surrounding a new moon, figuring if the full moon kept the bad weather way, then surely the lack of moon would mean it'd be the stormiest time, right?
The day of the new moon saw rain on... 12 of the 25 dates, so exactly the same! The day after? 13! The only difference was the day before was just-as-wet-14 days rather than the statistical quirk of 9 for the day-before-full-moon.
I'm betting what's actually going on here is that you notice the full moon more so you're brain is more likely to register that, hey, I can see the full moon, where as the moon might not be as memorable in its other phases.
Thus, I'd suggest sticking with the tried and true ways to know whether it will rain in Seattle on a future date: If the day of the week ends in a 'y', you've got a decent chance.