The Hepburn Tropo Index (HTI)
created by William R. Hepburn, WTFDA
"Hepburn Tropo Index"

  "The Hepburn Tropo Index is the degree of tropospheric bending forecast to occur over a particular area, which is an indication of the overall strength of tropospheric radio signal strengths (and hence interference) on a linear scale from 0 to 10."

Negative Tropo Index = Below normal conditions. Sub-refraction. Bending occurs, but skyward.
Tropo Index of 0 = Normal midday "dead-band" conditions (Standard Atmosphere). Tropo Scatter only.
Tropo Index of 1 = Some downward bending occurs. Marginal conditions. Mainly super-refraction.
Tropo Index of 2 = Weak to fair opening. Mainly super-refraction.
Tropo Index of 3 = Moderate opening. Ducting more likely.
Tropo Index of 4 = Good opening.
Tropo Index of 5 = Strong opening.
Tropo Index of 6 = Very strong opening.
Tropo Index of 7 = Intense opening.
Tropo Index of 8 = Very intense opening.
Tropo Index of 9 or 10 = Extremely intense opening.
(Yes, the Tropo Index can indeed exceed 10! is not a finite scale).

(Note: Remember that these descriptions are to be taken in context of a "global" scale. Regional differences must be taken into account. For example, an opening of Level 4 might be considered subpar for the Cape Verde area, but considered an excellent opening in Saskatchewan.) 

Notes on how to use the forecast maps..
The Hepburn Index is an indicator of the amount of signal bending occurring overhead.

Some general rules of thumb..

 A higher index = higher chance for an event and/or stronger signals.
 A larger area = longer distances.

 A higher index over a smaller area  =  stronger enhancement over shorter distances.
 A higher index over a larger area  =  stronger ducting over longer distances.
 A lower index over a smaller area  =  weaker enhancement over shorter distances.
 A lower index over a larger area =  weaker ducting over longer distances.

 Strongest path = centre of path with highest values.
 The longer the path,  the higher the index required to maintain it.

 Overland paths are usually the strongest at sunrise and weakest at mid afternoon.
 Water paths are usually the strongest at mid afternoon and weakest at sunrise.
 Combination land-water paths may peak at various times depending on the local weather conditions.

 The index does not take into account Tropospheric Scatter.  In the absence of any enhancement, and with only ever-present scatter occurring, the Tropo Index = 0.

 Physical barriers such as large hills and mountains may obstruct paths.

 Do not be overly concerned with the exact value of the index at your location. The purpose of the maps is to display potential duct paths. Often if you are in a void but are still within line-of-sight or tropo scatter range of a nearby duct, then you may be able to get in on the duct - although signals will likely be weak.

Unstable Signal Areas

  Unstable signal areas shown by areas enclosed with dotted lines labelled "U" are areas which may have isolated or scattered heavy rain showers or thunderstorms which could occasionally disrupt paths and cause unusual and sometimes rapid variations in signal strengths. This usually occurs when the lower atmosphere is stable, but the middle & upper atmosphere is unstable. This is most pronounced at night and also ahead of warm fronts.

  For a rundown of Tropospheric Modes, click here .

2008, Updated 2018 William R. Hepburn