Be Weather-ready: Marine Weather Maps Via Global Public Radio Network

Radio WEFAX (Weather Fax) transmits weather charts and satellite images via High Frequency (HF) radio waves, similar to a fax machine but without phone lines. A transmitting station scans the image, encodes it into audio tones for light and dark areas, and broadcasts these over shortwave radio. Receivers tuned to the right frequency capture these signals, and a decoder — often a computer or app — reconstructs the original weather map from the tones.

WEFAX uses HF frequencies (3–30 MHz, ~3 kHz bandwidth) to transmit images over long distances by bouncing signals off the ionosphere. Images arrive line by line, taking 10–15 minutes per chart, with start tones, sync pulses, data tones, and a stop tone. Only a radio, antenna, and simple decoding device (like a laptop or tablet app) are needed — no internet or satellite required.

In an age of high-speed internet and satellite forecasts, an almost century-old technology still quietly serves mariners as a lifeline for critical weather information. Radio WEFAX (Weather Facsimile) is an analog mode of transmitting weather maps over radio waves. Despite sounding like a relic of a bygone era, it remains free to use and remarkably resilient for those navigating remote oceans or facing communications blackouts.

If you want to stay informed about the weather but reside in a city environment where electronic devices cause significant radio interference, and you lack both a shortwave radio and the ability to install an effective receiving antenna, this project offers a suitable solution.

If you can read this instruction – you likely already have everything you need to receive WEFAX from any point in the world and be weather-ready. For this project you will need:

• A PC, SBC or tablet,

• Internet browser,

• Internet connection.

We will use KiwiSDR, a network-connected software-defined radio (SDR) device (a radio that uses software processing instead of specialized hardware) which allows you to listen to radio signals from around the world via the internet. KiwiSDR makes it easy to explore radio without owning a personal radio setup. Shortwave listeners use KiwiSDR to tune in international broadcasts and faraway signals across the globe.

Over 800 KiwiSDR units worldwide are available for public access, so you can easily find one in a specific region and tune it to hear local radio signals from that area. This unique worldwide access is what makes KiwiSDR popular among radio hobbyists and beginners alike.

In simple terms, KiwiSDR works like a web-based radio: you can tune and hear a wide range of radio frequencies using just a web browser, with no extra radio equipment or software needed. Each KiwiSDR covers low-frequency and shortwave bands (roughly 10 kHz up to 30 MHz) – this includes longwave (LW), AM broadcast (MW), shortwave (SW) and many ham radio (amateur radio) bands.

Multiple people can use one KiwiSDR at the same time – up to four users per device, each tuning a different frequency independently. The web interface provides simple controls: you can select the frequency and mode (type of signal modulation) such as AM for radio broadcasts or USB/LSB (single-sideband) for ham radio voice and digital transmissions. In essence, the public KiwiSDR network is a collection of these devices around the globe, all accessible through a central online directory.

Receiving Weather Fax (WEFAX) broadcasts using the KiwiSDR network via your web browser is a fantastic, completely specialised software-free way to grab real-time marine weather charts.

To get a good image, you need a strong radio signal. It is best to choose a KiwiSDR receiver that is geographically close to the transmitting station (try a few to choose the best).

1. Go to the official KiwiSDR List or map (e.g., http://rx.linkfanel.net/ or rx.kiwisdr.com).
2. Look at a WEFAX schedule (https://www.weather.gov/media/marine/rfax.pdf) to find a station currently transmitting (such as GYA in Northwood, UK; DDH/DDK in Hamburg, Germany; SVJ4, in Athens, Greece or JMH in Tokyo, Japan).
3. On the KiwiSDR map, click on a receiver located in that region and open its user interface in a new browser tab.

Every KiwiSDR has built-in digital decoders, meaning you don't need to route the audio into external software.

1. Look at the control panel in the bottom-right corner of the KiwiSDR interface.
2. Locate the drop-down menu labelled "Extensions".
3. Click the drop-down and select "Fax" from the list (if FAX option is grey, then choose another receiver from the map). A new control window specifically for decoding weather faxes will appear on your screen.

In FAX extension window select region of interest (Europe, Asia/Pacific, Americas, Africa), then select transmission site (DWD Hamburg, JMH Tokyo, etc.). Extension will tune to the right frequency and modulation (USB). Here some stations and frequencies:

1. Hamburg (DWD, Germany): 3855.0 kHz, 7880.0 kHz
2. Athens (SVJ4, Greece): 4481 kHz, 8105 kHz
3. Tokyo (JMH, Japan): 7795.0 kHz
4. Northwood (GYA, UK): 2618.5kHz, 4610.0kHz, 8040.0kHz, 11086.5kHz

In the Fax extension window that you opened in Step 2, adjust the following settings: LPM (Lines Per Minute): Set this to 120 (the global standard for weather fax).

Once the station begins transmitting a map, lines will start drawing downward in the Fax extension window. If the image looks split down the middle or the black border is running down the center of the page, the fax is "out of phase." Play with „auto align“ and „auto stop“. I prefer manual align with shift-mouse click, as written in Fax extension window.

