JMH weather fax from Tokyo on 7795 kHz: Pacific charts around the clock

What you hear

To the ear it is the same thing as any weather fax: a tone that rasps at two lines a second, a 300 Hz start tone before every chart, phasing pulses, then minutes of picture, then a 450 Hz stop tone. The format is 120 lines per minute with IOC 576, so a decoder set up for the German charts reads the Japanese ones without changes.

Decoded, the pictures are the charts of the north-west Pacific: a surface analysis with isobars, fronts and the positions of lows and highs over Japan, China, Korea and the ocean out to the date line; surface prognoses for 24 and 48 hours; analyses and forecasts of the 500 hPa level; wave-height charts; sea-surface temperature; and in the typhoon season, summer and autumn, track forecasts with the cone of probable positions. The charts are labelled in English with the usual international codes (ASAS for the surface analysis, FSAS for the surface forecast, AUPQ for the upper air), which makes them readable without any Japanese.

Who transmits and why

The station belongs to the Japan Meteorological Agency and signs JMH. It works on 3622.5, 7795 and 13988.5 kHz from a transmitter site in the Tokyo area, serving fishing fleets and merchant ships over a vast stretch of ocean where a fax receiver and a wire antenna are still the simplest way to get a synoptic chart. JMH is one of the few weather fax stations in the world that keeps going around the clock, chart after chart, which makes it a dependable test signal at any hour.

Which chart goes out at which minute depends on the season and on typhoon activity; the agency publishes the schedule and the station transmits a schedule page of its own, and those are the references, not this text.

When and where to listen

From Europe the signal is practically never readable: nine thousand kilometres and more of path, and even when a trace arrives the chart is noise. The way to see JMH is through a receiver in Japan, a few hundred kilometres from the transmitter, where 7795 kHz is strong at any hour. 7795 is the middle of the three frequencies and the safest choice day and night; 3622.5 kHz is for the night, 13988.5 kHz for the daytime. Think in Japanese time when you listen: summer nights there bring thunderstorm static that speckles the picture, winter days are the cleanest.

As with every fax station, the receiver has to be in USB 1.9 kHz below the nominal frequency, 7793.1 kHz, so that the black and white tones fall where the decoder expects them.

In ShackDeck

Station catalogue → Fax and digital signals → Fax JMH Tokyo 7795 kHz. The program picks a public receiver in Japan that has the frequency, tunes it to 7793.1 kHz USB, sets the WEFAX mode and starts the fax decoder; no receiver, antenna or account of your own is needed. The chart grows on screen line by line under the spectrum line; when you join in mid-chart the decoder locks onto the line rhythm and starts from where it is, and the next chart begins cleanly from its start tone. On the stop tone the picture is cut to its real height and can be saved as a PNG. For a listener in Europe this is the easy way to hold a chart of the Pacific weather at this very moment, taken straight off the air on the other side of the planet.