Spring Tides, Neap Tides — Reading a Tide Table Before You Go

2 hours ago

3 min read

Diving into a strong current is no fun. The good news is that you can tell ahead of time which days will have strong currents and which will be calm, once you understand spring and neap tides and know how to read a tide table.

Rising tide, falling tide, spring tide, neap tide: what causes them

Where Tides Come From

The moon and the sun are the main drivers of the tides. Wherever the moon (and the sun) lines up overhead, the water rises. Because the Earth is always turning, the place where the tide is high keeps moving.

The amount of water on Earth stays the same. If the tide is high in one place, it must be low somewhere else. As that mass of water moves from one place to another, it creates what we call a current.

Tidal currents are created by rising and falling tides

Days when the water rises and falls a lot are called spring tides, and the current runs strong.

Days when the water rises and falls only a little are called neap tides, and the current is gentle.

Spring Tides

The moon circles the Earth slowly, about once a month. If you watch, you'll notice it rises a little later each day. Its position relative to the sun is what decides whether the tide range will be big or small.

At full moon and new moon (the 15th day of the waxing or waning moon in the Thai calendar), the Earth, moon and sun line up. Their pull adds together, so the water climbs very high in some places and drops very low in others. A lot of water has to move, and the current runs fast.

Neap Tides

When you see a half moon (the 7th–8th day of the waxing or waning moon), the moon and sun are at right angles, each pulling the water a different way. The tide doesn't rise or fall much, and the current is gentle. Dives in this window are usually calmer.

Nature has other variables too. A storm can push sea water downwind, raising the water where the wind hits and dropping it unusually low on the other side. For example, when a storm hits Thailand's eastern coast, water in the lower Gulf of Thailand often drops unusually low.

The Gulf Isn't the Andaman

All of this is ocean theory. Real tides vary from place to place. The Andaman Sea rises and falls twice a day, like the Indian Ocean, while the Gulf of Thailand often rises and falls only once a day (a diurnal or mixed tide).

Gulf of Thailand tides once a day, Andaman Sea twice a day

That's because the Gulf reaches deep into the land and is shallow. When the Pacific rises and falls, water flows into the Gulf slowly. Sometimes the Gulf hasn't finished draining before ocean water pushes in again. This delay means the Gulf doesn't follow the ocean's rhythm, and its tides are more complex than the Andaman's.

Reading a Tide Table

The tool you need is a tide table: a forecast of water levels by day, time and place. You can get them from the Hydrographic Department of the Royal Thai Navy. Pick the station closest to where you plan to dive.

Each station's table lists the times and heights of high and low water for every day. Scanning a whole month of numbers is tiring, but if you know the range is smallest around the half moon, you can jump straight to those days.

Knowing the day helps you choose when to dive. Knowing the time helps you choose when to get in the water, because even on spring tide days there's a brief slack period at high and low water when the current pauses.

Near river mouths, a rising tide is usually better than a falling one. As the tide goes out, river water carries sediment into the sea and the water turns murky.

Apps Help, for the Big Picture

Tide apps are easy to use because they show a graph: steep curves on spring tide days, flat ones on neap tide days. Some also show a bar chart comparing each day's range, which makes picking a day very easy.

The catch is that most apps are foreign, rely on computer models, and Thailand has few stations. We suggest using an app to spot the trend and pick your day, then checking the Hydrographic Department's tide table for the detail.

At Koh Talu

Koh Talu sits in the Gulf of Thailand, so the tides are part of how we plan every day's activities. At certain times of year the morning tide drops so low that we can't take the sailboat out, while calm neap-tide days are our dive team's favourite.

If you're planning a dive trip, check whether your dates fall near a half moon, and message our Dive Center team. We're happy to look at the conditions with you before you travel.

Illustrations: WonderLens Studio