Press red arrow to start recording. As long as the audio is clear and the signal doesn't fade, the KiwiSDR will continue to draw a beautifully detailed map line-by-line.

Once the map is fully rendered, press red arrow to stop recording and start auto decoding. Thumbnail of decoded map will appear. Right click the on thumbnail inside the Fax extension window to save or view the map. The browser will download the captured image as a .gif or .jpg file directly to your computer.

Weather fax (WEFAX) maps (often called radiofax or synoptic charts) are vital tools used by mariners, aviators, and meteorologists to get a comprehensive view of large-scale weather patterns over oceans and continents.

Because these charts compress massive amounts of complex atmospheric data into a single black-and-white image, learning to read them involves breaking down their primary components. Here is a very short intro guide to understanding the elements shown in charts I received.

1. The Identification Block (The Metadata)

Every WEFAX map has a title or data block (usually in a corner or along the margin) that tells you exactly what you are looking at.

1. Issuing Agency: Look for acronyms like DWD (Deutscher Wetterdienst — German Weather Service, seen in my two images) or JMH (Japan Meteorological Agency, seen sideways in the other image).
2. Analysis vs. Forecast: A Surface Analysis (often labelled Analysis or Bodendruck) shows real-time observed conditions. A Forecast (labelled with a valid time, e.g., Vorhersage or VT: 20.05.26 00 UTC) shows predicted conditions.
3. Time Standard: Everything is recorded in UTC (Coordinated Universal Time / Greenwich Mean Time).

2. Pressure Systems and Isobars

The thin, curved lines looping across the maps are isobars—lines connecting points of equal atmospheric pressure.

1. The Letters: H (or A for Hoch) stands for a High-Pressure System (Anticyclone). These generally bring clear skies and calmer weather.
2. L (or T for Tief) stands for a Low-Pressure System (Cyclone). These are associated with clouds, rain, and strong winds.
3. Reading the Isobars: The numbers on the lines represent pressure in hectopascals (hPa) or millibars (mb), typically around 1013 hPa at sea level.
4. Wind Speed: The closer together the isobar lines are, the steeper the pressure gradient and the stronger the winds will be. (In one of images, notice the tight lines around the low pressure system in the North Atlantic — that means high winds).

3. Weather Fronts (The Heavy Lines)

Fronts mark the boundaries between different air masses. Moving along these lines is where you find the most dynamic weather.

1. Cold Front (Triangles): A heavy line with triangles pointing in the direction the front is moving. It brings rapidly dropping temperatures, sudden wind shifts, and heavy, localized rain or thunderstorms.
2. Warm Front (Semicircles): A heavy line with rounded bumps pointing in the direction of travel. It brings gradually rising temperatures and steady, widespread rain or fog.
3. Occluded Front (Alternating Triangles and Semicircles): Occurs when a fast-moving cold front catches up to a warm front, lifting the warm air entirely off the ground. This indicates a mature, often intense storm system.
4. Stationary Front: If triangles face one way and semicircles face the other, the front is stalled, often leading to prolonged periods of rain in one spot.
5. Trough Line: A simple heavy line labelled "TROUGH" (seen in one of images) indicates an elongated area of low pressure without a fully formed front, often causing unsettled, squally weather.

4. Specialty Maps (Wave/Swell, Humidity, Temperature)

WEFAX broadcasts multiple types of charts beyond basic surface pressure.

Swell Height and Direction

1. The Lines: These are contours of wave/swell height measured in meters (e.g., contours showing 1.5m, 2.5m, 3.5m).
2. The Arrows: The small arrowheads indicate the direction the swell is traveling. Tight circular contours with high numbers (like the 3.5m – 4m bullseye in the center Atlantic) represent dangerous heavy seas.

Upper-Air Maps / Relative Humidity

1. 850 hPa / 700 hPa: These maps aren't at sea level; they measure conditions higher up in the atmosphere (roughly 5,000 to 10,000 feet).
2. Relative Humidity: The shaded or contoured areas indicate moisture saturation in the upper air. High relative humidity aloft means thick cloud cover and a high probability of precipitation.

5. Station Plots (The "Spiders")

On detailed surface analysis maps (like image from the Japan Meteorological Agency), you will see tiny circles with lines sticking out of them clustered over landmasses and shipping lanes. These are Station Plots:

1. The Circle: The amount of shading inside the circle tells you the total cloud cover.
2. The Tail/Barb: The line sticking out points in the direction the wind is coming from. The "feathers" or barbs on the end of the tail tell you the wind speed (a half barb is 5 knots, a full barb is 10 knots, and a flag/pennant is 50 knots).
3. Text Codes: You may also see text warnings like [GW] for Gale Warning or FOG [W] for dense fog warnings area boundaries.

Stay informed and prepared for shifting weather by using WEFAX, whether you're planning a trip or just heading out for a walk. Keep seeking new knowledge!