Types of tides: semidiurnal, diurnal and mixed

Three tide patterns

Look at a day’s tide curve on different coasts around the world and you’ll see three basic shapes:

• Semidiurnal: two high tides and two low tides of roughly the same height, a little over six hours apart. The curve looks like the same wave repeated twice a day.
• Mixed: also two highs and two lows, but of clearly different heights. One high tide is big and the other falls short; one low tide drains the flats and the other barely drops.
• Diurnal: a single high tide and a single low tide in each lunar day of about 24 hours and 50 minutes. The curve is one long, slow wave.

All three keep the same lunar clock we explain in what causes tides. What differs is which part of the tide wins out at each place: the part that repeats every 12 hours and 25 minutes, or the part that repeats every 24 hours and 50 minutes.

Semidiurnal tides: two even tides a day

This is the tide most East Coast anglers grew up with. From Maine to Florida, the water rises and falls twice a day on a very regular schedule. Europe’s Atlantic coast works the same way, from Norway and Brittany down to northern Spain and Portugal.

A typical day in Charleston, South Carolina, might look like this (rounded example):

Time	Tide	Height
5:40 a.m.	High	5.6 ft
11:55 a.m.	Low	0.2 ft
6:05 p.m.	High	5.9 ft
12:20 a.m.	Low	0.1 ft

The two highs and the two lows are close to each other. There are small differences, but they don’t change how you plan the day: if the morning low works for wading the flats, the evening low will work almost as well.

Mixed tides: two a day, but uneven

Along the Pacific coast, from San Diego to Puget Sound, and in Hawaii, there are also two highs and two lows a day, but they don’t match. That’s why West Coast tide tables use four names:

• higher high water and lower high water;
• higher low water and lower low water.

Going by NOAA’s averages for Seattle, a typical day brings a higher high of about 11 feet, a lower low near or below zero, a lower high around 9.5 feet and a higher low near 6 feet. The two lows are several feet apart: one exposes entire tide flats, while the other hardly uncovers anything.

That “lower low” matters well beyond fishing. NOAA’s charts and tide predictions on US coasts measure heights from the average lower low water, called MLLW, which we explain in our guide to chart datum and MLLW.

Diurnal tides: one tide a day

In much of the Gulf of Mexico, including Pensacola, Florida, and Mobile, Alabama, and in parts of Southeast Asia, the water rises for about twelve hours and falls for about twelve more. You get one high and one low each day, and the next ones arrive about 50 minutes later.

These tides tend to be small. In Pensacola, even on the biggest days, the difference between high and low is only about 2 feet. They also have a quirk: when the moon crosses the equator, the daily tide nearly disappears. The table may show two small rises and falls, or water that hardly moves at all. On a coast like that, the wind often moves the water more than the tide does.

Why each coast has its own rhythm

Two reasons work together.

The first is the tilt of the moon. The moon doesn’t stay over the equator. Over a month, it swings north of the equator, then south, then back. Astronomers call that angle its declination. When the moon is north of the equator, the two bulges of water are tilted: the one facing the moon shifts toward the Northern Hemisphere and the opposite one toward the Southern Hemisphere. A harbor in the Northern Hemisphere passes through the thick part of one bulge and the thin part of the other, so its two daily high tides come out uneven.

That difference, called the diurnal inequality, is largest when the moon is farthest from the equator, twice a month (tropic tides). It almost vanishes when the moon crosses the equator (equatorial tides). The sun does the same thing over the year, with less strength.

The second reason is the shape of each body of water. A real tide can be broken down into many simple waves, each with its own rhythm. The most important are:

• M2, the moon’s 12-hour, 25-minute wave, and S2, the sun’s 12-hour wave, which make up the semidiurnal part;
• K1 and O1, with periods of about 24 and 26 hours, which make up the diurnal part and come mainly from the moon’s and sun’s declination.

Each ocean basin amplifies some of these waves and dampens others, depending on its size and depth, much like a room that makes some musical notes ring louder than others. The North Atlantic strongly amplifies the semidiurnal waves. In the Gulf of Mexico, the semidiurnal waves shrink to almost nothing and the diurnal ones take over.

The form factor: how to tell your tide type

Oceanographers sum all this up in a single number, the form factor (F). You get it by dividing the combined size of the two main daily waves by that of the two main twice-daily waves:

F = (K1 + O1) / (M2 + S2)

Each letter stands for that wave’s amplitude, meaning how much it raises and lowers the water at a given station. Then:

Form factor	Tide type
Under 0.25	Semidiurnal
0.25 to 1.5	Mixed, mainly semidiurnal
1.5 to 3	Mixed, mainly diurnal
Over 3	Diurnal

A rough example: on most of the East Coast, the semidiurnal waves are many times larger than the diurnal ones, and F lands under 0.25, a clear semidiurnal tide. In Pensacola, it’s the other way around: the diurnal waves are several times bigger than the semidiurnal ones, and F is above 3. Along the Pacific coast, F usually falls between 0.25 and 1.5.

You don’t need to work it out before heading out, but it explains why your local tide chart looks the way it does.

What it changes when you plan a trip

Knowing your coast’s tide type saves you surprises:

• With a semidiurnal tide, the day’s two lows are similar. If the morning low is good for wading or clamming, the evening one will be too. What changes is the strength over the month, with spring and neap tides.
• With a mixed tide, always find the lower low water. Only one of the two lows exposes the rocks and sandbars. On the West Coast, the minus tides that clam diggers and tide poolers wait for are lower lows.
• With a diurnal tide, you get one rise and one fall a day. The biggest tides don’t come with the full or new moon but when the moon is farthest north or south, and around the days it crosses the equator the tide nearly stalls.

Either way, the shape of the curve and the height of each high and low are in the tide chart for your spot. To get the most out of it, read how to read a tide chart, and if you wonder why the Mediterranean and the Gulf barely move, see why some seas have almost no tide.