The Hepburn Tropo Index (HTI)
created by William R. Hepburn, DX Info Centre
"Hepburn Tropo Index"

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

Negative Tropo Index = Below Normal. Sub-refraction. Bending occurs, but skyward.
Tropo Index of 0 = Normal.  "Dead-band" conditions (Standard Atmosphere). Tropo Scatter only.
Tropo Index of 1 = Marginal. Some downward bending occurs. Marginal conditions. Mainly super-refraction.
Tropo Index of 2 = Fair. Weak event. Mainly super-refraction. Chance of isolated ducts.
Tropo Index of 3 = Moderate. Patches of super-refraction with some ducts.
Tropo Index of 4 = High. Widespread super-refraction. Patches of ducting.
Tropo Index of 5 = Strong. Ducting over most of the area.
Tropo Index of 6 = Very Strong. Widespread Ducting.
Tropo Index of 7 = Intense. Widespread Ducting.
Tropo Index of 8 = Very Intense. Widespread Ducting. Strong signals.
Tropo Index of 9+ = Extreme. Widespread Ducting. Very strong signals.
(The Tropo Index can exceed 10 ! ... it's not a finite scale).

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 signals
/interference over shorter distances.
 A higher index over a larger area  =  stronger signals
/interference over longer distances.
 A lower index over a smaller area  =  weaker signals
/interference over shorter distances.
 A lower index over a larger area =  weaker signals/interference 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.

 Keep in mind that since tropospheric propagation occurs in the lowest part of the atmosphere, local terrain has a great impact. Significant variation in signal strengths/interference can be expected depending on local effects, often between locations only a few miles apart. This is especially true during elevated-ducting events, where elevation becomes increasingly important.

 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.

 For DXing purposes, don't 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, there may be some spillover - although signals/interference will likely be weak.

 In rare cases, it is believed that very distant signals can refract off of a strong ducting zone and come down into areas well away from that zone, rather than travelling mainly within a duct. This may be a form of "enhanced tropospheric scatter". At this point further study is required to confirm this mode of propagation. Signals are usually weak and of short duration.

 Another rare atmospheric condition that may support some enhanced tropospheric propagation is "slantwise" propagation - refraction off of a slanted, rather than horizontal, inversion boundary. More study is required to incorporate detection of this form of propagation into the models.

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 .

2020 William R. Hepburn