Glossary

Neap tide

A neap tide is a lower-than-average tide. The highs are smaller than usual, the lows shallower, the daily swing between them is the smallest of the fortnight. Neaps happen around the quarter moons — first quarter and third quarter — twice a month, year-round.

The reason is geometric. At the quarter moons the moon and sun sit at right angles relative to Earth. The moon's tidal pull is still bigger than the sun's, but the two are now working across each other rather than along the same axis. The resulting tide is the moon's pull minus part of the sun's, instead of the moon plus the sun. The big ranges of the new- and full-moon spring tides give way to the smaller ranges of neap.

Practically: at most coasts the range on a neap is roughly 70 to 80 percent of mean. At Boston the 2.9 m mean range compresses toward 2.4 m on neaps; at the Tagus mouth in Lisbon the 2.4 m mean compresses to about 1.5 m. For paddlers, anglers, and beach-walkers the practical effect of a neap is a less dramatic day at the water — less exposed flat at low, less inundation at high, calmer-feeling water on the change. For tidepool hunters and shellfish gatherers a neap is a quiet day; come back at the next spring. The forecasts on TideTurtle pages capture both. See the methodology for the source-by-source picture.