Tropospheric DX Modes
by William R. Hepburn, DX Info Centre

"Tropo" Modes 

  Tropospheric propagation is radio propagation that occurs in the lowest layer of the Earth's atmosphere - the troposphere. It is 100% weather-related. A Tropo DX mode is any abnormal condition that scatters, reflects or refracts VHF, UHF and/or microwave signals in the troposphere causing changes to their normal coverage. Another name for this is anomalous propagation, or AP for short. Tall mountain ranges form a physical barrier to tropospheric propagation, thus tropo is more rare in or across mountainous regions. Deserts are usually too dry to support the long distance tropospheric modes. There are six main tropospheric DX modes. The refraction and ducting effects are similar to those that cause visual mirages. Therefore, the distant signals received via tropo can be considered "radio mirages". Signals that are normally below the radio horizon and out-of-range instead appear above the radio horizon and receivable. (Note that due to the difference in wavelengths, the radio horizon is farther away than the visual horizon). A description of the tropo modes follows:

1) LINE-OF-SIGHT (GW) - Also known as Groundwave. Normal steady continuous reception where the receiving and transmitting antennas can see each other, taking into account the 4/3 Earth curvature of radio waves. Figure 1 illustrates normal conditions where the receiver is out-of-range. The illustration is not-to-scale.

2) TROPOSPHERIC SCATTER (TrS)  - An almost ever-present condition that brings in distant fluttery signals beyond normal line-of-sight. Scattering of the signals occurs in contact with discontinuities in the troposphere. These discontinuities can be small temperature or humidity variations, such as those that can be found around cloud layers, individual clouds, updrafts, downdrafts, small droplets such as drizzle and mist, or they can be large, such as the tropopause (the boundary between the troposphere and the stratosphere). The tropopause provides most of the longer-distance scatter due to its extreme height above the ground.

3) TROPOSPHERIC SUPER-REFRACTION (TrE) - Also known as Tropospheric Enhancement or Tropospheric Bending. Super-refraction occurs when the lower troposphere becomes stratified into two stable layers. A warm dry layer over a cool moist layer (with warm and cool being relative terms). This setup is called an inversion. Normally in the lower troposphere, temperatures decrease with height and humidity increases with height - thus why they are called inversions. Signals bend as they cross the inversion. When they start bending downwards, the signals can travel farther, reaching places that are normally beyond the radio horizon and out-of-range. The effective bending is now less than 4/3 Earth radius, but still more than 1 Earth radius. The base of the inversion is considered to be the ground. The top of the inversion is the airmass boundary. Figure 2 illustrates super-refraction.

4) TROPOSPHERIC DUCTING (TrD) - Ducting occurs when the super-refractive bending becomes so much that the signal hits the ground at a distance far away from the transmitter - and is then reflected back up to the inversion, to then be refracted back down again. In essence, the signals become trapped in a "trapping layer" or duct. The effective bending is now less than 1 Earth radius. The base of the duct is the ground. The top of the duct is the airmass boundary. Signals do weaken when reflecting off the ground. Signals that reflect off of a water surface instead (ocean/lake) retain much more strength. One side-effect of ducting is reduced range for aircraft flying above the inversion. Most ducting occurs below 3000 m (10,000'). Above that level, the air starts to get too sparse in density for inversions to be strong enough. The thickness of the duct (which equals the height of the inversion for a surface-based duct) can roughly determine the lowest usable (affected) frequency (LUF). Figure 3 illustrates ducting.

5) ELEVATED TROPOSPHERIC DUCTING - In cases where the top of the inversion is very high above the ground, it may become possible for the low level moisture to rise and pool beneath the inversion top. In these cases, 3 different airmass layers can form - with the surface layer being somewhat warmer and drier than the cool moist air higher up. This results in a duct that is elevated above the ground. Although the signals are being carried far from the transmitter, receivers at low elevation will not be able to receive them. Only tall masts or locations on high hills that "poke" into the duct will be able to receive the signals. Often this is the case with the transmitter as well, as tall masts or locations on high hills poking into the duct get direct access to it. Although the bulk of the trapped signals stay within the duct, they may occasionally escape allowing for random and spotty reception beneath the duct. Often, long ducts may consist of portions that are surface-based and portions that are elevated. An elevated duct that reaches the surface on both ends can result in distant reception without anything else in-between being received. These ducts are nicknamed "tropo wormholes" as they act like a wormhole from the sci-fi's. Figure 4 illustrates elevated ducting.
Elevated Ducting

(-Tr) - Also known colloquially as "Anti-Tropo". Sub-refraction occurs when the lower troposphere becomes unstable with a greater-than-normal dropoff of temperature with height. Signals gradually bend upwards. The effective bending is greater than the normal 4/3 Earth radius. This results in reduced range for all signals. The degree of abnormal bending during an extreme sub-refraction event is much less than that observed during extreme super-refraction. Figure 5 illustrates sub-refraction.

Main Types of Tropo, classified according to Meteorology

1) RADIATION TROPO [R/Tr] - Also known as Radiative Cooling Tropo or Nocturnal Tropo. A common nocturnal event that often occurs during clear, calm nights on land. Radiative cooling results in cooler more humid conditions near the surface which forms a shallow inversion. This inversion usually "burns off" shortly after sunrise. Due to its shallow nature, Radiation Tropo often follows the topography of the land.

2) HIGH-PRESSURE TROPO [H/Tr] - Also known as Subsidence Tropo. Sinking air (subsidence) in a high-pressure system warms and dries as it descends. Often cool moist air can become trapped underneath forming an inversion. High-Pressure tropo can last all day. Often, Radiation Tropo occurs simultaneously at night, blocking more distant signals from High-Pressure Tropo. As a result, conditions can often be better during the day.

