I planned coastal days from the ordinary weather forecast for years and could not work out why the forecast and the day so often disagreed.
The forecast was not wrong. It was answering a different question from the one I needed answered.
What the general forecast tells you and does not
A standard forecast gives temperature, precipitation, cloud and a wind figure, calculated for a general area.
On a coast, temperature is the least useful of those. What determines whether a day is pleasant is wind speed, wind direction, and what the sea is doing, and only one of those appears.
The wind figure that does appear is generally an average at a standard height over a general area, which can differ substantially from what is happening on an exposed shore or in a sheltered bay a kilometre apart.
Wind direction matters more than wind speed
The single most useful thing I learned.
A twenty kilometre per hour wind blowing offshore, from the land out to sea, produces a calm sheltered beach with flat water.
The same wind blowing onshore produces waves, spray, sand in the air and a beach that is unpleasant to sit on.
Which means the same forecast wind speed describes two completely different days depending on direction and on which way the beach faces.
The practical technique is to look at which way the beach faces on a map, look at the forecast wind direction, and choose a beach facing away from it. On any coast with variety, there is nearly always somewhere sheltered.
The caution is that offshore winds are dangerous for anything inflatable or floating, because they carry you out to sea. Calm-looking water with an offshore breeze is the specific condition in which people on inflatables get into serious trouble.
Swell is separate from wind
The concept that changed my planning most, and it is not in general forecasts at all.
Waves come in two kinds. Wind waves, generated by local wind, which arrive with the weather and leave with it. And swell, generated by distant storms, which travels enormous distances and arrives days later.
Which means you can have a completely calm, windless, sunny day with large waves breaking on the shore, from a storm a thousand kilometres away that you never experienced.
This is the situation that produces the most surprised swimmers, because everything looks benign and the water is not.
Marine forecasts give swell height and period separately from wind. A long period swell — waves arriving far apart — carries much more energy than a short period one of the same height, and is what produces powerful shore break and rip currents.
Where to find the actual information
Marine and inshore waters forecasts are published by national meteorological services, free, and are aimed at small boat users.
They give wind speed and direction, sea state, swell, visibility and weather, for defined coastal areas, usually with a longer outlook.
Surf forecasting services publish swell height, period and direction for specific breaks, which is useful even for people with no intention of surfing because it describes what the water will be doing.
And harbourmasters, lifeguard stations and coastguard notices post local conditions, which account for the specific geography in a way no model does.
The land and sea breeze cycle
A predictable daily pattern that explains a lot of coastal weather and is rarely mentioned.
Land heats and cools faster than sea. During the day, warm air over the land rises and draws cooler air in from the sea, producing an onshore breeze that typically builds through the afternoon.
At night the reverse happens, more weakly, producing a light offshore breeze.
Which is why so many coasts are calm in the morning and windy by mid-afternoon, on days with no weather system involved at all. It also explains why morning is generally better for swimming, snorkelling, small boats and photography.
On days dominated by an actual weather system this cycle is overwhelmed. On settled sunny days it is the dominant effect.
What I check now
Wind direction first, then speed, then swell height and period. Temperature last, because it is the thing I can dress for.
The inshore waters forecast rather than the general one.
And a plan with two beaches facing different directions, chosen on the morning rather than the night before, which costs nothing and has rescued a lot of days.
Fog, which appears from nowhere
The coastal weather phenomenon that is least well forecast and most disruptive.
Sea fog forms when warm moist air moves over colder water, which means it can arrive on a warm sunny day with no other change in the weather.
It can appear within minutes, reduce visibility to a few metres, and sit over a coast for days while a short distance inland is clear.
For walking on cliffs, for small boats and for anything involving navigation it is a serious matter, and the standard advice is to turn back rather than continue.
General forecasts handle it poorly. Inshore waters forecasts mention it explicitly, which is another argument for reading those instead.