NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Anomalous dip. was: [NAV-L] Testing pocket sextant.
From: George Huxtable
Date: 2006 Jun 16, 19:45 +0100
From: George Huxtable
Date: 2006 Jun 16, 19:45 +0100
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 george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.