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Name: Craig
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Question:
Last weekend was a beautiful and clear sunny day. I was facing south, and observed a jet contrail flying from the south to the north. What struck me was how there was NO upper atmosphere wind to disturb the trail and it completed a full line across the sky (south to north). But, I noticed that over time, the line was moving east (relative to us the observers). Showing off to my kids, I explained that we were witnessing the rotation of the earth! But, as I pondered my answer it was critically flawed: the jet's contrail was moving from the west to the east -- OPPOSITE of what would be expected if the earth was turning underneath the fixed atmosphere. PLUS, the atmosphere itself must be rotating along with the earth. I do not see how it could be upper atmosphere wind, because the contrail stayed intact the entire length of the sky for over 30 minutes. Other flights came and they created a series of these "meridians" across the sky. WHAT in the world is going on to produce this? It is like the upper atmosphere was rotating FASTER than the earth in order for the contrail to move easterly. Thank you so much for giving this your thinking.


Replies:
My guess is that the contrails were being pushed by a gentle, non-turbulent wind AT THE HEIGHT of the jet planes (25,000 to 35,000 feet). Those winds can be very different from the wind speed and direction at ground level. At that altitude the air could very possibly be moving in the same direction as the Earth's rotation, but faster. If you measure the angle change of the contrail vs. time and using a little trigonometry you might be able to estimate the speed of the air in the upper atmosphere.

Vince Calder


Craig,

The contrails were moving from west to east because of westerly winds at the altitude where the planes were flying. Even at that altitude, the atmosphere is well tied to the Earth's surface and does not slip as the Earth turns. It is very likely that the planes were flying in the lower part of the stratosphere, where the air temperature increases with height; this atmospheric condition leads to little turbulence, resulting in the contrails remaining straight for a long period of time. I have observed the same thing many times.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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