How Tides Work: Spring Tides, Neap Tides, and Why Hong Kong Gets Two Highs a Day

Tides feel chaotic the first time you plan a fishing trip or a coastal photo walk around them. They are not. Everything about them follows from a few simple ideas: two astronomical bulges, a coastline that amplifies or damps them, and a roughly two-week cycle between strong and weak tides. Once you see the pattern, a tide table stops being a wall of numbers and becomes a schedule you can plan around.

The engine: gravity and two daily bulges

The moon's gravity pulls hardest on the ocean nearest it and weakest on the ocean farthest away, so water piles into two bulges — one facing the moon, one on the opposite side of the Earth. As the Earth rotates beneath this pair of bulges, most places sweep past a bulge roughly twice a day. That is why "semi-diurnal" places like Hong Kong get two highs and two lows in about 24 hours and 50 minutes — the extra 50 minutes is the moon creeping along its own orbit, which is why each day's tides arrive about 50 minutes later than the previous day's.

The sun creates its own, weaker pair of bulges. When sun, moon and Earth line up — at new moon and full moon — the two forces add together and you get spring tides: higher highs, lower lows, bigger total range. When the sun and moon pull at right angles to each other — at the quarter moons — they partly cancel and you get neap tides: smaller range, less water movement. The strongest spring tides lag the moon phase by a day or two because the ocean takes time to respond.

Why Hong Kong is semi-diurnal (and mixed)

Hong Kong sits where the Pacific's semi-diurnal tide wave enters the South China Sea and works its way up the Pearl River estuary. The result at Quarry Bay is a classic pattern: two highs and two lows most days, with the two daily highs often at noticeably different heights — a "mixed" flavour of semi-diurnal. Sheltered inlets amplify the swing. Tai Po Kau, inside Tolo Harbour, shows a large spring range, while Tai O on the northwest coast sees the territory's biggest swings as estuary water funnels past Lantau.

Why Taiwan's tides change from port to port

Taiwan straddles two tide regimes. The west coast faces the shallow Taiwan Strait, where the tidal wave slows, stacks up and amplifies — so Taichung Port records ranges that dwarf most of the island, and the tide there leans toward one dominant high and low per day at neaps. The east coast drops into deep water almost immediately, so the tide arrives smaller and more irregular: Hualien and Keelung show mixed patterns with modest ranges. Kaohsiung in the southwest falls in between. This station-to-station variety is exactly why a national tide answer is useless — you need the table for your actual harbour.

Spring or neap? Which one you want depends on the game

Anglers often prefer spring tides: more range means stronger currents that stir up food and trigger feeding — our shore-fishing tide phases guide explains the reasoning and its limits. Photographers chasing exposed reefs and tide pools usually want a big spring low around sunrise or sunset — pair that with our golden-hour × low-tide planning guide. Beach walkers should be extra careful on springs: the water that retreated a long way comes back fast, which is the scenario in our rising-tide safety guide.

Reading the cycle in a real table

Pick any of our location tables — say Quarry Bay — and scan the high-water heights across two weeks. You will watch them grow to a peak (spring), shrink to a minimum about seven or eight days later (neap), then rebuild. If you know today's moon phase, you can predict whether the coming week trends bigger or smaller. Our guide to reading a tide table shows exactly which columns to compare.

Related reading

• How to read a tide table (times, heights, chart datum)
• Best tide phases for shore fishing
• Sun timing explained: golden hour, blue hour, solar noon