Spring and neap tides: how lunar phases affect tidal range

Spring tides and neap tides describe two phases of the recurring lunar cycle of the tides. During a spring tide, the average difference between high and low water is somewhat greater; during a neap tide, it is more moderate. “Tidal range” refers to the span between the two water levels, not the absolute level of the sea.

The Sun, the Moon and spring tides

At new and full moon, the Sun, Earth and Moon are approximately aligned. The gravitational forces of the Sun and Moon then act together in a similar direction. Average tidal ranges are somewhat greater during these phases: high water is a little higher and low water a little lower than average. This phase is called a spring tide. The term therefore describes a greater height difference over the course of the tide, not simply a single especially high tide.

Spring tides occur at new and full moon, but their influence is described in terms of the average tidal range. The term alone therefore indicates neither an exact high-water level nor a specific time for a particular port. For an explanation of terms relating to water levels and movement, see the article Tides and tidal currents.

Neap tides at half moon

About seven days after a spring tide, the Sun and Moon are approximately at right angles to one another as seen from Earth. Their solar and lunar effects then reinforce each other less, producing more moderate tides. This phase is called a neap tide and coincides with the first or third quarter of the Moon. On average, high and low water are therefore closer together than during a spring tide.

Spring and neap tides are therefore not two different kinds of water level. They refer to phases in which the average tidal range is greater or smaller. For more context on water levels and coastal influences, separate articles also cover storm surges, wind setup and tides as well as chart depth and datum.

Why location still matters

The shape and depth of a sea basin affect how tides develop in a given location. The North Sea largely oscillates in time with the adjoining oceans. Local water-level observations capture conditions at a port more accurately than the lunar phase alone. Wind and air pressure also play a part: they can cause additional currents and changes in water level. Such short-term weather influences are not included in long-term astronomical tide predictions. Actual high and low waters may therefore differ from these predictions both in timing and in height.