Glossary

Spring tide

A spring tide is a higher-than-average tide. The highs go higher, the lows go lower, the swing between them is the biggest of the fortnight. The name has nothing to do with the season — it comes from the old English sense of "spring" meaning to leap or surge. Spring tides happen twice a month, year-round, around new and full moons.

The mechanism is gravitational. The moon's pull dominates the ocean tide; the sun's pull adds about 46 percent more. Around new moon the sun and moon are on the same side of Earth; around full moon they are on opposite sides. In both alignments the two pulls add together, and the resulting tide is bigger. Around the quarter moons the two pulls work at right angles and partly cancel — that's a neap tide.

How big the spring boost is depends on the coast. At San Diego the mean range of about 1.7 m climbs to roughly 2.1 m at spring; at Newquay in Cornwall a 4.5 m mean range stretches close to 7 m on the largest springs. The biggest spring tides of the year are the perigean springs, when the moon is near closest approach to Earth at the same time as new or full phase — typically in early spring and early autumn. For the underlying method behind these predictions, see the methodology page.