Reading a sea with almost no tide: when the pools show themselves

A tide you can barely measure

The Mediterranean is almost enclosed, and the narrow opening at Gibraltar lets very little of the Atlantic's tidal swing through. What reaches the Catalan coast is a rise and fall of somewhere between ten and twenty-five centimetres, twice a day, which on a sloping beach moves the waterline by a stride and on a rocky ledge by barely a finger. If you arrive from a big-tide coast expecting the sea to go out and reveal a world, you stand there for an hour and nothing happens.

The tide tables are published and accurate, and I still glance at them, but the astronomical tide is the smallest of the forces moving the water here. The ones that matter are air pressure and wind, and between them they can move the sea level by more than the moon does on its best day. Learning to read those two took me most of a year and changed which mornings I leave the flat.

Pressure and wind do the real work

High pressure pushes down on the sea and lowers it. The rough figure I have seen quoted, and which matches my own rough observations, is about one centimetre of sea level for every hectopascal of pressure above average. A settled winter high sitting over the peninsula at a thousand and thirty or more can drop the water by fifteen centimetres or so, which on this coast is as much as a spring tide. The ledges I care about start to appear from under the water, and pools that are usually just part of the sea become separate, still and clear.

Wind is the other half. A wind from the land, which on this coast means from the north or north-west, pushes surface water away from the shore, flattens the swell and lowers the level along the edge. A wind from the east or south-east does the opposite, piling water against the beaches and filling the pools with churned sand. My best mornings come after two or three days of high pressure with a light land breeze overnight, the sort of weather where the sea looks like oil at dawn.

How I choose a morning

The evening before, I look at three things: the pressure chart for the next morning, the wind forecast for the coast rather than the city, and the wave height. If the pressure is high, the wind is light and from the land, and the waves are under half a metre, I set an alarm. If any one of the three is wrong, I usually stay home, because a flat sea with poor visibility or a clear sea with a swell running are both disappointing, and the second can be dangerous on wet limestone.

Winter is the best season by a wide margin. The highs are more frequent and stronger, the water is clearer because there are fewer people and boats stirring it up, and the low sun from the south-east lights the pools on the east-facing ledges without glare. Summer mornings can be calm too, but the sea is higher on average, the water is full of suspended sand from swimmers by nine, and the animals in the shallow pools are stressed by the heat.

What the notebook records

For every outing I note the pressure at the nearest coastal station, the wind direction and strength, the wave height, the water temperature from my own thermometer and a rough visibility figure measured by how deep I can still read the markings on a white spoon held under the surface. It sounds fussy, and the first few months of it felt that way, but after five winters the columns tell me more about when to go than any single forecast does.

The pattern the notebook keeps repeating is that the pools are best on the second or third morning of a settled spell, not the first, because the sea needs time to calm and clear after whatever came before. I used to rush out on the first calm day and come home disappointed. Now I wait a day, check the barometer, and take the early train on the morning it has held.

The notebook has also taught me to be modest about the forecasts. Twice in the last year a settled morning turned out to have a leftover swell from a storm far out to the east, invisible on the wind forecast and only showing in the wave model. Both times I turned back at the first ledge, where the water was sucking in and out of the gullies in a way that made the pools unreadable and the rock unsafe, and both times the log for that day is a single line.