Complete Guide to Tide Times
Everything you need to know about how tides work, how to read tide charts, and how to use tide predictions safely — whether you're a sailor, angler, surfer, or simply planning a beach visit.
1. What Are Tides?
Tides are the periodic rise and fall of sea level at coastal locations around the world. They are one of the most predictable natural phenomena on Earth, driven by the gravitational forces of the Moon and the Sun acting on the world's oceans. Unlike waves, which are caused by wind, tides are slow, large-scale movements of water that affect entire coastlines and ocean basins.
At high tide, the sea is at its highest level, often covering beaches, rock pools, and the lower sections of harbour walls. At low tide, the sea retreats to its lowest level, exposing sand flats, rocks, and the seabed in shallow areas. The difference in water height between high and low tide is called the tidal range.
Tidal ranges vary enormously around the world. The Bay of Fundy in Canada has the world's largest tidal range — over 16 metres in some locations. In contrast, enclosed seas such as the Mediterranean have very small tidal ranges of less than half a metre. Most open coastlines fall somewhere between these extremes, typically between 1 and 6 metres.
2. What Causes Tides?
Tides are primarily caused by the gravitational pull of the Moon on Earth's oceans. Although the Sun is far more massive, the Moon has roughly 2.2 times more tidal effect because tidal forces decrease steeply with distance, and the Moon is much closer to Earth.
As the Moon orbits the Earth, its gravity creates a tidal bulge — a slight mounding of ocean water — on the side of Earth facing the Moon. At the same time, a second bulge forms on the opposite side of Earth, caused by the centrifugal effect of Earth and Moon orbiting their common centre of mass. As the Earth rotates, different parts of the coastline pass through these bulges, experiencing two high tides per day.
The Sun also exerts a tidal pull, roughly 46% as strong as the Moon. When the Sun and Moon align (at new moon and full moon), their forces combine to produce larger tides. When they are at right angles (at quarter moons), their forces partially cancel, producing smaller tides.
Local geography also plays a major role in the character of tides. The shape of ocean basins, the depth of water, the width of continental shelves, and the geometry of bays and estuaries all amplify or reduce the tidal signal arriving from the open ocean. This is why adjacent locations can sometimes have very different tidal ranges.
3. Types of Tides
Coastal locations around the world experience three main types of tidal patterns, determined by their geography and position relative to oceanic tidal systems:
Semidiurnal tides
Two nearly equal high tides and two nearly equal low tides per tidal day (24 hours 50 minutes). This is the most common pattern worldwide and is typical of most of the Atlantic coastline — including the UK, much of the US East Coast, and most of Europe.
Diurnal tides
One high tide and one low tide per tidal day. Common in parts of the Gulf of Mexico, the Philippines, parts of Alaska, and Vietnam. The single daily tide cycle is often linked to specific resonance characteristics of the ocean basin.
Mixed semidiurnal tides
Two high tides and two low tides per day, but with significant differences in height between successive tides. Common along the US West Coast, parts of Australia, and much of the Pacific. One high tide each day is notably higher (the higher high water) and one low tide is notably lower (the lower low water).
4. Spring Tides and Neap Tides
The monthly cycle between larger and smaller tides is one of the most important patterns for anyone using tide predictions for practical purposes.
🌕 Spring Tides
Spring tides occur at new moon and full moon, when the Sun, Moon, and Earth are roughly aligned (a configuration called syzygy). The combined gravitational pull of the Sun and Moon acts in the same direction, producing:
- Higher than average high tides
- Lower than average low tides
- Larger tidal range
- Stronger tidal currents
Despite the name, spring tides happen year-round — the term comes from the Old English word "springan" meaning to jump or leap, not the season.
🌗 Neap Tides
Neap tides occur at the first and third quarter moons, when the Sun and Moon are at right angles to the Earth. Their gravitational forces partially oppose each other, producing:
- Lower than average high tides
- Higher than average low tides
- Smaller tidal range
- Weaker tidal currents
Neap tides are approximately 10–30% smaller in range than spring tides at the same location, depending on local geography.
