Large Wildfire threatened Danabay near Mossel Bay!! (26 November 2025)

Image: Danabay Fire - Jacobus ZS1JDJ  (Click for larger view.)

During the Sonsakspan Net on the 26 November 2025 at 16h30 the following interesting question was asked by Jacobus ZS1JDJ.

What impact does ash, smoke and flames have on radio communications while a veldfire is burning?

Danie ZS1DJ reported that there was a lot of interference on his radios while the emergency personal attended to the fire and other incidents. 

Videos:  Provided by Danie ZS1DJ

Herewith a short explanation:

Wildfire ash and the associated smoke and flames can significantly impact amateur radio signals

Wildfire ash and the associated smoke and flames can significantly impact ham radio signals by causing signal attenuation and refraction, particularly at higher frequencies like VHF and UHF. The effect is dependent on the density and chemical composition of the smoke/ash, and the frequency band used. 

Effects on Radio Signals

• Signal Attenuation: The primary effect is a degradation of signal strength. Smoke and ash contain particulate matter (like graphitic carbon and potassium) and ionized gases (plasma) that can absorb or scatter electromagnetic waves.

• Frequency Dependence: Higher frequencies (UHF and above, such as 70cm amateur bands, Wi-Fi's 2.4 GHz, and cell signals) are generally more affected by the particulate matter than lower frequencies (VHF and HF bands).

• Refraction and Ducting: The intense heat from a wildfire creates a "thermal bubble" or "thermal lens" in the atmosphere, leading to temperature inversions and changes in the air's refractive index. This can sometimes cause unexpected signal bending (refraction) and even enhance long-distance propagation (tropospheric ducting) in certain conditions, where radio waves bounce further than normal.

  
  
  What is a Thermal Bubble?

  A thermal bubble is a concept used to describe a buoyant mass of hot air and emissions that rises from an intense fire. 

• Formation: Continuous heating of the surface air mass by the fire causes the air to bulge upwards into a "bubble". The intense heat creates strong thermal buoyancy, making the air less dense than the surrounding cooler air.

• Ascension: Eventually, this hot air mass can break away from the surface and ascend into the atmosphere as a distinct cell.

• Atmospheric Dynamics: As the thermal bubble rises, cooler air from aloft moves downward to replace the ascending warm air, initiating a circular motion or horizontal roll vortices at the edges of the plume. This process is part of the formation of a pyrocumulus cloud (fire cloud) and is most prominent in strong vertical wind profile environments. 

  
  
  Image:  Pyrocumulus cloud, or fire cloud - Wikipedia
  

         What is a Thermal Lens? 

A "thermal lens" in the context of wildfires

refers to the atmospheric distortion caused by a fire's heat, which can negatively impact radio communications. The intense heat creates a "thermal bubble," which acts like a physical lens, bending radio waves and causing signal degradation. This phenomenon requires specialized communication strategies and technologies for effective firefighting and operational coordination during a wildfire. 

How it works

• Atmospheric distortion: Wildfire heat creates a plume of hot air, smoke, and debris, which alters the air's density and refractive index.
• Radio wave bending: This change in the atmosphere causes radio waves to bend or refract, similar to how a glass lens bends light.
• Signal disruption: The bending can cause the signal to be weaker, distorted, or lost entirely, making it difficult for emergency responders to communicate using standard equipment.

Impact on radio communications

• Coverage gaps: Areas directly under or near the plume experience significant radio dead zones.
• Interference: Other radio signals can interfere with one another, leading to unreliable communication.
• Operational challenges: Firefighters need to have a backup plan, such as using different radio frequencies, satellite phones, or establishing direct line-of-sight communication.

Solutions and strategies

• Specialized equipment: Firefighting agencies often use specialized radio equipment that can better penetrate or adapt to the challenging conditions.
• Backup systems: Relying on multiple communication systems, such as satellite phones and handheld radios, can ensure redundancy.
• Strategic planning: Planning for the thermal lens effect is crucial. This includes identifying areas likely to be affected and developing contingency communication plans.
• High-frequency radios: Some high-frequency radios are less affected by atmospheric conditions and can provide more reliable communication over longer distances.

