Thursday, July 19, 2007

The Rest Of The Story

"What kind of cloud is that?" she said.

"Where?" he said. I could view him leaning toward the glass of the train window, one hand shielding his eyes, like an Indian scanning the horizon in those old Cowboys and Indian films of the fifties and sixties.

"There!" she said, apparently pointing somewhere in the sky. "That long straight one."

I have to explain, I could not see them because they were sitting behind me. The train was crowded and I was sitting on the second row of backward-facing seats in the front of the train.

"What?" he said, apparently spotting what she meant "That one that is pointing toward the aeroplane and growing as the plane moves?"

"Yeah." She said. "That one."

"That's smoke coming out of the aeroplane's engines."

What!? Ah. They are definitely not the teachers I had thought they were earlier in the trip. Or at least they are not teachers of mechanics of meteorology. Were they mechanically inclined, they would have known that to leave that much smoke behind the plane would have to burn a hell of a lot of oil or would be really inefficient at burning its fuel. Besides, were it actually smoke coming out of the engine it would be visible coming out of the engine and not just as a trail that forms behind the plane.

Were they meteorologically inclined, they would have noted that contrails only form under certain conditions, generally when the air is cool and still. My understanding is that they are caused when a jet has gone through a patch of cold, still air and are formed by the difference in pressure that is caused either by the hot, high pressure jet exhaust suddenly expanding in the rarified atmosphere or by the sudden clash of pressures in the wake of the wings.

Were I a bit more gregarious (or if I wanted to let them know that I had been eavesdropping) I would have explained it. Oh well, they'll be blissful in their ignorance.

Wherever you are, I hope that you have a wonderful day!

Don Bergquist - 19 July 2007 - Thames Ditton, Surrey, UK

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