Using Weather Faxes While Cruising
Marine weather faxes, or Radiofaxes, from NOAA are a valuable resource while cruising. In Weather, Part 3, I discussed how we use weather charts that we download from the internet. In this post I talk about how we get weather faxes on Questeria when we have no internet or cell phone service, such as our last Bahamas cruise.
What You Need to Get Weather Faxes
To receive weather faxes, you need a SSB (single-sideband) receiver (you don’t need a transmitter), a way to decode the signal and a way to display the fax. We currently decode the signal with a pactor modem, but we have also used our smartphones and tablets to decode weather faxes. You can display the fax on a computer screen, mobile device or print it on paper. We generally like to look at the fax on a large screen tablet.
Getting Ready to Use Weather Faxes
We got ready for our Bahamas cruise by going on-line deciding what data we needed to look at. We started by going to http://tgftp.nws.noaa.gov/fax/marine.shtml and deciding what station was best for our cruising area. For us it was GULF of MEXICO, CARIB, TROP ATLANTIC, and SE PAC (New Orleans/NMG). Next, we looked at the Schedule and made note of the broadcast times and frequencies. All times are in UTC (Universal Coordinated Time) and all broadcasts are in the USB (Upper Side Band) mode.
Why All the Frequencies?
The faxes are broadcast on multiple frequencies for different propagation conditions. Propagation is the principle that allows HF radio signals to travel long distances. Propagation is complicated, but for our purposes we just listen to each frequency and decide which one is clearest. You must tune your receiver 1.9 kHz. lower than the frequency on the schedule. For example, if you want to receive a fax on 4317.9 kHz, set your receiver to 4316.0 kHz. Make sure your radio is programmed to receive all frequencies so you can quickly scan through them and listen for the clearest signal. My ICOM M802 came with weather stations already programmed in. New Orleans station 4316.0 kHz. is channel 54.
What Charts Should I Get?
There are many charts available, but what do you really need to understand the forecast? We rely on the Surface Analysis charts to see what is happening and Surface Forecast charts to see what will happen. The four charts that are most interesting for the Bahamas are the U.S./TROPICAL SURFACE ANALYSIS (W HALF) and 24, 48 and 72 HR SURFACE FORECAST charts.
Sometimes we look at the WIND/WAVE charts to verify our analysis of the surface forecast charts. These charts have wind barbs that show wind speed and direction, but wind speed is only in 5 knot increments. Also, the charts don’t always have wind barbs in the exact locations you care about. With practice you can extract the wind speed and direction from the SA/SF charts and use WIND/WAVE charts to verify your analysis.
Familiarize yourself with these charts before you start cruising. You can click on the times or contents of the Hyperlinked Schedule to see what each transmission looks like. The charts you get over the air will not be as clear as what you download and there will be times when you can’t get readable charts at all.
Decoding the Signal
Once we have the SSB receiving the signal, we need to decode it. On Questeria we have decoded it two ways; with an iPhone/iPad or with a pactor modem. Before we had a pactor modem we used the HF FAX app with an iPad or iPhone. At first, we would hold the device next to the speaker or an ear bud. Later I built a an interface based on the instructions at this website.
We found it easier to decode weather faxes when we got a pactor modem. We installed the Viewfax software, which comes with the AirMail program. We got AirMail on a CD that came with the pactor modem we bought from DockSide Radio. We highly recommend DockSide Radio. They sell systems already set-up with the right cables, software and documentation for your SSB and they will help you resolve any problems.
If you have AirMail on your computer you just open the GetFax window, select your station and frequency, and start receiving faxes. We listen to each frequency and choose the clearest. We periodically listen to make sure we are receiving the clearest signal, as propagation condition change over time.
Viewing the Weather Fax
When we have decoded the faxes that we want, we need to view the files. The files are in one of several formats, depending on how you receive and decode them. If downloaded from the internet they could be in .gif (Graphics Interchange Format) or .tif (Tagged Image File Format). If decoded with HF FAX they will be in .png (Portable Network Graphics) format. When received with a pactor modem they will be in .tif format. Files in .gif and .png format are easily viewed as photos, but files in .tif format need a special viewer. AIrMail includes a TIFF viewer, but we like to transfer the files to an android tablet so we can easily rotate and zoom into the image. There are many android apps to view TIFF files, but we use Multi-TIFF Viewer.
When we use HF Fax on the iPad, we use the iPad to display the charts. The displays on our iPhones are too small, so we transfer the files to the computer with iTunes. Make sure your iTunes is up to date before you start cruising off the grid. When we were in Dry Tortugas we were unable to transfer files until we got back home because we had a back-level version of iTunes on our computer which refused to talk to our phones.
Examples of Weather Faxes
In Weather, Part 5, I talked about tropical storm Ana forming on top of us. Here are some real weather charts we received while in the Bahamas showing this forecast.
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In this chart you can see the surface trough at about 25N 76W.
This chart shows the low starting to form at 30N 76W.
In this chart the low continues to form.
Weather Log
In Weather, Part 3, I described how to analyze the weather fax by measuring the isobar lines and latitude lines. But what do you do with this data? I like to keep track and see how the actual weather compares with the forecast. I keep a weather log book that I update daily with forecast and actual weather data.
Weather Log Book
This is the page when I received the faxes shown above. It starts with the date and current location. Next is observed weather, at 6:13am there was 17 knots at 95 degrees, barometric pressure was 1017.0 and it was raining. The next line was from the Bahamas Zone forecast, Winds Southeast at 10-15 knots, seas 5 to 7 feet and 2 to 3 feet. Numerous showers and isolated thunderstorms.
The next three lines are the 24, 48 and 72 hour Surface Forecast at 00 UTC. the 24-hour forecast calls for 1017 millibars and winds of 11 knots from the ESE. The actual barometric pressure was measured at 1017.3. I was not able to calculate wind speed or direction for the 48 or 72 hour forecasts. Nor did I record actual observations.
Following that are notes from the zone marine forecast and Chris Parker.
What’s Next?
Receiving weather faxes on your SSB radio is pretty simple once you get started. Analyzing them is a bit more difficult, but with practice it becomes easier and easier. We started treating it like a hobby and now they have become a resource that we rely on while cruising. In a later post I’ll talk about how we use zone forecasts, synopsis-es, and GRIB files.