Wednesday, 15 October 2014

Weather forecasts offshore

Aurigas SSB receiver - with space
 for a smartphone on top
Our recent passage from La Coruna to Porto Santo took just over a week. Even the best pre-departure weather forecasts can hardly be relied upon over this time period. Receiving weather information at sea allows us to re-evaluate our route planning, ensuring a fast and safe passage.
But what are the options? Mobile networks only help within a few miles from civilization. The forecasts received using our VHF radio can only be heard up to 40 nautical miles from the shore. After that, additional, and, typically, expensive equipment is needed: either a satellite phone (or modem), or a long-wave radio receiver. The choice is bewildering. Moreover, one has to consider whether one is content with text-only forecasts (like the shipping forecast), or looking for more detailed wind weather maps (like the Metoffice pressure charts or Passageweather wind maps).

For Auriga, we where looking (as usual) for a reliable, redundant, power-efficient and affordable system.
All communications rely on electromagnetic waves. The mobile networks use waves with a length of centimetres, and corresponding frequencies in the GHz (gigahertz - a billion oscillations per second) range. The yacht's standard VHF radio operates using waves in the meter range, and ("very high") frequencies of about 100 MHz (megahertz - a million oscillations per minute).  As the frequency used decreases, the wavelength increases, and, for earth-based stations, so does the range - longer waves can "bend", following the Earth curvature or reflecting off certain layers in the atmosphere. Satellite systems, on the other hand, rely on having a direct "line of sight" between the yacht and the satellite overhead, and can therefore use GHz frequencies similar to those used by the mobile phones.

But enough theory. Satellite phones are expensive, and data costs for downloading images exorbitant. Plus, a PC is required to complete the system. Cheap stand-alone solution? Hardly. On the plus-side, you get reliable, worldwide coverage. Satphones can be used in an emergency, but then we have an EPIRB and a PLB already. After reviewing the options, we settled for a half-way solution: a Yellowbrick Tracker/Messenger. It can send and receive short text messages, and our weather girl Giulia, who for now is regretfully stuck in Cambridge, has been supplying us with regular updates like "L980 N50W15, 968 N55W10 by Fri. SW4-5, Fri var 2-3, then NW4". We agreed that these would relate to our current and anticipated position, as reported by the tracker here. The downside (or the upside?) is that you need a shore contact with some skill in interpreting the available weather information. However, with a little effort one can also set up automatic weather emails to be forwarded to the Yellowbrick. For weather maps, however, we had to turn elsewhere.

When we bought Auriga, she was equipped withe a long-wave SSB receiver shown above - a dated but reliable model. A new one can be picked up for under £200, and a Grundig SSB world radio receiver providing similar capabilities retails at under £100. Initially, we didn't think much of it, and I used it to listen to BBC Radio 4 on sleepless nights in the Channel (speaking of which, the shipping forecast on 198kHz moved to 1754 daily, and we could hear it all the way down to Spain). However, it can do much more - an SSB reciever can be tuned into one of the several Weatherfax stations. These transmit image data as a series of audible beeps, the changes in frequency encoding greyscale values between black and white. A list of stations worldwide, together with broadcast schedules, can be found here. Of those, the UK Northwood and the German DWD Hamburg stations cover European waters. The series of beeps is usually decoded using a PC - it sounds something like this. However, Weatherfax apps are now available for both Android and iOS phones and tablets (I use "HF Weather Fax for marine" on a Nexus 4 Android phone). Put the smartphone from your pocket close to a £100 SSB receiver - and a few minutes later, you have a complete weather map! A few examples are shown below. Quality is weather dependent, however, with several frequencies to choose from and several transmissions per day (the DWD, for example, will update the forecasts twice daily, and transmit surface analysis, +24h/+48h surface pressure forecasts, wind arrows and more), we have always been up to date with the weather developments. The quality is not always perfect, but here are some real-life examples (I believe they have been compressed when I copied them, and black/white contrast can also be adjusted in the app):
Surface pressure chart showing isobars and fronts
Sea state and swell forecast (this one is for the Baltic)
A more detailed analysis chart with pressure trends, winds and precipitation.
In general, the finer text is only legible under ideal conditions. The pressure systems and fronts, on the other hand, can be clearly seen and are generally sufficient to make informed decisions. A better antenna would probably help - we use a wire connected to one of the chainplates, thus turning the entire rig into one. However, because we do not have a transmitter, we don't need to worry about ground plates, rig insulators and high current demands. 

With both text forecasts and surface pressure charts, we feel we have most bases covered. Read soon about how we used the weather information and decided to sail WNW at 90 degrees to the rhumb line to punch through the cold front, got drenched and knocked down on our way to Madeira, and arrived well ahead of another Ballad that chose to head S on the opposite tack (sorry Jonathan)!

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