The spring-neap cycle repeats roughly every 14–15 days, completing two full cycles each lunar month. For practical purposes: if you need the largest possible low tide to access rocks, sand flats, or shallow areas, aim for spring tides. If you want calmer, less extreme tidal conditions, neap tides are preferable.
5. How to Read a Tide Chart
Tide charts (also called tide curves or tidal graphs) display the predicted rise and fall of sea level at a specific location over time. Here's how to read them:
Time, usually shown in local time for the location. The chart typically covers 24 hours, with each day shown separately. All times on Tide Times Today are displayed in the local timezone of the station.
Water height in metres, measured above Chart Datum (CD). Peaks are high tides; troughs are low tides. The shape of the curve between them shows how quickly the tide rises and falls.
High Water — the time and height of each high tide. This is the peak of the tidal curve. Two HW events typically occur each day at most locations.
Low Water — the time and height of each low tide. This is the trough of the tidal curve. Low water height is always positive (above Chart Datum) unless an extreme storm surge is occurring.
The rising section of the curve, from low water to high water. The tide is coming in. Tidal currents generally flow onshore or up estuaries during the flood. The fastest currents occur around mid-tide.
The falling section of the curve, from high water to low water. The tide is going out. Tidal currents generally flow offshore or down estuaries during the ebb.
6. What Is Chart Datum?
Chart Datum (CD) is the reference level from which all tide heights and water depths on nautical charts are measured. It is defined as close to the Lowest Astronomical Tide (LAT) — the lowest sea level that can be predicted to occur under any combination of astronomical conditions, in the absence of meteorological disturbances.
By anchoring the reference level near the lowest possible tide, Chart Datum ensures that:
- • Tide heights shown on charts are almost always positive numbers
- • Depths shown on nautical charts represent the minimum water depth a sailor can expect (under normal conditions)
- • Adding the predicted tide height to the charted depth gives the actual depth of water at any point in time
For example: if a nautical chart shows a depth of 2.0 m at a particular spot, and the tide prediction shows 1.5 m of water above Chart Datum, the actual depth of water at that time is 3.5 m. This calculation is fundamental to safe coastal navigation.
7. How Accurate Are Tide Predictions?
Astronomical tide predictions — generated by harmonic analysis of historical tidal data — are highly accurate under normal weather conditions. At most locations, predictions are typically within 15–30 minutes of the actual time of high or low water, and within a few centimetres of the actual height.
However, actual water levels can differ significantly from predictions due to weather effects:
🌬️ Wind effect
Strong onshore (offshore) winds push water towards (away from) the coast, raising (lowering) the actual sea level. In exposed locations, wind effects can exceed 0.5 m.
🌀 Atmospheric pressure
Low atmospheric pressure raises sea levels (the "inverted barometer effect") — a 1 hPa drop in pressure raises sea level by about 1 cm. A deep Atlantic storm can raise coastal sea levels by 0.3–0.5 m.
⚠️ Storm surges
The combination of low pressure and strong onshore winds can produce storm surges of 1–3 m or more in shallow seas. These are especially dangerous when they coincide with spring high tides, and are the primary cause of coastal flooding.
Always check local weather forecasts alongside tide predictions. For safety-critical activities — sailing, diving, working in the intertidal zone — monitor both the predicted tides and current weather conditions.
8. Tides and Coastal Activities
🎣 Fishing and angling
Many species feed most actively in the two hours either side of high and low water, when tidal currents are at their weakest (slack water) or when baitfish concentrate around tidal rips. Check the tide table for your chosen location and target the tide change periods for the best results.
⛵ Sailing and boating
Tide times are critical for planning departures from and arrivals into tidal harbours, and for navigating shallow channels and estuaries. Use the tide table to ensure you have sufficient water depth, and plan to arrive or depart on a rising tide if crossing a bar or entering a tidal harbour.
