Tides on the Texas coast, explained

If you fish the bays, jetties or flats, the tide decides a lot: where the water is deep enough, where it is moving, and when it is not. A tide table gives you times and heights, but a table does not explain itself. This guide explains what is behind the numbers, so you can read a table, notice when the water is not following it and know what to look at before you go.

What a tide is

NOAA calls tides very long-period waves that move through the ocean in response to the forces exerted by the moon and sun. They begin in the ocean and travel toward the coast, where they show up as the regular rise and fall of the sea surface. When the crest of the wave reaches a place, it is high tide, and when the trough arrives, it is low tide. The difference in height between a high and a low is the tidal range.

The tide day is longer than your day

The tides follow the moon, not the clock. A lunar day, the time for a spot on the Earth to rotate from directly under the moon to directly under it again, is 24 hours and 50 minutes, which is 50 minutes longer than a solar day. The reason, NOAA explains, is that the moon moves around the Earth in the same direction the Earth rotates, so the Earth takes an extra 50 minutes to catch up. Where there are two high tides a day, they come 12 hours and 25 minutes apart, and the water takes a little over six hours to go from high to low or low to high. It also means the tides shift later by roughly 50 minutes every day.

One tide or two a day: the Gulf is different

NOAA describes three basic patterns. In a semidiurnal pattern there are two highs and two lows a day, about equal in height. In a mixed semidiurnal pattern there are two highs and two lows of different heights. In a diurnal pattern there is only one high and one low a day. NOAA says that many areas of the Gulf of Mexico have diurnal tides, so people who learned their tides on the East or West Coast can be surprised on the Texas coast. Pick a pattern below to see the day, and slide the days forward to watch the tide move later.

An interactive drawing goes here in the app.

NOAA says many areas in the Gulf of Mexico have one high and one low a day. That is not a promise for every Texas station, so look at the predictions for the station nearest where you fish.

Spring tides and neap tides

The sun matters too, though its pull is about half as big as the moon's. At the new moon and the full moon, the sun, the Earth and the moon line up and the sun's pull adds to the moon's, so the high tides are a little higher and the low tides a little lower than average. These are called spring tides, a very old term from the idea of the tide springing forth, with nothing to do with the season. About a week later, at the quarter moons, the sun and moon pull at right angles and partly cancel each other, making the weaker neap tides. Each happens twice a month. NOAA adds that the moon's changing distance matters as well: once a month, when it is closest to the Earth, the tide-raising force is higher than usual.

An interactive drawing goes here in the app.

The shape of the coast matters

The size of a tide on any shore depends on the shape of the land. NOAA says wide continental margins can magnify tides, funnel-shaped bays can change them a lot, and narrow inlets and shallow water tend to dissipate incoming tides. Its example from our own coast: inland bays such as the Laguna Madre in Texas have areas classified as non-tidal even though they have ocean inlets. That is why a bay can show a much smaller tide than the gulf side of a pass.

Wind and pressure can overrule the table

A tide table is a prediction from the positions of the moon and sun. It does not know the weather. NOAA says strong offshore winds can move water away from the coastline and exaggerate low tide exposure, onshore winds can pile water onto the shore and virtually eliminate it, high-pressure systems can depress sea levels and lead to clear sunny days with exceptionally low tides, and low-pressure systems are typically associated with tides much higher than predicted. Strong wind, such as the wind that follows a front, can make the water noticeably lower or higher than the table says. Move the sliders to see.

An interactive drawing goes here in the app.

Flood, ebb and slack

The rise and fall of the water level is the tide. The movement of the water is the tidal current, and it is what you feel at a pass or a jetty. NOAA defines a flood current as the movement of a tidal current toward the shore or up a tidal river or estuary, and an ebb current as the movement away from shore or down a tidal river or estuary. In between is slack water, when the current is at a minimum. NOAA notes that recreational and commercial fishermen use their knowledge of tides and tidal currents to improve their catches: depending on the species and the water depth in an area, fish may concentrate during the ebb or the flood, and in some areas strong tidal currents concentrate bait and small fish, which attract larger fish.

An interactive drawing goes here in the app.

Heights are measured from a reference level

A tide height is always measured from a reference level, called a datum. NOAA's glossary defines mean lower low water (MLLW) as the average of the lower low water height of each tidal day over the National Tidal Datum Epoch, and says that since 1980 the chart datum has been implemented as mean lower low water for all marine waters of the United States. So the depths on a chart are measured from a low-water level, not from the water that is there when you arrive.

A quick tide check

An interactive drawing goes here in the app.

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