What's a Diurnal Tide?
A diurnal tide means one tidal cycle - one high tide, one low tide - within a lunar day (about 24 hours and 50 minutes). Around the globe, diurnal tides are quite rare. More commonly, coastlines experience semidiurnal tides (two highs and two lows) or mixed tides (also two highs and two lows, but with differing heights).
It's also an interesting word, too. The word 'diurnal' comes from the Latin diurnus, meaning 'of the day', which itself stems from dies, or 'day.' It neatly describes a tide cycle that repeats just once within a day.
The Science Behind It: Why Do Diurnal Tides Happen?
It all comes down to how ocean basins interact with gravitational forces. If the Earth were a perfect, water-covered sphere with no continents, tides would be neat and uniform; two highs and two lows everywhere. But continents block those tidal bulges and set up unique wave patterns in each ocean basin. This all influences diurnal tides.
Some regions line up in such a way - thanks to the bathymetry (that's the study of underwater depth, mapping the shape and features of the seafloor to you and me), coastline shape, and ocean basin resonance, which means that only one bulge (a high tide) passes each lunar day. That explains why places like the Gulf of Mexico, parts of southern Alaska, and segments of Southeast Asia experience diurnal tides.
Diurnal vs. Mixed vs. Semidiurnal Quick Comparison
Now that we know about diurnal tides, let's also explore mixed and semidiurnal tides, where they're found and what they each mean. The NOAA provides us with a simple explanation, but I'll go into a little more detail.
| Tide pattern | Highs/day | Lows/day | Common regions |
|---|---|---|---|
| Semidiurnal | 2 equal | 2 equal | East Coast USA, UK, Europe |
| Mixed | 2 unequal | 2 unequal | West Coast USA, parts of Oceania |
| Diurnal | 1 | 1 | Gulf of Mexico, parts of Asia, Alaska |
The graphs below show the pattern for each, with the peaks representing high tide and the lows representing low tide.
Where to see diurnal tides today
As mentioned, the regions you'll see diurnal tides include Alaska, the Gulf of Mexico, and parts of Asia. Specifically:
Gulf of Mexico
Alaska
- 🇺🇸 Pyrite Point, Bristol Bay, Alaska, United States
- 🇺🇸 Middleton Island, North End, Alaska, United States
South Asia
Quick questions and their answers
What is a diurnal tide?
A diurnal tide is a pattern with one high and one low tide in each lunar day (~24h 50m).
Can a place switch between diurnal and semidiurnal tides?
Yes! tidal patterns can shift slightly across the lunar month, including times where minor secondary tides appear or vanish, depending on local hydrodynamics.
Why do some places only have one tide a day?
Because of the ocean-basin shape, coastal geometry and resonance that favour a single tidal bulge passing once per lunar day.
Where do diurnal tides occur?
They're most common around parts of the Gulf of Mexico, sections of southern Alaska, and pockets of Southeast Asia.
Why does the Gulf of Mexico have one tide?
Its semi-enclosed basin and amphidromic system set up a resonance that favours a single daily tidal maximum/minimum.
How long is it between tides in a diurnal area?
There's roughly 12h 25m between the day's high and low, and ~24h 50m between consecutive highs (or lows).
Are diurnal tides more dangerous?
They're not inherently dangerous, but the single window can catch people out. Always check the nearest station's predictions before planning.
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