For radio amateurs and maritime history enthusiasts, few things are as evocative as the rhythm of a weather report sent via Morse code. Before satellite data took over, the safety of life at sea depended on the Radio Officer (“Sparks”) tapping out observations in a standardized, condensed numeric format.

This format is known as FM 13 SHIP (or WMO-No. 306). It is a masterpiece of efficiency, allowing a ship to transmit its position, wind, pressure, and temperature to weather centers worldwide using five-digit groups.

I use this format in my digital correspondence to keep a small part of this maritime tradition alive.

The Code Structure

A typical FM 13 message looks like a string of random numbers to the untrained eye, but it follows a strict logic. Here is the breakdown of the groups used in a standard report:

BBXX [CALLSIGN] YYGGiw 99LaLaLa QcLoLoLoLo Nddff 1snTTT 4PPPP

Legend

• BBXX: The identifier. It tells the receiving station: “This is a surface weather report from a ship.”
• YYGGiw (Date & Time):
  • YY: Day of the month.
  • GG: Hour in UTC.
  • iw: Wind measurement indicator (4 = measured in knots).
• 99LaLaLa (Latitude): The ship’s position North/South in tenths of a degree.
• QcLoLoLoLo (Longitude): The ship’s position East/West and the quadrant of the globe.
• Nddff (Wind & Cloud):
  • N: Cloud cover in oktas (eights).
  • dd: Wind direction (in tens of degrees).
  • ff: Wind speed (in knots).
• 1snTTT (Temperature): The air temperature in degrees Celsius (to the tenth).
• 4PPPP (Pressure): The barometric pressure in hectopascals (hPa).

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Essential Documentation & Resources

If you want to dive deeper into maritime weather reporting or decode historical logs, these are the three most important resources available.

1. The Official Standard (The “Bible”)

WMO Manual on Codes (WMO-No. 306) This is the primary source from the World Meteorological Organization. It defines every single code used internationally. It is technical and exhaustive.

Download Volume I.1 from the WMO Library
Look for FM 13-XIV SHIP in Part A.

2. The Practical Guide (Best for Learning)

NWS Observing Handbook No. 1 Published by the US National Oceanic and Atmospheric Administration (NOAA) for their “Voluntary Observing Ships” (VOS) program. Unlike the dry WMO manual, this handbook explains how to observe the weather and encode it, complete with diagrams and clear examples.

Download the Handbook (PDF)

3. The “Cheat Sheet”

VOS Ship Code Card A two-page quick reference guide designed to be kept next to the radio or on the bridge. It summarizes all the groups and tables needed to quickly encode or decode a message.

Download the Ship Code Card

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Why this matters for Ham Radio?

While we no longer rely on CW to send weather data to the Met Office, understanding these codes connects us to the history of wireless telegraphy.

In the future, I plan to expand this section to cover:

• Receiving Weatherfax (WEFAX) on HF.
• Understanding NAVTEX transmissions.