A Spectral Two-Fer!

Here in Tucson, our Summer rainy season is winding down - right on schedule! You can usually start planning on astronomical observing opening up again in mid-September after the rain starts (and threatens most every day) around the Fourth of July. Our "monsoon" season provides a nice break during the hottest (and most humid) time of the year, so we don't have to worry about fighting discomfort of warm temps and bugs that seem to wait till then to "bug" you!

Yesterday the weather forecasters were saying there was a 50% chance of possibly severe storms, and the next week looked to be a drying trend as weather patterns changed. True to their word - clouds thickened and threatened after lunch. We found ourselves watching from the second floor of the Cancer Center, where Melinda was getting her occasional topping-off of fluid. With the desert dryness and depending how she feels, it is tough to drink enough, so the liter of fluids on about a weekly schedule is welcomed. While we used to enjoy the view of the mountains and the employee parking lot in years past, we now enjoy the mountains and the construction zone as the treatment center expands.

Yesterday, even the mountains disappeared as storms moved in from the southwest. It never rained more than sprinkles at the Cancer Center as they moved around us, but the conditions were fine for rainbow formation! The thing to note is how low the rainbow arc is in the sky. Of course, if you know how they are formed, you know that they appear at a constant angle of 138 degrees from the sun. Since it was about 2:30pm, the sun was still high in the sky and as a result, the bow peaked out at a low angle, just clearing the trees around the construction zone. With the sun higher in the sky, the bow would be even lower - I was on a winter bike ride when we ran into showers and the bow didn't clear the horizon! Of course, you can make your own "rainbow" with a garden hose at high noon and you can see the nearly full circle of the bow around the shadow of your head projected on grass. The close-up at right shows the acute angle it forms with the ground...

A few hours later, after we got home, another band of intense showers moved over us again and the process was repeated. This time, just 45 minutes before sunset the sun was much lower and correspondingly, the rainbow appeared much higher in the sky. At left is shown a panorama-mode image of the rainbow, taken with my IPhone 6S from our back yard. The shower was intense - about the hardest I've ever seen it rain for all of about 2 minutes! But as soon as it passed, the sun appeared, so I knew there was likely a bow... In the close-up at right, you can also spot a fainter secondary bow outside the main one. This second bow is caused by a second reflection inside a raindrop, instead of just the single reflection that causes the main bow. Note also that the colors are reversed - in the outer secondary bow, the red color appears on the inside - on the primary rainbow, red is on the outside! Rain and rainbows are rare enough in the desert that a simple comparison of the heights of the arc are difficult to come by, so it was neat to see this pair in the same afternoon.

And right at sunset about 6:30, some more dramatic lighting appeared - direct sunlight bathing the clouds in sunset-colored clouds while the gibbous moon and shadowed dark clouds also remained in the view. Nice contrast, nice colors, but tough to underexpose enough to not saturate the highlights. Will have to re-read the 6S manual again!