Tides
Ocean tides are the periodic rise and fall of sea levels caused primarily by the gravitational pull of the Moon and, to a lesser extent, the Sun. As Earth rotates, coastal locations experience a rhythmic cycle of flooding and ebbing waters that repeats roughly every 12.4 hours in most parts of the world.
High Tide
The moment when the sea reaches its maximum height at a given location, driven by the direct gravitational pull of the Moon overhead and by the corresponding bulge on Earth's opposite side. High tides typically occur twice per day, separated by approximately 12 hours 25 minutes. The water height is measured in metres above mean sea level.
Low Tide
The moment when the sea retreats to its minimum height, occurring in the trough between two high-tide bulges. Low tides follow each high tide by roughly 6 hours 12 minutes. In areas with a large tidal range, significant stretches of seabed, mudflats, or rocky shoreline are exposed at low tide.
Tidal Range
The vertical difference in water height between consecutive high and low tides, expressed in metres. Ranges vary enormously by geography — from under 0.3 m in semi-enclosed seas like the Mediterranean to over 16 m in the Bay of Fundy. Coastal shape, ocean basin resonance, and the lunar phase all influence the range on any given day.
Tidal Cycle
The repeating pattern of highs and lows at a location. Most coastal areas experience a semidiurnal cycle — two high tides and two low tides every ~24 hours 50 minutes, which is the time it takes the Moon to return to the same position in the sky. Some regions have a diurnal cycle (one high and one low per day) or a mixed cycle due to local basin geometry.
Spring Tides
Exceptionally large tides that occur near New Moon and Full Moon, when the Sun, Earth, and Moon are roughly aligned. The gravitational forces of the Sun and Moon reinforce each other, increasing the tidal range — highs are higher and lows are lower than average. Despite the name, spring tides occur twice per month, not only in the season of spring.
Neap Tides
Smaller-than-average tides occurring near First Quarter and Last Quarter Moon, when the Sun and Moon are at approximately 90° to each other as seen from Earth. Their gravitational forces partially cancel, reducing the tidal range — highs are lower and lows are higher than usual. Neap tides also occur twice per month.
Coastal Cities Only
Tide data is only meaningful for locations within tidal reach of the ocean. The Tides tab is therefore available only when the selected city is identified as coastal. Inland cities do not experience ocean tides and are excluded from this tab.
Tide Model
Predictions are computed using the EOT20 (Empirical Ocean Tide model 2020) — a global harmonic tide model developed by DGFI-TUM and distributed under a CC-BY 4.0 licence. EOT20 decomposes the ocean tide into harmonic constituents fitted to satellite altimetry data and provides coverage from 66°S to 66°N.