3) FRONTAL TROPO [F/Tr] - Frontal inversions can be found in the area ahead of an approaching warm front, behind a departing cold front, or north of a quasi-stationary front. Inclement weather often accompanies fronts and may hinder duct formation. Due to the normally fast motion of cold fronts, cold frontal tropo events are often short-lived.

4) ADVECTION TROPO [A/Tr] - Advection Tropo comes in two forms:
 4A) Warm Air Advection Tropo
[WA/Tr] occurs when warm dry air overrides cooler moist land (example: recently rain-soaked land) resulting in a shallow inversion.
 4B) Cold Air Advection Tropo
[CA/Tr] occurs when cool moist air undercuts warmer drier air aloft. This can often occur along the northern and western flanks of tropical cyclones as they advance into the temperate zones.

5) DOWNSLOPE TROPO [D/Tr] - Also known regionally as Chinook Tropo, Santa Ana Tropo, Fhn Tropo, Bora Tropo, Zonda Tropo, etc. Downslope Tropo is caused by air descending down a mountainside that warms and dries as it descends. If the pre-existing airmass is cool enough, it may become trapped under an inversion.

6) VALLEY TROPO [V/Tr] - Warm dry air can override cooler moist air trapped in a valley under the resulting inversion. This is different from topography-conforming Radiation Tropo in that the inversion can persist all day long, long after any radiative effects have dissipated.

7) MARINE TROPO [M/Tr] - Also known as Maritime Tropo, Oceanic Tropo or Lake-Effect Tropo. Marine Tropo occurs when warm dry air overrides a cooler body of water. Marine inversions often extend the entire breadth of lakes and can extend for thousands of miles over the ocean. It also spreads into coastal areas by way of sea or lake breezes. Marine tropo can become enhanced or combined with other types such as High-Pressure Tropo. It normally peaks during the afternoon when the inversion is the strongest. Outside of the equatorial zone, spring and early summer is the best season.

What about evaporation ducts?
Evaporation ducts are an omni-present phenomena over bodies of water that affect microwave frequencies. They are caused by the sharp dropoff in humidity above water. Because of their omni-present nature, they should not be considered anomalous propagation (in the sense of a mode of DX). The thickness of these surface ducts varies between 0-100' (0-30 m). These shallow thicknesses will only support frequencies > 3 GHz. Evaporation ducts do not spread inland. RACONs (radar beacons) and ship radars are transmitters whose ranges are typically extended by evaporation ducts. This sometimes causes difficulties with interpreting radar echoes.

Thicker inversions and ducts that occur over or near bodies of water, that affect frequencies < 3 GHz, are the result of weather systems (both synoptic and mesoscale) producing Marine Tropo (noted above), High-Pressure Tropo, etc. At microwave frequencies, it is also possible for other tropo modes to link into and be extended by the omni-present evaporation ducts.

What is meant by an "inversion"?
In a normal troposphere (the lowest layer of the atmosphere), the temperature decreases with altitude and the relative humidity increases. In an inversion, the opposite happens; the temperature increases with altitude and/or the relative humidity decreases with altitude.

Exotic Tropospheric DX Modes

  Here are some other tropospheric modes that are more rare and exotic :

1) RAIN SCATTER (RS) - Rain scatter mainly occurs in the higher UHF and microwave bands. A band of very heavy rain at a distance can scatter or even reflect signals. The effect is the one used for Meteorological Radars. (Note that snow is not an useful reflector). Signals may appear to arrive from the wrong direction - that of the rain band rather than the transmitter.

2) HAIL SCATTER (HS) - Similar to Rain Scatter but caused by Hail.

3) ICE PELLET SCATTER (SS) - Known as Sleet Scatter in the USA. Similar to Rain Scatter but caused by Ice Pellets.

4) LIGHTNING SCATTER (LS) - Lightning Scatter is a seldom-reported mode that affects UHF and microwave. It is very similar to Meteor Scatter (MS). Quick short bursts of reception are caused by reflection off of the ionized trails forged by lightning strikes.

5) AIRCRAFT SCATTER (AS) - Caused by reflections off of aircraft. Aircraft scatter is more common within 10 miles of an active airport.

Omni-present modes.
Reflections off of hills, mountains, buildings and Knife-Edge Diffraction by itself are not considered true DX modes since they are omni-present. However, when present in combination with tropo they can help to extend signal range even further.

There are also conditions in the ionosphere that produce distant reception via a whole different set of modes (mainly affecting frequencies below ~ 120 MHz). Ionospheric Skip and Scatter are not caused by the weather, but instead by the interaction between the Sun and the Earth's outer atmosphere, by ionospheric wind convergence that increases the concentration of ionic nuceli such as meteoric or terrestrial dust (volcanic, etc.), by thunderstorm sprites, and other possible phenomena. Ionospheric propagation can also be classified by character. These include Sporadic E-Skip (ES), Auroral E-Skip (AE), Auroral Scatter (AU), F2-Skip (F2), Trans-Equatorial (TE), Meteoric Scatter (MS) and Ionospheric Scatter (IS). For more information on these modes, consult Wikipedia's article on VHF-UHF Propagation.

When discussing radio propagation, an effort should be made to not call tropospheric propagation "Skip", as this is reserved for ionospheric propagation. The colloquial term "Tropo" (TR) is more appropriate and technically correct.

2021 William R. Hepburn