Weather Blog

How a noon rainbow on the 4th of July is like a unicorn

How a noon rainbow on the 4th of July is like a unicorn
Photo by Gilbert Hannah (Butch) Kelly Rd, Bellingham, taken Jan. 17, 2011.

Q: Why do leprechauns love the summer time?

A: Because they always get extended lunch breaks.

The Northwest is famous for its "liquid sunshine" -- days when heavy rain showers run around the region, followed by brilliant sunbreaks, and when those two get together, it's like a rainbow outbreak.

But did you know the typical gloomy fall and winter months in the Northwest are the best time of year for rainbow hunting because you can see them all day long.

Now, you might be thinking to yourself, "Wait, you mean there are days you can't?"

Yes, indeed. Here is a bet you can make with your friends that you're sure to win: Challenge them to find a rainbow in the sky (created by nature, as in sun and rain, not a hose or waterfall, etc.) around solar noon in the summer. (Solar noon is when the sun is at its highest point in the sky. It's typically around 1 p.m. on our clocks during Daylight Savings Time)

It'll never happen in Seattle. In fact, between March 18 and September 23, you'll never see a midday rainbow in Seattle. And as you get closer to the June, the range of time around solar noon where rainbows are missing expand, peaking on the summer solstice.

Why? When the raindrops reflect the sunlight into the colors of the spectrum, it does so at a 42 degree angle. But when the sun is higher than 42 degrees above the horizon, any rainbows arc created would be displayed horizontally on ground, not across the sky.

(You'd still be able to see a rainbow if, say, you were on a cliff looking down at a waterfall or ocean spray, but as far as up above your head level against the sky? Nope.)

It's not until the sun gets lower in the sky that the rainbow gets projected vertically -- with the best shows at sunset when the sun is lowest in the sky.

In the winter time, the sun never gets anywhere near 42 degrees above the horizon -- in fact around the solstice in Seattle, it's only 18 degrees high at noon. Thus, this is technically the best time to see rainbows as far as window of opportunity, although climatologically speaking, spring is when the weather pattern is most conducive to "liquid sunshine."

Just don't go out at noon after the equinox and expect to find a pot of gold. You'll have as much luck as finding a unicorn!

Great illustration of rainbow in action

Dr. Dale Ireland of Silverdale captured a great time lapse of a "traveling rainbow" on Jan. 14. Look how the rainbow travels as the sun moves:

* Note that other ways to create rainbows still work as usual. For instance, seeing a rainbow in a spray from a hose, provided the spray is lower than you head at noon in the summer.