🏄 Surfing
Most surf breaks perform differently at different tide heights. Reef and point breaks often work best at mid-tide, while some beach breaks may favour low or high tide. Learn the optimal tide for your local break, and use tide charts to plan your sessions around the best conditions.
🏊 Swimming and water sports
Tidal currents are strongest at mid-tide. If swimming in tidal waters, check the state of the tide and be aware that currents can increase rapidly as the tide turns. Many beach rip currents are influenced by tidal flow. At low tide, rocks and shallow areas that are covered at high tide become hazards.
🔬 Rock pooling and foraging
The best rock pooling is at low water, especially on spring tides when the tide falls further than usual, exposing lower shore habitats. Plan your visit for the 2 hours either side of low water. Always check that the tide is falling — not rising — when you arrive.
📐 Marine engineering
Coastal construction, dredging, and marine surveying work must be carefully scheduled around tidal windows. Engineers use tide predictions to identify when areas are accessible at low water, and to calculate extreme high water levels for structural design.
9. Tide Safety
Tides are powerful and predictable — but they can be dangerous if you don't check them before heading to the coast. Here are the most important safety points:
Always check before you go
Check the tide table for your specific location and the day of your visit. Tides shift by 50 minutes every day, so yesterday's tides do not apply today.
Know your escape routes
On tidal beaches, estuaries, sand bars, and cliff paths, the tide can cut off your route back surprisingly quickly. Identify your exit before the tide starts rising, and never assume you can outrun the sea.
Spring tides are more extreme
Around new moon and full moon, spring tides are significantly larger than average. Low water falls much lower than usual, but the incoming flood tide then rises much higher and faster than neap tides. Be especially cautious around spring tides.
Weather affects actual sea levels
Storm surges caused by low pressure and onshore winds can raise the sea significantly higher than predicted. In severe conditions, flooding can occur even when tides are predicted to be moderate. Check weather forecasts as well as tide predictions.
Don't rely solely on digital devices
Phone batteries die and mobile coverage fails at sea. When sailing or diving, always carry a printed tide table as a backup, and make sure crew members also know the tide times for the day.
10. Frequently Asked Questions
Why do tide times change every day?
The Moon's orbital period is 24 hours and 50 minutes, which is 50 minutes longer than the Earth's rotation. This means each successive tide occurs approximately 50 minutes later than the day before. Over a month, this adds up to a complete shift through the clock, which is why the tide table always shows different times day by day.
What is slack water?
Slack water is the brief period around high and low water when tidal currents are at their weakest before reversing direction. It is an important concept for divers (who often time their dives to slack water to avoid strong currents) and for sailors transiting tidal race passages. Slack water duration varies from minutes to over an hour depending on location.
Can you predict tides far into the future?
Yes — because tides are driven by the predictable orbital mechanics of the Moon and Sun, they can be predicted years or even centuries in advance. The astronomical component of tide prediction is extremely accurate. However, meteorological effects (storm surges, wind, pressure) cannot be predicted more than a few days ahead, so short-term accuracy is always better than long-term accuracy.
What is the Rule of Twelfths?
The Rule of Twelfths is a simple method for estimating water depth between high and low tide. Divide the tidal range into 12 equal parts. In the first hour after low water, the tide rises by 1/12 of the range. In the second hour, it rises by 2/12; in the third and fourth hours, 3/12 each; in the fifth hour, 2/12; in the sixth hour, 1/12. This shows that the tide rises fastest in the middle hours, which is important for timing shore access.
Why are some tides higher than others?
Several factors cause tides to vary in height: the monthly spring-neap cycle (driven by the Moon's phases), the Moon's distance from Earth (tides are larger when the Moon is at perigee — its closest point), the Sun's declination (seasonal effect), and local weather conditions including atmospheric pressure and wind.
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