High tide is the stage of the tidal cycle when the sea reaches a local high water level, while low tide is when it reaches a local low water level. Between them, the water level gradually rises or falls. The vertical difference between high and low tide is known as the tidal range.
What is high tide?
High tide occurs when the tidal cycle reaches a local maximum water level. At the coast, this means more of the beach, rocks, reefs or other shoreline features may be covered by water than at lower stages of the tide.
High tide does not mean the ocean reaches one universal height. The actual water level depends on the location, the astronomical tide and other influences on local water level.
What is low tide?
Low tide occurs when the tidal cycle reaches a local minimum water level. As the tide falls, parts of the shoreline that were underwater can become exposed.
Depending on the coast, low tide can reveal sandbanks, rock platforms, reefs, tidal pools and large areas of intertidal habitat. The intertidal zone is the coastal area that lies between the levels reached by high and low tides.
What is the difference between high tide and low tide?
The fundamental difference is water level. High tide represents a local maximum in the tidal cycle, while low tide represents a local minimum.
The difference can change much more than the position of the waterline. Water depth over sandbanks, reefs, rocks, harbour entrances and other coastal features changes as the tide rises and falls.
How dramatic that change appears depends strongly on the local tidal range and the shape of the coastline. A gently sloping beach can show a large horizontal movement of the waterline even when the vertical change in water level is relatively modest.
The complete astronomical process behind these changing water levels is explained in how tides work.
What happens between low tide and high tide?
After low tide, the water level generally begins rising toward the next high tide. This is commonly called a rising or incoming tide.
After high tide, the water level generally begins falling toward the next low tide. This is commonly called a falling or outgoing tide.
Horizontal tidal currents can accompany these changes. Water moving landward or into bays and estuaries is commonly described as a flood current, while water moving seaward is described as an ebb current.
The timing of flood, ebb and slack water does not necessarily correspond exactly with the timing of high and low tide. Their relationship varies between locations, so water level and tidal-current predictions should be treated as related but distinct information.
What is a rising tide?
A rising tide is the part of the tidal cycle during which water level increases from low tide toward high tide. It is also commonly described as an incoming tide.
The rising water can progressively cover beaches, rocks, sandbanks and other coastal features. The speed and pattern of this change depend on the location and the shape of its tidal cycle.
What is a falling tide?
A falling tide is the part of the cycle during which water level decreases from high tide toward low tide. It is also commonly called an outgoing tide.
As water levels fall, more of the intertidal zone becomes exposed. Tidal currents associated with the falling stage may flow seaward in bays, estuaries and other constrained coastal environments, although the exact relationship between water level and current varies by location.
How long is it between high tide and low tide?
There is no universal interval that applies to every coastline. Many locations experience a semidiurnal pattern with two high tides and two low tides during a lunar day of approximately 24 hours and 50 minutes. In a simplified semidiurnal cycle, a high tide and the following low tide are therefore separated by roughly six hours.
Real tidal patterns vary. Some locations experience mixed tides in which successive highs and lows differ substantially, while others experience only one high and one low during a lunar day.
For a specific coast, use a local tide prediction rather than assuming a fixed six-hour interval.
Are there always two high tides and two low tides each day?
No. Two highs and two lows are common, but they are not universal.
Three broad tidal patterns occur around the world's coastlines:
- Semidiurnal: two high tides and two low tides of approximately similar height during a lunar day.
- Mixed: usually two highs and two lows, but their heights differ noticeably.
- Diurnal: one high tide and one low tide during a lunar day.
The pattern experienced at a location is influenced by the way astronomical tidal forcing interacts with ocean-basin geometry, water depth, coastlines and other regional factors.
Why are two high tides not always the same height?
Successive high tides can reach different levels because real tidal behaviour is more complex than a perfectly symmetrical rise and fall of the ocean.
The relative positions of the Moon and Sun matter, but so do ocean depth, basin geometry, coastlines and the way tidal waves propagate through each region. This can produce unequal successive high and low tides.
That is why a tide table may show a higher high tide followed later by a lower high tide at the same location.
What is tidal range?
Tidal range is the vertical difference in water level between high tide and low tide. A coast with a large tidal range experiences a much larger vertical change in sea level during the tidal cycle than a coast with a small tidal range.
Tidal range is not constant. It changes through the lunar cycle and can also differ significantly between locations because of coastal and ocean-basin geography.
For a closer look at why these differences can be small at one coastline and much larger at another, see what tidal range means.
Do high and low tides change during the lunar month?
Yes. The difference between high and low water changes as the relative positions of the Sun, Moon and Earth change.
Around new moon and full moon, their tidal effects combine to produce spring tides, which generally have a larger tidal range. Around the first and third quarter moon, neap tides generally produce a smaller tidal range.
This means that high tide and low tide are not fixed water levels. The height of successive tides changes throughout the lunar cycle as well as between locations.
See spring tide vs neap tide for the relationship between the lunar cycle and tidal range.
Does high tide mean the current is strongest?
Not necessarily. Tide height and tidal-current speed describe different things. High tide describes water level, while a tidal current describes horizontal movement of water.
Depending on the location, maximum flood or ebb current can occur at different stages relative to high and low water. Slack water, when horizontal tidal-current speed becomes very low, also does not universally occur exactly at high or low tide.
The relationship depends on how tidal waves and currents behave within the local coastline, bay, harbour, estuary or channel. For that reason, you should not infer current strength solely from the time of high or low tide.
Is high tide always at the same time?
No. High and low tides occur at different clock times from day to day. A lunar day is approximately 24 hours and 50 minutes, which contributes to the shifting timing of the tidal cycle.
Local geography also affects when the tidal wave reaches a particular coastline. High tide therefore does not occur everywhere simultaneously, nor does it simply occur whenever the Moon is directly overhead.
How do you read high and low tide on a tide table?
A tide table normally gives a predicted time and water level for each high and low tide at a specific location. The sequence shows when the water is expected to reach local highs and lows throughout the day.
Do not look only at the time. The predicted height is also important because successive high tides and low tides may not reach the same level.
Tide predictions are location-specific. A prediction for one harbour or beach should not automatically be treated as the exact tide for another part of the coast.
How do high and low tide affect surfing?
For surfing, the important effect of the tide is the changing depth of water above the seabed. As the water rises or falls, waves can interact differently with sandbars, reefs, rocks and other underwater features.
At a beach break, changing water depth can alter where waves interact with the sandbars and where they eventually break. At a reef break, lower water can reduce the depth above a fixed reef, while higher water can change how strongly incoming waves interact with that structure.
Point breaks can also respond strongly to changing water level because the takeoff, breaking sections, currents and access routes can all depend on tide.
There is therefore no universal rule that high tide is better than low tide for surfing. The useful tidal window depends on the individual break, its bathymetry, the incoming swell and the conditions on that day. Our guide to beach breaks, reef breaks and point breaks explains why different seabed and coastline structures respond differently to incoming waves.
Why do high and low tide matter at the coast?
The changing tide continuously changes the boundary between land and sea. Water depth changes over sandbanks, reefs and rocks, intertidal areas become covered and exposed, and tidal currents move water through bays, estuaries and channels.
For swimmers, surfers, sailors and other coastal users, this means the same location can behave differently at different stages of the tide. Understanding whether the tide is high, low, rising or falling gives more useful information than treating the coastline as a fixed environment.
For surfers, tide is only one part of the picture. Swell height, period and direction, wind and local bathymetry also matter. The next practical step is understanding how to read swell and surf forecasts and applying the predicted water level to the characteristics of the individual break.