A warm, sunny afternoon in late April, a clear blue sky, and a whole bunch of contrails. As I've said before, it's sometimes difficult to grasp just how much air traffic passes over a major city in even a short space of time, but, on a calm, clear day, the evidence speaks for itself.
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This picture of contrails covers just a small portion of the sky and a short period of time, and yet a significant proportion of the sky is obscured by fuzzy white trials. It's long been suggested that abundant contrails may, cumulatively, be sufficient to have an adverse effect on surface temperatures, as the article
"Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate" (Science Daily, April 28, 2004) points out.
I'm not qualified to know whether there's any truth in such theories, but it's difficult to ignore the sheer number of contrails drifting overhead, day and night!
PP