TIDAL GLOSSARY

APOGEAN TIDE: A monthly tide of decreased range that occurs when the Moon is at apogee (farthest from Earth).

CURRENT: Generally, a horizontal movement of water. Currents may be classified as tidal and nontidal. Tidal currents are caused by gravitational interactions between the Sun, Moon, and Earth and are part of the same general movement of the sea that is manifested in the vertical rise and fall, called tide. Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability.

DIURNAL TIDE: A tide with one high water and one low water in a tidal day of approximately 24 hours.

MEAN LOWER LOW WATER: The arithmetic mean of the lesser of a daily pair of low waters, observed over a specific 19year cycle called the National Tidal Datum Epoch.

NEAP TIDE: A tide of decreased range that occurs twice a month, when the Moon is in quadrature (during its first and last quarters, when the Sun and the Moon are at right angles to each other relative to Earth).

PERIGEAN TIDE: A monthly tide of increased range that occurs when the Moon is at perigee (closest to Earth).

RED TIDE: Toxic algal blooms caused by several genera of dinoflagellates that usually turn the sea red or brown. These pose a serious threat to marine life and may be harmful to humans.

RIP CURRENT: A potentially dangerous, narrow, intense, surf-zone current flowing outward from shore.

SEMIDIURNAL TIDE: A tide with one high water and one low water every half-day. East Coast tides, for example, are semidiurnal, with two highs and two lows during a tidal day of approximately 24 hours.

SLACK WATER (SLACK): The state of a tidal current when its speed is near zero, especially the moment when a reversing current changes direction and its speed is zero.

SPRING TIDE: A tide of increased range that occurs at times of syzygy each month. Named not for the season of spring but from the German springen ("to leap up"), a spring tide also brings a lower low water.

STORM SURGE: The local change in the elevation of the ocean along a shore due to a storm, measured by subtracting the astronomic tidal elevation from the total elevation. It typically has a duration of a few hours and is potentially catastrophic, especially on low-lying coasts with gently sloping offshore topography.

SYZYGY: The nearly straight-line configuration that occurs twice a month, when the Sun and the Moon are in conjunction (on the same side of Earth, at the new Moon) and when they are in opposition (on opposite sides of Earth, at the full Moon). In both cases, the gravitational effects of the Sun and the Moon reinforce each other, and tidal range is increased.

TIDAL BORE: A tide-induced wave that propagates up a relatively shallow and sloping estuary or river with a steep wave front.

TSUNAMI: Commonly called a tidal wave, a tsunami is a series of long-period waves caused by an underwater earthquake or volcanic eruption. In open ocean, the waves are small and travel at high speed; as they near shore, some may build to more than 30 feet high, becoming a threat to life and property.

VANISHING TIDE: A mixed tide of considerable inequality in the two highs and two lows, so that the lower high (or higher low) may appear to vanish. ■