Ocean Tides: High, Low, and the Forces That Drive Them

Ocean tides are the periodic rise and fall of sea level caused by gravitational forces of the Moon and Sun combined with Earth's rotation. There are two primary tidal phases:

The Two Main Tides

• High tide: Water level reaches its maximum height at a given location.
• Low tide: Water level reaches its minimum height.

Most coasts experience two high and two low tides daily (semidiurnal) — roughly every 12 hours 25 minutes. Some areas have one high/one low (diurnal) or mixed patterns.

What Causes Tides?

Lunar Gravity (Dominant — 68%)

The Moon's gravity pulls water toward it, creating a bulge on the near side. Inertia creates a second bulge on the far side. Earth rotates under these bulges.

Solar Gravity (32%)

The Sun's effect is 46% of the Moon's (greater mass but 390× farther). It modulates lunar tides.

Spring and Neap Tides

• Spring tides (max range): New and Full Moon — Sun, Moon, Earth aligned (syzygy). Forces add.
• Neap tides (min range): First and Third Quarter — Sun and Moon at 90°. Forces partially cancel.

Coastal Amplification

Tide range varies dramatically by coastline geometry:

• Funnel-shaped bays/estuaries: Bay of Fundy (16 m / 53 ft range), Bristol Channel, Ungava Bay
• Narrow inlets, continental shelves — resonance amplifies
• Open ocean: <1 m range
• Mediterranean, Baltic, Caribbean: Small (cm-level) — restricted connection to ocean

Ecological and Human Importance

• Intertidal zone: Unique ecosystems (barnacles, mussels, shorebirds) adapted to alternating immersion/emersion
• Navigation: Chart datum, clearance, grounding risk
• Energy: Tidal stream turbines, barrage systems (La Rance, Sihwa Lake)
• Culture: Fishing schedules, recreation, folklore