Reference
OBX Tide Glossary
Plain-language definitions of the tide and ocean terms you’ll see on tide charts, NOAA forecasts, fishing reports, and weather alerts for the Outer Banks.
- Apogee
- The point in the moon’s orbit when it is farthest from Earth. Tidal forces are weakest at apogee, producing smaller-than-average tide ranges.
- Astronomical tide
- The portion of the tide caused purely by the gravitational pull of the moon and sun. This is what NOAA tide tables predict — it does not include effects of wind, pressure, or storms.
- Bathymetry
- The shape and depth of the seafloor. Bathymetry shapes how the tidal wave moves across the continental shelf and through OBX inlets.
- Constituent (tidal constituent)
- One of the individual harmonic components that, added together, produce the predicted tide curve. The biggest one — M2, the principal lunar semi-diurnal constituent — has a 12.42-hour period and dominates OBX tides.
- Current (tidal current)
- The horizontal movement of water associated with the rise and fall of the tide. Strongest in inlets like Oregon Inlet and Hatteras Inlet, where peak ebb and flood currents reach 3-5 knots.
- Datum (tidal datum)
- A reference water level. Common datums on OBX charts include MLLW (Mean Lower Low Water — the average of the lower of each day’s two low tides) and MHW (Mean High Water). Tide chart heights are measured relative to a datum.
- Diurnal tide
- One high and one low tide per day. Not the OBX pattern — we’re semi-diurnal.
- Ebb tide
- The outgoing tide, when water is flowing from high to low. On the OBX oceanfront, ebb tide drains the beach and lengthens the surf zone.
- Estuary
- A partially enclosed body of water where freshwater rivers meet the ocean. Pamlico Sound is the second-largest estuary in the U.S.
- Flood tide
- The incoming tide, when water is flowing from low to high. Fish often move shoreward on the flood, making it a productive surf-fishing window.
- Fetch
- The distance of open water over which wind blows. Pamlico Sound’s long fetch lets even moderate winds build a meaningful wind-driven tide.
- Gulf Stream
- The warm, fast ocean current that passes closer to Cape Hatteras than to any other point on the U.S. East Coast. Influences offshore sea level, water temperature, and fishing patterns.
- High tide
- The maximum water level reached during a tidal cycle. The OBX oceanfront sees two high tides per day, separated by about 12 hours and 25 minutes.
- Harmonic prediction
- The method NOAA uses to forecast tides — summing dozens of sine-wave constituents derived from years of measured water levels at each station.
- Inverse barometer effect
- Sea level rises about 1 cm for every 1 mb drop in atmospheric pressure. A deep low-pressure system can add a foot to OBX water levels just from the pressure drop.
- Low tide
- The minimum water level during a tidal cycle. Best time for shelling and beachcombing.
- M2
- The principal lunar semi-diurnal tidal constituent, with a period of 12.42 hours. The dominant signal in every OBX tide chart.
- Mean range
- The average difference between Mean High Water and Mean Low Water at a tide station. About 3.4 ft at Duck, 1.6 ft at USCG Hatteras.
- Mean Lower Low Water (MLLW)
- The reference datum used on U.S. nautical charts — the average of the lower of each day’s two low tides, averaged over 19 years. Tide heights on charts are typically given above MLLW.
- Mixed tide
- Two highs and two lows per day, but with noticeably different heights. The OBX experiences mild mixed semi-diurnal tides — the second high of the day is usually a few inches different from the first.
- Neap tide
- The smaller-range tides occurring at first and third quarter moons, when the sun and moon pull at right angles to each other.
- Nor’easter
- An extratropical storm forming offshore between October and April, with strong northeast winds and onshore push. Can hold OBX water levels 2-4 ft above normal for several days.
- Perigee
- The point in the moon’s orbit when it is closest to Earth. Tidal forces are about 20% stronger at perigee.
- Perigean spring tide / King tide
- When a new or full moon coincides with the moon’s perigee, producing the largest tides of the year. Often causes nuisance flooding on NC-12 even without storms.
- Semi-diurnal tide
- Two highs and two lows per day, roughly equal in size. The OBX oceanfront pattern.
- Slack tide
- The brief period of minimal current at the turn between flood and ebb. The safest window for boats transiting Oregon Inlet or Hatteras Inlet.
- Spring tide
- Larger-than-average tides occurring around new and full moons, when the sun and moon align gravitationally. Nothing to do with the season “spring.”
- Storm surge
- The abnormal rise in seawater driven entirely by a storm (wind setup + pressure drop). Distinct from “storm tide,” which is surge + the regular astronomical tide combined.
- Tidal range
- The vertical distance between high tide and the following low tide.
- Tidal prism
- The total volume of water that moves into or out of an inlet during one tide cycle. Oregon Inlet’s tidal prism drives those famously strong inlet currents.
- Wind setup
- The tilt of a water surface caused by sustained wind. The dominant non-astronomical effect on Pamlico Sound water levels.