Written by Craig West, CEO Europe, Weathernews
The Saffir-Simpson hurricane scale on its own cannot provide a full picture of a storm. Two hurricanes in the same category can bring very different storm surge, rainfall and tornado risk, and in 2009 the US National Hurricane Center dropped pressure and surge from the category definition altogether, because folding in more variables was making the scale less useful, not more.
What made it work instead was the opposite principle; meteorologists sacrificed scientific completeness for a number simple enough that any insurer, port authority or family deciding whether to evacuate could all act on it the same way, at the same time. A storm stopped being "dangerous" and became a Category 3 – something that is easily understood - without anyone needing details of the storm explained to them.
Shipping hasn’t had a comparable metric agreed for its own weather challenges.
“Heavy weather” sounds like a term that is shared and widely understood by all. In practice, it is anything but. Owners, charterers, underwriters and P&I clubs can look at the same conditions and reach different, equally defensible conclusions about their severity. Without a common benchmark, there is no consistent basis on which to reconcile them.
Part of the challenge is that the tools available were not built for this job. The Douglas Sea Scale, created in 1920 for visual observation from the bridge, was never designed to stand up to the rigours of a modern charter party agreement. Yet Douglas Sea State 3 still sits inside a huge number of speed and performance warranties, and depending on how it is interpreted, the same threshold can support wave-height readings ranging from around 1.25 to 2.4 metres. Three recent LMAA awards have required tribunals to adjudicate this ambiguity. In a recent case, the High Court found that a captain's own calculation of wave height was “misplaced” because it did not reflect the definition agreed in the charter party.
Deck logs and weather routing reports can diverge by at least one Beaufort force across much of a voyage's steaming days. When that happens, a tribunal can be left choosing between competing accounts, with no common severity benchmark against which to assess them. Speed and performance claims are therefore frequently settled by negotiation rather than precedent, while performance benchmarking remains inconsistent, with the underlying weather language giving commercial parties little solid common ground.
This ambiguity becomes more significant as operating conditions grow more variable. Weathernews' analysis of North Atlantic winters shows no clear long-term increase in overall severity, but a widening range between mild and highly disruptive periods. Three recent Bay of Biscay winters, December to February 2011/12, 2013/14, and 2025/26, would each be logged as a heavy weather season. 2011/12 had three storm events, covering just over 10% of the season; 2013/14 had eight, storm conditions for nearly half the season, including one 20-day storm; 2025/26 had twelve, more frequent but shorter, covering roughly a quarter.
A vessel that has to sail through one 20-day storm behaves very differently - commercially and operationally - from one that encounters a series of shorter events. Recovery windows differ, as do route, speed and operational decisions. Weather persistence matters just as much as peak severity, yet “heavy weather” captures none of these nuances.
Following the closure of the Strait of Hormuz in late February 2026, tanker traffic through it fell by 76%, while Cape of Good Hope routing increased by 20%, adding 6,000 to 8,000 nautical miles to voyages. The Cape route carries routine gale-force winds and wave heights of 8 to 12 metres, and Weathernews' Accumulated Wave Energy (AWE) data shows wave energy off South Africa can vary by more than 60% between the calmest and most severe seasons on record. Thousands of vessels are now transiting this part of the world’s oceans, many with limited historical experience of it, yet materially different seasons can still be described contractually in much the same terms.
Clearly, a shared language and definition of weather severity has its merits. A shared starting point lets an insurer, a port authority and an operator skip past what "dangerous" means and get straight to deciding what to do about it. This is admittedly a challenge for shipping, because the industry faces chronic, persistent weather variability. That is what the AWE Index is built to measure: combining the intensity and persistence of ocean conditions into one number, benchmarked against data back to 1979.
Weather data is already used by arbitrators alongside deck logs to settle disputes after the fact. AWE extends that principle by providing a continuously updated, regionally comparable severity benchmark across voyages and regions.
Of course, a common language only works if it is genuinely common. The AWE Index is Weathernews' contribution to that framework: a consistent measure of severity built on decades of historical data. The goal is not another proprietary description of weather, but a way of thinking that allows operators, charterers, insurers and other stakeholders to work from the same baseline.
