Good seamanship dictates that you always have a back up. Navigating, you correlate your GPS fix with a depth reading, and maybe a visual sight. You carry spare filters for your engine, and there’s an extra handheld VHF stowed somewhere down below, just in case. When it comes to weather, however, it’s easy to be sucked in by the apparent precision of forecast apps on your phone. As Simon Keeling explains, this is raw data and a long way from a real forecast, but with a few simple steps, we can turn it into a prediction we can trust.