What does high tide and low tide mean?
High tide is the point in the tidal cycle when the water level at a station reaches its peak; low tide is the point when it falls to its lowest. The cycle is driven by gravitational interactions among the sun, moon and earth, and NOAA predicts its timing and height for stations with harmonic constituents.
Tides are the periodic rise and fall of a body of water resulting from gravitational interactions among the sun, the moon and the earth. High tide is simply the top of that rise: the moment in the cycle when the water level at a given spot is at its highest before it starts dropping again. Low tide is the bottom of the cycle, when the water level is at its lowest before rising once more. Between those two points, the water is either flooding in or ebbing out, and the horizontal movement of water that accompanies the vertical rise and fall is what NOAA describes as the tidal current.
[1]Where the numbers come from
When you look up a tide prediction, you are not reading a live measurement. NOAA describes its tide predictions as a calculation of what the water level will be, based on harmonic constituents. Each constituent represents a periodic change in the relative positions of the earth, moon and sun, and together they produce the predicted times and heights of the highs and lows. Predictions are offered at six-minute and hourly intervals for stations with harmonic constituents, along with high and low data for stations in NOAA's published tide and current tables.
[1]What the height figure actually means
A predicted height is meaningless on its own. Water level, as NOAA defines it, is the height of the water relative to a datum, which is a base elevation used as a reference for reckoning heights or depths. NOAA's station map includes a water level display option referenced to Mean Lower Low Water, or MLLW, so a figure you see there is height above that station's reference datum, not a depth of water across the whole shore. A high tide of six feet does not mean six feet of water everywhere on the beach; it means six feet above the datum at that location.
[1][3]One caution worth knowing: tidal datums are local. NOAA states plainly that tidal datums should not be extended into areas with differing hydrographic characteristics without substantiating measurements. In practice, that means the numbers for one harbour do not transfer to the next bay along, even if they look close together on a map. Always check the specific station you care about.
[1]A worked example of reading a prediction
Here is an invented example to show how the pieces fit together. Suppose you are planning a morning walk along a shore and you see a listing for a nearby station: low water at 6:12 am at 0.3 feet, high water at 12:38 pm at 5.9 feet. The first figure tells you the water will be near its lowest for that cycle, measured against the station's datum, so flats and sand that are normally covered may be exposed. The second tells you the water will be close to six feet above the datum by early afternoon, so the same flats will be well underwater. The exact times and heights are specific to that station, and a station a few miles away could show noticeably different figures.
[1][2]Prediction versus what actually happens
A prediction is an astronomical calculation, and actual water levels can differ from it. NOAA notes that at Great Lakes stations, for example, changes in water level are usually due to weather events rather than the gravitational tide. On the coast, weather can also push water higher or lower than the predicted curve, and NOAA provides real-time water level observations, meteorological readings and a Coastal Inundation Dashboard showing real-time and historical coastal flood information for checking conditions as they unfold.
[1]A practical next step
Find the station nearest your stretch of coast by searching on the NOAA tide predictions page or the Tides and Currents map, using a station name, station ID or latitude and longitude. Read the high and low times and heights for that station specifically, note which datum the heights are reckoned from, and if your plans depend on conditions on the day, compare the prediction with the station's latest observed water level before you set out.
[1][2][3]Sources and dates
- https://tidesandcurrents.noaa.gov/products.html · accessed 2026-10-18
- https://tidesandcurrents.noaa.gov/tide_predictions.html · accessed 2026-10-18
- https://tidesandcurrents.noaa.gov/map/ · accessed 2026-10-18