Radio Operations During Wildfires

Image: The illustration above shows high altitude troposheric ducting - VK3SF.

Despite these challenges, amateur radio operators play a vital role in emergency communications during fire events, often filling in when traditional cellular or landline infrastructure fails. They adapt their operations by: 

• Utilizing lower frequency bands (HF or 2m VHF) which are less susceptible to severe attenuation from smoke than UHF.

• Employing tactical positioning of antennas to work around dense smoke plumes or terrain features.

• Leveraging enhanced (though unpredictable) propagation events like tropospheric ducting to make long-distance contacts.

Temperature inversions significantly disrupt radio communications during wildfires by creating

anomalous propagation conditions that can cause signal path loss and unpredictable signal patterns. 

Effects on Radio Communications

• Signal Trapping and Bouncing: A temperature inversion creates a "lid" of warm air over cooler, denser ground-level air, which traps smoke and pollutants. Radio waves get bounced or "trapped" within this cooler surface layer, leading to signals ricocheting around rather than propagating normally through the atmosphere.
• Signal Path Loss: The environment created by the fire plume itself and the inversion is "sub-refractive," which can cause significant radio signal path loss, especially over long distances.
• Unpredictable Behavior: The turbulent, ionized, and smoke-laden atmosphere within and around a fire can cause radio beams to spread out or bend in unpredictable ways, making consistent communication difficult.
• Increased Range for Some Signals: In some instances, the "sandwich effect" of an inversion layer can cause sound and radio waves to travel further than normal, leading to unexpected signal reception from distant sources, though this is generally unreliable for operational communication within the fire zone.

Operational Impact During Wildfires for Firefighters

 

For firefighters, these disruptions mean that traditional radio communication systems can become unreliable, particularly in valleys or areas where inversions settle overnight. This communication failure poses a significant safety risk during active fire suppression efforts. 

Mitigation Strategies:

• Situational Awareness: Fire crews are trained to observe and report weather conditions, including signs of an inversion (e.g., low-hanging smoke, hazy atmosphere), to anticipate potential communication issues.
• Alternative Technologies: Modern approaches utilize technologies that do not rely on traditional, centralized radio infrastructure. Mobile mesh networking, where each device acts as a relay station, creates a more resilient network with no single point of failure.
• GPS and Situational Apps: Firefighters use smartphones with downloaded maps and situational awareness applications (like ATAK) that use GPS, which functions independently of cellular networks, to maintain awareness of their location and the fire's movement even when voice communication is lost.
  

Amateur Radio and Wildfires 

Amateur radio is a crucial tool for communication during wildfires because it is independent of traditional infrastructure like cell towers and power grids, providing a reliable backup network. Amateur radio operators can provide vital information to emergency services and the public when other communication systems fail, report fires, coordinate logistics, and even assist in long-range communication for isolated communities. This independence makes them essential for public safety and personal communication in disaster situations. 

Benefits of ham radio for wildfire communication

• Infrastructure independence: Amateur radio does not rely on the cellular or power grids, so it remains functional even when they are down due to damage or overload.
• Reliable communication: Operators can communicate directly or use repeaters to extend range, enabling them to report fires, provide status updates, and request assistance.
• Emergency support: Trained ham radio operators can integrate with official emergency response efforts to provide communications support, as seen with programs like California's Provincial Emergency Radio Communications Service (PERCS).
• Long-range capability: With appropriate equipment and antennas, ham radios can facilitate long-distance communication, which is crucial for large-scale disasters or reaching remote areas.
• Sustained operation: Radios are designed for long-term use, making them ideal for extended emergencies where other devices may run out of power.

Examples in practice

• Reporting fires: An amateur radio operator used his radio to report a wildfire, and within minutes, emergency services were dispatched, a helicopter arrived, and firefighters extinguished the fire.
• Evacuation support: In the 2017 Mossel Bay Heiderand Fire, radio amateurs helped residents receive timely updates and evacuate safely, even as there houses were under threat.
• Official integration: HAMNET are officially incorporating amateur radio operators into their disaster plans, training volunteers and establishing radio stations to ensure communication continuity.

 Interesting reading:  Radio Propagation in Fire Environments