How to read the sea before you drive to the coast

The commonest disappointment on this coast is a beautiful still morning with a sea that is completely unworkable. People look out the window at home, see sunshine and no wind, drive two hours, and find the Atlantic heaving.

Wind sea and swell are two different things

Local wind makes short, steep, messy waves — they arrive with the wind and they leave with it. Swell is different: it is wave energy generated by a storm hundreds or thousands of miles away, which has organised itself into long, smooth lines and travelled here.

That is why a windless day can still be a bad day. The wind that made your swell was in the mid-Atlantic three days ago, and the calm you are standing in has nothing to do with it. On a west-facing coast like ours, swell is usually what decides whether we can dive.

A flat calm morning tells you about today's wind. It tells you nothing about last week's storm, which is what is arriving at the rocks.

Period matters as much as height

This is the number most people ignore, and it is the one that separates a workable day from a wasted drive. Wave period is the gap in seconds between one wave and the next.

• A short period, around 4 to 6 seconds, is local windchop. Unpleasant on the surface, but it does not carry much energy down deep.

• A long period, 12 seconds and up, is ground swell that has come a long way. It carries far more energy, it reaches much deeper, and it will stir the bottom and wreck your visibility even when the surface looks tolerable.

One metre at 5 seconds and one metre at 14 seconds are not the same sea. The second one will be breaking heavily on exposed rock.

Significant wave height is an average, not a maximum

When a forecast says 1.5 metres, that is the significant wave height — the average height of the biggest third of the waves. It is not the biggest wave you will see. Individual waves run noticeably larger, and sets come through bigger again.

This is exactly how people get caught on rocks and slipways: they read 1.5 m, see waves around that size for ten minutes, relax, and then a larger set arrives. Never judge a rock platform on one minute of watching.

Wind direction, not just speed

An offshore wind — blowing from the land out to sea — flattens the surface and can make a big day look deceptively tidy. An onshore wind piles water in and makes the same swell far worse. The same 15 knots can be the difference between a good day and a cancelled one purely on direction.

It also decides which sites work. Part of the reason we dive different sites on different days is simply that some are sheltered from a northwesterly and others are not.

What to check, in order

• Swell height and period first. This is the one that decides most days.

• Wind speed, gusts and direction. Gusts matter more than the average for small boats.

• Tide times, if you are going anywhere that depends on them.

• Rain in the last day or two, which drops visibility through run-off near river mouths.

• Any small craft warning or gale warning in force.

Our home page carries live sea temperature, air temperature, wind, wave height, tide and UV for Kilkee, each with the next 48 hours, so you can check the whole lot in one look before you commit to the drive.

And the last check is a phone call

No model knows what a particular reef does on a particular swell direction. We do, because we are looking at it. If you are travelling and it is marginal, ring us on 085 255 2064 before you set out — we would far rather tell you not to come than have you drive from Limerick for nothing.

Forecasts here are a convenience, not a safety service, and never a substitute for the official Met Éireann sea area forecast or your own judgement at the water's edge.