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From: George Huxtable (no email)
Date: Fri Jun 16 2006 - 14:45:43 EDT
I had written, in an earlier posting-
| > Normally, with increasing
| > height, the air temperature falls.
Here, I was speaking of the atmosphere over the Earth's surface, in
general terms.
and Alex replied
| I thought this was not the case in the lower layer over a sea
surface
Alex is correct, in the conditions we are discussing, of warm
non-turbulent air sliding on top of a cooler sea. Then there is a
reversed temperature gradient, an "inversion", in which the cool sea
surface cools the lower layers of air.
| I was confused by your example of a desert where sand
| is hot and air is cool. I always thought that it is this
| combination which gives reversed dip.
| (You do not even have to go to the desert, just look
| along a long, straight highway on a hot day. You will see
| a strong reversed dip).
When I was discussing desert sand, that was just to give a picture of
the way air gets heated at the Earth's land-surface, not to discuss
the dip over it.
When you observe a highway surface on a hot day, you can see a bright
"pool" on the surface, which is an image of reflected sky. But this is
NOT reversed dip, it's locally ENHANCED dip, in the normal direction
of dip. It implies that even looking somewhat downward, into the
highway ahead, you are still seeing the sky, and you have to look more
downward still until you see a view of the ground, at an enhanced
angle of dip. That's because the temperature gradient over the tarmac
is hotter below, warmer above (the opposite way round to what we are
hypothesising occurs over a sea surface). There is no inconsistency.
| I thought that the combination of hot air and cold water
| would give the opposite effect. That is normal dip.
No, hot air and cold water reduces the dip, and in an extreme case
changes its sign.
| Now I depart to my sailing trip to Denmark,
| hope the weather will permit some Cel Nav:-)
Enjoy your trip, Alex.
George
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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