By Paul M. J. Suchecki – Updated Mar 24, 2022
Tides are the predictable rise and fall of ocean water levels driven by the gravitational forces of the moon and the sun. While large lakes such as the Great Lakes also experience minor tidal variations, this article focuses on the pronounced tidal cycles found in the world’s oceans.
The moon exerts a stronger gravitational pull on the side of Earth facing it, creating a bulge of water that results in a high tide. On the opposite side, a secondary bulge forms, though it is smaller. Because water has significant inertia, the highest tide lags behind the moon’s position by roughly a quarter of the daily cycle—about an hour or more after the moon sets at a given location. This lag explains why the highest tide does not coincide with the moon being directly overhead.
In addition to the near-side bulge, the far side of Earth experiences its own, smaller high tide, while the remaining points on the globe experience low tides. Due to Earth’s rotation, most places record two high and two low tides each day.
Tides peak twice each month when the moon and sun align. These are known as spring tides, which exhibit the greatest difference between high and low water. Spring tides occur after every full and new moon, when the sun’s pull on Earth is in line with the moon’s gravitational pull.
The smallest tidal range, called a neap tide, occurs when the moon and sun are positioned at right angles relative to Earth. Neap tides happen during the first and third quarters of the lunar month. Spring tides reach their maximum variation around the equinoxes—approximately March 21 and September 21—when day and night are of equal length worldwide.
An ebb tide describes the period when sea level decreases over several hours. The point where the tide changes direction is called slack tide or slack water. The interval between slack and high tide is referred to as flood tide.
Local geography can significantly alter tidal patterns. For instance, Panama City, Florida, experiences only one high and one low tide each day. In most of the world, the interval between high and low tides is roughly 12 hours 25 minutes, causing the high tides to shift forward by about an hour each day.
One of the most dramatic tidal phenomena occurs in the Bay of Fundy between New Brunswick and Nova Scotia. Here, the high tide produces a rapid “tidal bore” that travels up river against the prevailing current, a result of the tide funneling into a shallow, narrow inlet from a wide bay. The Bay of Fundy records some of the largest tidal ranges globally.
If you’re boating, surfing, or swimming, consult a local tide table to stay informed about the timing and magnitude of tides.
