Monday, September 7, 2009

Tonight's Deathmarch...

Melinda won't walk with me anymore - she says I'm too fast (it is exercise, after all!), so mostly I go out on the nights that she works, like tonight. Normally I don't bring my camera along, but this time of year, Melinda heads off to work right about sunset, and as I headed out, what I spotted made me go back in to get it!

This time of year as the monsoons wind down and the thunderstorms dry up, you still get widely separated thunderheads, and with the right conditions, you can get some spectacular effects. Tonight a thunderhead formed right over the setting sun, and even though the sun had gone down, the sun's rays cast a shadow into the upper atmosphere, where scattering made it visible. Called crepuscular rays, they appear to diverge from the sun. of course, the rays are parallel and their apparent divergence is a trick of perspective, like the rails of a railroad track appear to converge into the distance. Here you can see the thunderhead on the western horizon, lit up by the already-set sun casting a shadow into the sky.

Oh, and another thing I like to watch on my walks - do you see the shadowy spots against the sky (I see 4)? They are bats that also come out at sunset, feeding on flying insects, blurred by the longish exposure.

Of course, if you have crepuscular rays, there is also a chance of seeing anticrepuscular rays! As the clouds projected shadow continues in the direction opposite sunset, it appears to reconverge in the east. Sure enough, you can see the anticrepuscular rays tonight as well in this shot looking east. And as a bonus, if you look at the full-resolution image (click on the image), the caret marks the bright planet Jupiter, now low in the SE at sunset. It appears here over the palm tree.


Anonymous said...

i have seen this phenomenon before and thought it curious but never knew exactly what caused it. thank you for the photos and explanation.

as for the bats see also:

now when you walk with your spouse, you're supposed to enjoy yourself and call it a toner, leaving the serious sweat walk for other occasions of self infliction. ha just kidding

so what's with the title anyway?

Dean said...

As for the title, one of the last times Melinda walked with me, I made her go to far, thus was even longer going home (my standard 2 mile route) - she called it a deathmarch and the name stuck (with tongue in cheek).

I had heard about the batfest (only about a mile from me), but I only mentioned it because 4 of them were caught in that one photo. There is also a colony that live in the football stadium, where the Mirror Lab is located. You can occasionally see folks watching them come out at dusk...


Anonymous said...

dude, poor mel, i agree with her. kind of like going on a hike and not realizing how steep how far and the "there and back" can feel like a deathmarch if one is not ready for it. have been there before.

have seen far more bats in tucson this year than in other years, not sure why. shouldn't be seeing clouds of them where i am but do, very odd.

Dean said...

I agree - more bats this year, it seems. You gotta love em - munching bugs all night long! I don't know if it is the central air I put in or the wimpy monsoons, but have seen no mosquitos this year - nice to see them on the decline like they are supposed to be in the desert.