Posted by Dennis E. on July 09, 2001 at 12:04:26:
In Reply to: Re: good VHF/UHF ducting reported for thurs 12:00 GMT posted by Rich on July 03, 2001 at 21:59:02:
I have a bit of a weather forecasting background so I'll add to what Rich said...
A significant factor in ducting is a warmer air layer on top of a cooler layer of air. It's called an inversion because the temperature normally decreases the higher you go (from approximately 3-5 degrees F/1,000 feet. From the surface to X,XXX feet the temp is decreasing but at Y,YYY feet it starts to have an "inverted" profile, thus warming aloft. Typical inversion layers can be 1,000-3000 feet thick and start at 2,000 feet and higher. How does it develop? When high pressure moves into the area, the air aloft start subsiding...This warms up the lower atmosphere and also removes a significant amount of moisture, thus resulting in mostly clear skies. What's been happening lately is that a very strong high-pressure system has been stalled for some time over the Four Corners region of the Southwest, moving slowly East, and it's this system that has been causing significant inversions aloft. By the way, a really easy way to tell there is an inversion aloft in Western WA is when the air starts to dirty up. Normally, pollutants are pretty well "mixed" out of the atmosphere by horizontal winds and also vertical winds (called thermals). When there is an inversion, tho, the pollutants are trapped under the inversion layer because so little 'mixing' is occurring, thus leading to the stagnant, "dirty" air.
As Rich points out, these ducts can signals thousands of miles as long as these types of high-pressure systems dominate with inversions. Again, this isn't the only way ducting occurs but I've read that it is the most prevalent type.
: : OK my question is. What is ducting? And what do you listen to/for? Thank you.
: : Nukeboy1977
: I'm certainly no expert but heres what I know.
: It kinda works like this, normally VHF/UHF goes line-of-sight to the horizon and then the signal flies off into outer space. VHF actually bends some and goes a little past the visible horizon. Either way thats about as far as it normally goes.
: Ducting is when theres a space between two layers of moist air (like a warm layer and a cooler layer) so the signal bounces between the two layers, think of it like a long heating duct or a hollow tube. Depending on the length of the duct the signal can go hundreds of miles well past the horizon.
: This happens in the lower part of the atmosphere (the troposphere). As I understand it, the higher layers of the atmosphere doesn't have enough moisture density to make frequencies at the high end of VHF + UHF reflect and skip like the lower frequencies do.
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