Capri Sea Guide

The data. Waves, swell height, period and direction come from Open-Meteo's marine models (ECMWF WAM, GWAM, Météo-France and NCEP WaveWatch), blended together. Wind, gusts and pressure come from six forecast models (ECMWF, GFS, ICON, GEM, Météo-France and UKMO) plus ECMWF's 51-member ensemble. All of it is hourly, 7 days out. Morning = the 09:00–13:00 hours averaged, afternoon = 13:00–18:00. The page re-fetches every hour on its own.

The percentages. Each one is the chance that activity is closed or cancelled in that time slot. Every activity has its own breaking point: the Blue Grotto's mouth is barely a metre high, so very little rise at the entrance is enough to stop the boats. It faces northwest, so N/NW/WNW swell hits it head-on. Swell from the west and southwest reaches it too, across open water; short wind chop from that side does not, and a sea from the south or east is blocked by the island altogether. A steady wind from the north shuts it on its own, in a sea that looks flat on every chart: it blows straight down the Gulf of Naples onto the cave, and over the recorded days a northerly of even a few knots nearly always kept the grotto closed. Its odds also lean up for the three days from each new or full moon, when the tides are largest: on otherwise calm days it shut about two days in five then, against one in seven the rest of the time. The grotto's figure never drops below 15%, because it was closed about one hour in seven even when nothing gave a reason, and it runs cautious: at 30–60% it was closed about four hours in five. Boat tours give up around 1.2 m or 18 kt. Hydrofoils cancel before car ferries; Amalfi Coast routes before both.

Today's bar. Solid up to the last check is what the boatmen actually did, recorded every 10 minutes. The pale stretch after it is forecast — but it starts from the last call instead of ignoring it, because a closure carries: across the recorded days a grotto shut at noon was still shut 89% of the time an hour later, and 73% three hours later. An "open" reading fades within the hour and the sea model takes over. Past days are never forecast this way; with no recorded call they simply read "No data".

The verdict chips. Calm (under 13% chance sea activities are cancelled), Good (to 30% chance sea activities are cancelled), Uncertain (to 50% chance sea activities are cancelled), Likely off (to 72% chance sea activities are cancelled), Off (above). The figure weighs the Blue Grotto at 62% and a boat tour at 38%, since those are the plans that break first.

The confidence line. Starts high and drops with each day further out, when the forecast models and the ensemble disagree on wind, when pressure is falling fast (a system moving in), and when conditions sit right on a threshold. Pulling in six models plus a 51-member ensemble means disagreement is measured across many independent forecasts, not just two. It's about the forecast, never about how open something is.

The written summaries. Generated from the numbers by fixed rules, so the wording changes as conditions do: grotto odds above 80% closed produce a "shut" line, 55–80% a "boatmen often refuse" line, 30–55% a "borderline" line, and below that a calm line or a wind warning for boat trips. A note is appended when the afternoon is meaningfully better or worse than the morning.