Today (Thursday Oct 6th, 2016) marked the first significant contrail morning for several months here in El Dorado County, Northern California. The above photo on the right of criss-crossing contrails was taken at 7:30AM, and planes continued to leave trails for the next few hours. I took photos of the jets, and they were just regular air traffic, mostly Alaska or other regional jets flying north or south, often to or from Los Angeles. They were all in the range 36,000 to 39,000 feet.
But it's Thursday, and the air traffic on Wednesday was pretty much identical. So why were there trails this morning, but not yesterday morning?
The answer is quite straightforward. It's the upper air weather!
Upper air weather is different to the weather on the ground. At 35,000 feet up it's very cold (usually below -40°F/-40°C)...
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