Intermediate 8 minutes
Reading a marine forecast
Height, period, and sector: the three numbers that decide whether the day is a go
Two forecasts both announce a one-meter wave. On the first, the day is perfect; on the second, a shore entry is impossible. The difference isn't in the height: it's in a number almost nobody looks at: the period.
In this article you'll see what a marine bulletin contains, how to read significant wave height and swell period, why ground swell is read separately from wind sea, and how far out you can reasonably trust a forecast.
The forecast is the only information you have about the diving day before you commit to a trip, an outing, or a booking. Reading it well turns a gamble into an informed decision; reading it poorly produces the two classic surprises: canceling a day that would have been good, or going out on one that wasn't.
What a marine bulletin contains
Five blocks, always the same ones, whatever the source.
The first is wind, and as you saw in its own article, it's expressed as a sector of origin and a speed in knots. A good bulletin also distinguishes mean wind from gusts: the gap between the two tells you how unstable the day will be, and a maximum gust well above the mean announces an uncomfortable surface even if the average looks fine.
The second is sea state, expressed as significant wave height and its period. Good bulletins also separate two components: wind sea, generated locally at that moment, and ground swell, swell that arrived from far away, with its own height, period, and direction.
The third is atmospheric visibility, which is not underwater visibility but surface visibility: it matters for the boat's navigation and for being seen, especially in fog or heavy rain.
The fourth is official warnings. When a maritime authority issues a warning (gale, storm, combined sea) that warning outweighs any interpretation you might make of the rest of the bulletin.
And the fifth, often forgotten, is the tide. It isn't part of the weather bulletin, but in channels, straits, and reef passes it determines the real diving window, as you saw in the tides article. A perfect wind forecast doesn't save a site that today can only be dived at slack tide.
Keep reading with full access
This article is part of the full Encyclopedia. A donation from USD 10 unlocks every article, every species photo and the downloadable app, and helps sustain the Scuba Sign Paperless Diving Community project.