Continuing onward with our series on how to access the internet while you're at sea, the next question is how to get the important weather information you need to make good decisions about where to go and how to react to weather situations. There are dozens of software products out there to fill this void but, after spending time with most of them, I've found one that I think is the best, ZyGrib. Gribs (short for GRidded Binary files ) are a great resource for your decision process and access to good grib data is IMHO (in my humble opinion) paramount to proper passage planning. Read on to learn more about it…
Gribs or Image Files?
When you want to look at weather, you might download a prepared graphic file that depicts the current or forecast conditions. There are lots of place online to grab a prognostic chart. More likely you would download a number of forecast time periods to see how the weather will change over time. While this would appear to be wonderful at first (at least while you are at home with fast internet access), the problem is the graphic files are usually very large and, over a slower or expensive internet connection while at sea, a real concern as they will take a long time to download and cost you a bunch.
Enter the Grib file. It is a compressed file of data that will allow your computer to draw a graphic representation of the weather using a data source rather than grabbing an image file created by the data. Much more efficient, much smaller and much less expensive to download. The result is a wonderful, graphic of the weather generated by data downloaded rather than an image generated by some server somewhere. In fact, they use the same data to generate the image file so you really aren't losing anything except the large file size of the chart images.
As an example, let's look at a comparison between the downloaded graphic file for the eastern Atlantic and the grib data needed to make the same graphic. The image file is from the Ocean Prediction Center and is a chart of the current surface conditions for pressure and winds. It weighs in at about 74 kilobytes. The same image in ZYGrib created from downloading grib data is only 9 kilobytes. Regardless of the connection you are running while at sea, it is a significant savings in time and money.
I think by examining the image presented in ZYGrib you can see that it is in full color and much more detailed to look at than the plain B&W hand-drawn chart from the Ocean Prediction Center. It's also important to note that the underlying data for both charts is from the same source, the US National Weather Service so the images they both show should be close to identical. The big difference, of course, is in the size.
Another difference is that the Grib file has room for 5 days worth of data rather than one single image so you can see the weather that is coming over time rather than just a snapshot of what is happening now. This is really important. When downloading a Grib file, you can choose aspects of the data you want and this affects the size of the resulting file.
ZYGrib allows all these choices and shows you an approximation of what the file size will be so you have an idea of their relationship. While offshore and using an Iridium phone, for example, you might only want to get the sea-level pressure and winds at 12 hour periods with a resolution of 2 degrees for the next 2-3 days. This will result in a small file size easily handled by the Iridium connection (note the red circled data about showing this would be about 2 kb to download). While in a marina with WiFi, you might choose a higher resolution data adding in things like sea height, precipitation, cloud cover and the CAPE (an index of the potential for convective activity).
Using ZYGrib, you have a fantastic tool to gather your weather data for free. It is also, in my opinion, the best out of all the Grib programs; even the ones you pay for.