Tides: Why the Same Spot Is a Different Place Morning and Evening

A rider arriving at the ocean from a lake usually learns about tides the unpleasant way: in the morning the spot was shallow and flat, by lunchtime the depth had risen by a metre and brought current and swell with it. Here is the mechanism.

Where tides come from

The Moon pulls water on the near side of the Earth into a bulge. On the far side a bulge forms by inertia — the water effectively lags behind. The Earth rotates beneath those two bulges, and your coastline passes through them.

Hence two high tides and two low tides a day in most places, shifting by about 50 minutes each day because the Moon is moving too.

The Sun adds its own contribution, half as strong. When the Moon and Sun line up — at new and full moon — tides are largest. At right angles, they are smallest.

A practical conclusion that saves trips: tidal range varies through the lunar cycle, peaking every two weeks.

What changes at the spot along with the water

Four things, all affecting riding.

Depth. Obvious but underrated. A spot that is knee-deep at low tide can be deep at high — and vice versa. For teaching that is fundamental: shallow water is half the methodology.

Current. Water arrives and leaves through channels, river mouths and around headlands. At peak flow the current can reach several knots — and if it runs against the wind, you will drift downwind even while going upwind properly.

Wave shape. Waves break where the depth roughly equals their height. Change the depth and the break point moves. A spot with clean waves at low tide can be flat water at high, or the reverse.

The bottom is exposed. At low tide rocks, reefs and sandbars appear that were invisible at high.

How to read a tide table

A table gives the time and height of each high and low. Three things matter practically.

One: the timing. Roughly six hours and a bit between high and low.

Two: the range. The difference between high and low. Half a metre is barely noticeable. Two metres is a different spot.

Three, and most often missed: current is fastest not at high tide but midway between. The water stands still at the peak and the trough and moves fastest halfway between them. If current is your problem, plan around slack water.

Mui Ne and Crimea: why the difference is enormous

A practical section for anyone riding our spots.

The Black Sea is effectively tideless. The range is around ten centimetres and does not affect riding. In Crimea the water level changes mostly with wind: sustained strong wind can push more water than the astronomical tide does.

The South China Sea at Mui Ne has noticeable tides. A change in level alters both the depth over the shallows and the character of the swell at the cape.

Hence a travel rule: if you are coming from Crimea to Vietnam, tides are a new variable to factor into when you go out. Ask at the station — locals know which phase suits their spot.

On the region's seasons and wind, see our monsoon article, and on the spot itself the Mui Ne guide.

Tides and wind: a non-obvious link

A tide does not change the wind by itself. But it changes the shoreline, and the shoreline shapes the flow.

An example: at low tide a wide sandbar is exposed. Air over dry sand heats faster than over water, strengthening the local thermal effect. At high tide the same bar is submerged and the effect is gone.

A second example: at low tide you launch from a point a hundred metres further out than at high. If there are dunes or trees behind you, those hundred metres can be the difference between broken and smooth wind — we covered turbulent wakes in the gustiness article.

What to do about it

• Check the tide table for the day before your session. Every weather app has one.
• Ask at the spot which phase works best. This is local knowledge and it is not online.
• Factor current into your tacks. Especially if it runs against the wind.
• Watch the bottom at low tide. Rocks and reefs that are safe at high tide become a problem.
• For lessons, choose the shallow phase. Being able to stand up is half the comfort in early sessions.

Common questions

How many tides are there a day?

In most places two highs and two lows, shifting by about 50 minutes each day.

When is the current strongest?

Midway between high and low. At the peak and the trough the water almost stands still.

Does the Black Sea have tides?

Astronomical ones of around ten centimetres, which do not affect riding. Level there is driven mostly by wind.

Do tides affect the wind?

Indirectly: the shoreline moves, changing surface heating and the distance to obstacles onshore.

How do I learn which phase suits a spot?

Ask locally. It is local knowledge and depends on the specific bathymetry.

What to read next

Reading gustiness, wind seasons in Southeast Asia, and sea breeze and thermal wind.