Graph Description
Skew-T with Radio Propagation Plot
Bob Larkin W7PUA  Rev 25 Feb 2008
Jimmy Oldaker W7CQ Rev 24 Sept 2009

This composite of two graphs is designed to relate weather in the troposphere to
VHF/UHF/Microwave propagation. The data comes from two different web sites so the
composit graph makes it is possable to have two graphs that do not match in time
due to web access time constraints.  The times displayed at the bottom of each side
of the composit graph are correct for each graph. Please note the times, if the graph
information seems not to match from side to side.

This note explains the origins of the plot and some details about the Radio 
Propagation plot.

In a nutshell:
  * The right plot is the standard skew-T plot of balloon soundings.
  * The left plot is the radio refraction in N-units per kilometer.
  * The data at the top of the left plot is attenuation and sky temperature.

Background:
Weather Balloons are launched twice daily from small number of sites in the 
United States.  The information returned, as the balloon rises is air pressure, 
dry bulb temperature, and wet bulb temperature. In addition, the balloon is 
tracked to determine wind direction and speed.  One resulting plot is the 
"skew-T" showing wet and dry temperatures. See reference 1, for a description 
of the Skew-T plots.  The original skew-T plots for two western Oregon and 
western Washington balloons are linked from the W7CQ web site.

To provide indicators for the refraction and attenuation in the atmosphere, 
the ballon data is processed.  The refraction methods are ones widely used, 
and can be found in references 2, 3 and 4, below. Radio wave refraction occurs 
when the wave encounters a change in the amount of water vapor.  Rapid changes 
with height cause more refraction than does a slow change.  The measure of 
water vapor refractivity is generally referred to by the letter N. It is 
related to the index of refraction, n by N=10^6*(n-1.0).

The rate of change of N with height, called N-Gradient, is plotted to the 
left of the skew-T. Negative values correspond to refraction back towards 
the Earth's surface, and the reverse for positive values.  Two values of 
refraction have special meaning. A level of -40/km is roughly the level 
for a mixed atmosphere within a few thousand feet of the surface. This has 
sufficient refraction to look past the geometric horizon, and has the same 
effect as if the Earth had a diameter 4/3 of the physical value. This is 
indicated  with a green line on the plot, and only shows for the first 
12,000 feet, where this concept applies.  A second line, shown in pink, 
is for a refractivity of -157/km, supporting sufficient refraction to have 
the same effect as if the Earth was flat.  

This -157/km level of N-Gradient is referred to as a duct, since it is a 
minimum requirement for the radio wave to be able to be trapped between 
the atmospheric refraction and the Earth's surface, where it is reflected.

The actual refraction that occurs is found from a detailed ray tracing 
through the atmosphere.  Our refractivity plot is only an indicator of 
potential propagation.  For instance, if the refractivity is less than 
-157 N-units/km a duct cannot be formed. But the higher level may not 
support a duct for the following reasons:
  * The frequency of operation may be too low for the thickness of the 
         refracting layer
  * The layer may not extend geographically
  * The angle of entry to the layer may be too great to be refracted enough.
  * This latter condition is a problem for high layer elevations, and short paths.

Atenuation for 24 GHz is shown in the upper corner of the left plot.  This is 
found from the balloon measured amounts of water vapor. Included with the 
attenuation is loss due to oxygen, but this does not vary greatly with the 
weather.  The attenuation does not include attenuation due condesed water, 
such as rain, snow, fog, clouds, etc.  These attnuations can be very high.

The total sky temperature is shown for 24 GHz, as "Temp."  This is includes 
the effects of oxygen, water vapor and the cosmic background temperature. 
The units are Kelvins (i.e., above absolute zero).

Attenuation and sky temperature are given for elevation angles of 90 and 15 
degrees. The attenuation can be found for other angles by A90/sin(E), where 
A90 is the attenuation in dB at 90 degrees, and E is the new elevation angle.  
This is reasonably for elevation angles above about 15 degrees.

More refraction and attenuation information, plus numerical tables of most 
parameters are available in text files. This includes N values, grad N values, 
attenuation at 24 GHz on a layer-by-layer basis.  Also included is a table of 
so-called M values for entering to the EREPS ray-tracing program RAYS.EXE.  
These can all be seen by left-clicking on the dual balloon plot.

The text balloon data, with the extra information, is available twice-daily 
in e-mail form.  Contact Jimmy by e-mail 'W7CQ at-sign teleport.com'  to sign 
up for this.  Archives of many items are also available from Jimmy, on request.

References
	1- http://www.star.nesdis.noaa.gov/smcd/opdb/goes/soundings/skewt/html/skewtinf.html  
		is a summary of the skew-T plot.
	2- G. R. Jessop, G6JP, "VHF UHF Manual," fourth edition from Radio Society of 
		Great Britain, pages 2-2 to 2-15.
	3- "The ARRL UHF/Microwave Experimenters Manual," American Radio Relay League, 
		1990, pages 3-12 to 3-22.
	4- B. R. Bean and E. J. Dutton, "Radio Meteorology," National Bureau of Standards 
		Monograph 92, 1966.