Springs and neaps, and why your session moves an hour a day

Some weeks your break works every morning before work. The next week it does not, and the swell has not changed. That is the tide, on two clocks at once: one that turns every day, and one that turns every fortnight.

The daily clock: fifty minutes later

Two high tides a day, roughly, each about fifty minutes later than the day before. A window that opened at seven this morning opens at about ten to eight tomorrow, and by the end of the week it has left your morning altogether.

The fortnightly clock: springs and neaps

• Springs come around new moon and full moon, when the sun and moon pull along the same line. Bigger range, faster water.
• Neaps come at the quarter moons, when they pull at right angles. Smaller range, slower water.
• About seven days separate one from the other, and a full cycle from spring to spring takes a little under fifteen days.

What that does to your window

On springs, a lot of water moves quickly. Your break's working band goes past fast, the window is short, and currents are stronger. On neaps, the water barely moves: the window is long, but the tide does less to shape the waves.

Neither is better in general. It depends on the bank or the reef. What is general is that a rule of thumb learnt on a neap week will let you down on a spring week.

Why mid-tide goes so fast

The tide does not rise at a steady rate over its six hours. The rule of twelfths gives the shape of it: of the total rise, one twelfth comes in the first hour, two in the second, three in the third, three in the fourth, two in the fifth, one in the last.

Half the water moves in the middle two hours. So:

• A window around mid-tide is the shortest. On a big spring, it can be over in an hour. Be early.
• A window around low or high water lasts longest, because the water is hardly moving. You have more slack.

Most British beach breaks are at their best on the push from low to mid, which is exactly the stretch where the water speeds up. On springs, that is the hour you cannot afford to miss.

Why it matters more in Britain than almost anywhere

The Bristol Channel has a mean spring range of about twelve metres, and more than fourteen in places, among the largest in the world. Most British coasts are nowhere near that, but seven to ten metres is common enough. With that much water moving, the tide does not just nudge the session. It decides it.

Planning the week

1. Know which state of tide your break works on. That does not change.
2. Know whether this is a spring week or a neap week. That tells you how long the window will last.
3. Walk the window forward about fifty minutes a day, and see which mornings it still falls on.

Step three, for every break you surf, every evening, is the tedious part.

Frequently asked

How often do spring tides happen? Twice a month, around new and full moon. A full cycle from one spring to the next takes a little under fifteen days.

Are spring tides better for surfing? Not in general. The window is shorter and the water faster. Some breaks like that, some do not.

Why does the tide move so fast at mid-tide? Because half the rise or fall happens in the middle two hours. That is the rule of twelfths.

How much later is the tide each day? About fifty minutes.