What Are Tides?
Tides are the periodic rise and fall of sea levels caused by the gravitational pull of the Moon and, to a lesser extent, the Sun on Earth's oceans. The Moon's gravity creates a bulge of water on the side of Earth closest to it, and a corresponding bulge on the far side due to inertial forces. As Earth rotates through these bulges, coastal locations experience high and low tides.
The Moon orbits Earth roughly every 29.5 days (the lunar month). Its gravitational pull is the dominant driver of tides, but the Sun — despite being far larger — exerts only about 46% the tidal force of the Moon due to its greater distance.
Types of Tides: Diurnal, Semidiurnal, Mixed
The pattern of tides at any given location depends on local geography, the shape of ocean basins, and resonance effects. Three patterns exist:
- Semidiurnal: Two high tides and two low tides per day of roughly equal height. Most of the Atlantic coast experiences this. The tidal period is approximately 12 hours 25 minutes.
- Diurnal: One high tide and one low tide per day. Parts of the Gulf of Mexico, South China Sea, and certain Pacific coasts experience this pattern.
- Mixed semidiurnal: Two tides per day, but with significant inequality in height. Much of the US Pacific coast, Southeast Asia, and Australia falls in this category — successive high tides may differ by a metre or more.
Spring Tides vs. Neap Tides
The alignment of the Sun and Moon relative to Earth modulates the tidal range (the difference between high and low water) over the lunar cycle.
- Spring tides occur during new and full moons (approximately every 14 days), when the Earth, Moon, and Sun align. The gravitational forces of the Moon and Sun add together, producing the largest tidal ranges — the highest highs and lowest lows of the lunar cycle. "Spring" has nothing to do with the season — it derives from the Old English word meaning to leap or surge.
- Neap tides occur at first and third quarter moons, when the Moon and Sun are at right angles relative to Earth. Their gravitational forces partially cancel out, producing the smallest tidal ranges of the month.
The amplified water levels of spring tides are a critical planning factor for coastal communities. A storm arriving during a spring tide has a much higher starting water level to amplify, producing greater coastal flooding than the same storm during a neap tide.
Tidal Ranges Around the World
| Location | Approximate Tidal Range | Type |
|---|---|---|
| Bay of Fundy, Canada | 14–17 m (46–56 ft) | Semidiurnal — world's largest |
| UK/Bristol Channel | 10–14 m (33–46 ft) | Semidiurnal |
| Amazon River mouth, Brazil | 7–10 m (23–33 ft) | Semidiurnal |
| US Atlantic Coast (NY) | 1.5–2 m (5–7 ft) | Semidiurnal |
| Gulf of Mexico (New Orleans) | 0.3–0.5 m (1–1.6 ft) | Diurnal |
| Mediterranean Sea | 0.1–0.4 m (4–16 in) | Near-tideless (enclosed basin) |
| Sydney, Australia | 1.0–1.5 m (3–5 ft) | Mixed semidiurnal |
Storm Surge: When Tides Become Dangerous
Storm surge is the abnormal rise of water above predicted tidal levels, generated by a tropical storm or extratropical cyclone pushing water toward the coast. It is the most destructive aspect of tropical cyclones and the primary cause of storm-related coastal deaths. Hurricane Katrina's storm surge reached 8–9 metres (28 feet) along parts of the Mississippi coast in 2005. Hurricane Sandy's 4.2-metre surge in New York Harbour in 2012 remains the highest ever recorded there.
Storm surge is driven by three mechanisms: wind-driven water piling (sustained onshore winds push surface water toward the coast); low barometric pressure (each 1 hPa drop below normal raises sea level approximately 1 cm, or the "inverse barometer effect"); and wave action on top of the surge layer (adding further inundation).
⚠️ Storm surge + high tide = compound flood: When storm surge arrives at the same time as a spring high tide, the combined water level can exceed individual predictions by metres. This compound effect — known as "tidally resonant storm surge" — is why meteorologists urgently warn of storm timing relative to tidal cycles during hurricane forecasts. Never underestimate surge risk based on storm category alone.
Sea Level Rise and Changing Flood Risk
Global mean sea level has risen approximately 21–24 cm (8–9 inches) since 1880, with the rate accelerating in recent decades to about 3.6 mm/year (2006–2015) and further to approximately 4.0–4.4 mm/year in the 2010s–2020s. The causes are thermal expansion of ocean water as it warms, and accelerating ice loss from Greenland and Antarctic ice sheets.
Even small increases in mean sea level dramatically increase the frequency of what were previously rare extreme water level events. A flood that historically occurred once every 100 years at a particular location may now occur every 10–30 years due to sea level rise, and could be annual or more frequent by mid-century under high-emission scenarios.
Coastal Safety: Tides, Currents, and Cut-Offs
Understanding tidal cycles is essential for safe coastal recreation, particularly in areas with large tidal ranges:
- Tidal cut-offs: Causeway roads, island footpaths, and sandy spits that are accessible at low tide can become submerged or cut off within minutes on a rising tide. Always check tide tables before crossing tidal causeways. In the UK alone, dozens of people are stranded or drowned each year at tidal causeway crossings.
- Tidal rip currents: Outgoing tides through narrow channels or across sandbars create powerful seaward-flowing currents. If caught in a tidal rip, swim parallel to shore until out of the current, then swim back.
- Quicksand and mud: Large inter-tidal flats expose unstable sediments at low tide. Estuary mud can entrap people. Never walk on unfamiliar inter-tidal mud without local knowledge and never alone.
- Coastal cliff erosion: Cliffs at the high-tide line are actively undercut by wave action and are subject to sudden collapse, especially after storm events or prolonged rainfall.
Tidal Data Sources: NOAA CO-OPS and Beyond
NOAA's Center for Operational Oceanographic Products and Services (CO-OPS) operates the primary US network of tide gauges and publishes free, real-time water level data from more than 200 stations. Tide predictions are available years in advance based on astronomical harmonic analysis at tidesandcurrents.noaa.gov.
Global tide predictions are available through several free services: the UK National Tidal and Sea Level Facility (NTSLF), the European Copernicus Marine Service, and the open-source TPXO Global Tidal Model. For navigational purposes, national hydrographic offices publish official tide tables for their coastal waters.
How EarthIntelPro Monitors Tide Levels
EarthIntelPro integrates tide level data from Open-Meteo's marine API, which provides predicted tide heights at coastal locations worldwide based on global tidal models. Users can see current and forecast tidal conditions for their region alongside storm and flood risk indicators, allowing compound event risk to be assessed at a glance.
During tropical storm or extratropical cyclone events, the platform displays storm surge warnings issued by national meteorological agencies alongside tidal predictions, helping users understand the total expected water level and flood risk for their location.
🌊 Monitor Coastal Water Levels Live
Tidal conditions and storm surge risk — updated continuously during coastal hazard events.
Open Live Monitor →