Monday, August 08, 2011
Sharp shadows, contrail dictaphone
This morning the sunlight had a special quality. All vertical shadows were exceedingly sharp, bringing out some details of brickwork and windows that I hadn't seen before. If I had to put a name on the effect, I'd call it vertical polarization, though I don't know if that's possible.
Thinking about that, I started looking closely at the clouds to see if there was something Polaroidish about them. Hard to tell.
But in the process of examining the clouds closely, I noticed something else.
All the clouds were clearly formed from jet contrails, in a pattern of fairly regular stripes. This didn't make sense, because I know there's only one jet path in that direction, not several widely spaced parallel paths. (Widely spaced parallel paths couldn't all lead to the same airport anyway!) In fact, a jet was running along that one path as I watched, so I could spot it definitely.
The first part of the contrail was clear, then it blurred out; and each line of clouds to the left was unquestionably formed by an even more blurred remainder of an earlier contrail.
Finally I got the picture. Jets run along the same path toward the airport at regular intervals. After each jet passes, the contrail blurs out, and simultaneously the prevailing upper-level wind carries all the earlier and fuzzier contrails steadily toward the left. It's like recording an old dictaphone cylinder. More accurate but more strained metaphor: It's like plowing a field by running the tractor north and south in one line, with long pauses between runs, while slowly and steadily towing the whole field to the west... in a strong wind that smears each furrow after it's plowed. (Told you it was strained!)
This morning the sunlight had a special quality. All vertical shadows were exceedingly sharp, bringing out some details of brickwork and windows that I hadn't seen before. If I had to put a name on the effect, I'd call it vertical polarization, though I don't know if that's possible.
Thinking about that, I started looking closely at the clouds to see if there was something Polaroidish about them. Hard to tell.
But in the process of examining the clouds closely, I noticed something else.
All the clouds were clearly formed from jet contrails, in a pattern of fairly regular stripes. This didn't make sense, because I know there's only one jet path in that direction, not several widely spaced parallel paths. (Widely spaced parallel paths couldn't all lead to the same airport anyway!) In fact, a jet was running along that one path as I watched, so I could spot it definitely.
The first part of the contrail was clear, then it blurred out; and each line of clouds to the left was unquestionably formed by an even more blurred remainder of an earlier contrail.
Finally I got the picture. Jets run along the same path toward the airport at regular intervals. After each jet passes, the contrail blurs out, and simultaneously the prevailing upper-level wind carries all the earlier and fuzzier contrails steadily toward the left. It's like recording an old dictaphone cylinder. More accurate but more strained metaphor: It's like plowing a field by running the tractor north and south in one line, with long pauses between runs, while slowly and steadily towing the whole field to the west... in a strong wind that smears each furrow after it's plowed. (Told you it was strained!)
