Percent chance of rain

percentage of rain on a given day, data from over 112 years. Click for a bigger image

I was playing around my visualization software and graphed the historical likelihood of rain in Seattle over the last 100 years. (I have a way’s to go until I can do something as totally cool as this.) As noted earlier, the data’s useful if you’re planning a long time in advance. For example, for camping, July 30-August 1 is probably a good choice. If you want to scare off some friends from LA hoping to “take advantage of the cheaper housing,” invite them over for Thanksgiving. November 19th is the day it’s most likely to rain. And be dark, too. And if that doesn’t work, you can trot out the totally depressing graph #2 that shows the likelihood it will rain at least once during the week.

Downpours like the one seen during last year’s RSVP are uncommon, and the rain is typically less than one hundredth of an inch. As a comparison, here are graphs (Source: Western Regional Climate Center) showing the likelihood that it would rain at least a certain amount. For example, the first graph is 0.01″ — a light, misty drizzle. The second is 0.10″, or as we say “rain.” The last graph is 1.00″, aka “heavy showers.”

As a comparison, August 6, the first day of RSVP, showed 0.93″ of precipitation. The historical probability of that happening is small.)

	Precipitation >=0.01″
	Precipitation >=0.10″
	Precipitation > =1.00″