Signal guide

Weather fax (radiofax)

Weather maps and charts sent as sound, one line at a time, mainly for ships.

I've heard this My life list

Weather fax sounds like a steady, rhythmic scraping or swishing, about two strokes per second, that goes on for ten minutes or more while a chart is sent line by line.

What does it sound like?

Plate 75Picture

A steady, rhythmic scraping or 'swishing', about two strokes per second, starting with distinct start tones.

0 Hz1,0002,0003,000 Hz

12-second example. Recorded off the air on the developer's own receiver.

Field marksA steady tick, 120 lines a minuteTones between 1,500 and 2,300 HzStart and stop tones frame each chart

Hear it, step by step

This weather fax clip, recorded off the air from the U.S. Coast Guard station at Point Reyes, California, catches the signals that come before a chart.

0 Hz1,0002,0003,000 Hz

Tap a step to hear that moment. The box shows where it is on the waterfall.

0.0 sThe start signalFor the first 4.4 seconds the tone jumps back and forth 300 times a second, drawing a row of lines 300 Hz apart on the waterfall. This start signal tells an automatic fax receiver to wake up and get ready.4.4 sSteady blackAt 4.41 seconds the tone settles at 1,502 Hz. In radiofax, 1,500 Hz means black and 2,300 Hz means white.4.9 sA white blip every half secondFrom 4.89 seconds, a 25-millisecond burst of white at 2,300 Hz arrives every 0.5 seconds. Each half second is one scan line, which makes 120 lines per minute, the rate this station lists for its charts.4.4 sPhasing: finding the edgeThis is the phasing signal: every line is 95 percent black and 5 percent white. Stacked up, the white slivers form a vertical line that shows the receiver where the edge of the picture is.

Who uses it?

National weather services and coast guards; received by sailors and hobbyists.

Where and when can I hear it?

Shortwave, on published schedules. In the US, NOAA charts are sent from Point Reyes, California (for example 8682 and 12786 kHz) and from other coast stations. In June 2026 the National Weather Service proposed ending these US broadcasts toward the end of 2026, so check the current schedule.

The science

Radiofax sends a picture one scan line at a time by shifting a tone between about 1,500 and 2,300 Hz as the shade changes, a frequency deviation of about plus or minus 400 Hz around a 1,900 Hz center on shortwave. The US National Weather Service's radiofax schedule lists nearly every chart at 120 lines per minute, two strokes a second, with an index of cooperation of 576, a figure that sets the picture width at about 1,810 pixels per line. An automatic start tone (usually 300 Hz switching), a phasing signal that marks the image edge, and a 450 Hz stop tone let unattended receivers print charts on their own.

Why use it?

A ship far from any internet connection can print detailed surface analyses, wave forecasts and satellite pictures with a shortwave receiver and a fax recorder or computer, and published schedules tell crews when each chart will air.

Lore

The idea of sending images over wires goes back to Alexander Bain, who proposed scanning a picture line by line in 1843. The National Weather Service schedule lists assigned frequencies and notes that receivers feeding a computer run in upper sideband on the carrier frequency, 1.9 kHz lower.

What do the decoded parts mean?

• Chart: The image: surface analysis, forecasts, wave heights, satellite pictures and more, as listed in the station's schedule.

Easily confused with

• Slow-scan TV (SSTV): SSTV warbles in pitch and finishes a picture in minutes
• Weather satellite pictures (APT): APT came from satellites at 137 MHz

How to recognize it

Radiofax sends a picture one line at a time as a tone that moves between about 1500 Hz for black and 2300 Hz for white. Most stations send 120 lines per minute, two lines per second, which produces the regular "swish-swish" rhythm. Dense text and coastlines make the sound rougher; empty white areas sound like a smooth steady tone.

A transmission begins with a start tone of a few seconds, then about half a minute of phasing signal (short regular clicks that line up the picture), then the image, and ends with a stop tone.

Easily confused with:

• SSTV: pictures too, but with a musical header and a sweeping, faster sound, lasting from about half a minute to a few minutes.
• NOAA weather satellites (APT): the same two lines per second, heard during satellite passes on 137 MHz until NOAA switched off the last of them in August 2025.
• RTTY: a constant two-tone chatter with no line rhythm.

Where and when to hear it

In the United States, the Coast Guard broadcasts National Weather Service charts from Point Reyes, California (NMC, including 4346, 8682, 12786, 17151.2 and 22527 kHz), Boston (NMF), New Orleans (NMG) and Kodiak (NOJ), and a Defense Department station sends them from Honolulu (KVM70). In June 2026 the Weather Service proposed ending all five of these HF radiofax broadcasts, toward the end of 2026 if approved, so check the current schedule before listening. Many other countries run their own, such as Germany's DDH/DDK and Japan's JMH.

Radiofax frequencies are published as the assigned frequency; tune your receiver in upper sideband 1.9 kHz lower so the tones fall in the right place. Exact times for each chart are listed in NOAA's worldwide radiofax schedule.

Because charts follow a fixed timetable, radiofax is one of the most predictable signals on shortwave: look up the next surface analysis or wave forecast, tune in a minute early, and listen for the start tone. Nighttime favors the lower frequencies and daytime the higher ones, so stations send on several at once.

A little history

Sending pictures over wires dates to the mid-1800s, when Alexander Bain patented an early facsimile idea in 1843. Radio facsimile developed in the 1920s and 1930s, first for news photos and then for weather maps sent to ships and airfields. For decades, ships carried dedicated fax recorders that burned or printed charts onto paper.

Radiofax remains in service because it needs nothing but a shortwave receiver, works far from any internet connection, and covers whole oceans, which is why offshore sailors still use it.

Common questions

Sources

• NOAA: Worldwide Marine Radiofacsimile Broadcast Schedules (PDF)
• NWS: Radiofax charts
• NWS: marine broadcasts through the U.S. Coast Guard
• NWS PNS 26-48 (June 2026): proposed end of US HF radiofax broadcasts
• SSTV Handbook ch. 11: Facsimile - Radiofax (M. Bruchanov)
• Deutscher Wetterdienst: radiofax schedule (Sendeplan Faksimile, September 2023, listed as current)
• NWS: Point Reyes (NMC) radiofax schedule