How We Handle Weather: Routing, Forecasting and Reading the Sky

Weather is the variable that most determines how a passage feels. The difference between a well-timed departure and a poorly timed one can be the difference between three weeks of comfortable trade wind sailing and three weeks of hard work in conditions that the boat and crew can handle but that nobody aboard would choose if given the option. Getting the timing right is not luck. It is the product of good information, sound judgement, and experience that tells you which forecasts to trust and which ones to treat with scepticism.

This article is about how we think about weather on the Naora route: the tools we use, the approach Sven brings from 25 years of offshore sailing, and what it actually looks like to make a weather routing decision on a five-year circumnavigation.

The Tools

Modern weather forecasting for offshore sailing is genuinely excellent by historical standards. The global numerical weather prediction models, particularly the GFS and the European ECMWF, produce five to seven day forecasts of wind, pressure, and sea state that are accurate enough to make reliable passage planning decisions. We access these models through dedicated routing software that displays them as animated wind and pressure fields over the chart of the passage, allowing us to see how conditions will evolve over the days ahead and to choose the departure time and routing that puts the boat in the best conditions throughout.

We also receive professional weather routing advice from shore-based meteorologists for the major ocean crossings. On the Atlantic crossing, the Pacific crossing from Panama, and the Indian Ocean passage, a qualified weather router with access to the full suite of models provides regular updates and routing recommendations throughout the passage. This is the same service used by ocean racing programmes and commercial vessels, and it gives us access to interpretation of the models that goes beyond what any individual, however experienced, can produce alone.

During passages, weather is received daily via satellite communication: GRIB files downloaded directly to the navigation laptop, updated twice daily as the models run, giving us a continuously refreshed picture of the conditions ahead. If the forecast changes significantly during a passage, we discuss it at the navigation station and adjust routing or speed accordingly.

What 25 Years Adds

The models are good. They are not perfect. They miss squalls. They underestimate the strength of local effects in mountainous island chains. They occasionally produce confident forecasts of conditions that do not materialise, and they occasionally fail to predict conditions that do. What 25 years of offshore sailing adds to the model data is the ability to read those discrepancies before they become surprises.

Sven reads the barometer every watch. Not because the model does not tell him the pressure, but because the trend of the barometer, the rate at which it is rising or falling, tells him something about the speed of an approaching system that the model expresses in forecast hours but the barometer expresses in immediate physical reality. A steady fall of three millibars over three hours is a different situation to a fall of one millibar over twelve hours. The number is the same at any given moment. The trajectory is entirely different.

He also reads the sky, the sea surface, and the wildlife. The way cumulus clouds build and flatten over an atoll tells you something about the wind gradient. The way the swell changes direction relative to the wind tells you something about a distant disturbance that has not yet appeared in the forecast. The behaviour of seabirds in the approach to a frontal system is observable before the front arrives. These are not superstitions. They are observations that the ocean has been communicating to sailors for thousands of years, and that remain valid despite the existence of numerical weather prediction.

How We Make the Go or Wait Decision

The most consequential weather decision on any passage is the departure decision: go now, or wait. On a long ocean crossing, a poorly timed departure can mean sailing into the tail of a low pressure system that was avoidable with 48 hours of patience. A well-timed departure catches the high pressure that follows it and rides the favourable gradient all the way to the destination.

We make departure decisions conservatively. If the weather window is not clearly favourable, we wait. This sometimes means staying in a port for an extra day or two when members are ready to leave. We explain the reasoning, we show the forecast, and we take the time. A passage that begins in the wrong conditions costs more than the day saved by leaving early, both in discomfort and in fuel used motoring against headwinds that should not have been there.

We also maintain flexibility during passages. If conditions differ significantly from the forecast, the routing adjusts. Speed is increased or decreased to time arrivals at tidal passes. Course is altered to stay in better wind. We do not follow a pre-planned route as though the weather has no say in the matter. The route is always a suggestion. The weather is always the final word.