Tides
0 of 4 hands-on
01 Introduction
Tides are the regular rise and fall of the sea. If you dive anywhere with a significant tidal range, the tide will affect the depth, the current, the visibility and even whether you can get in and out of the water. Understanding it is one of the most useful skills in dive planning.
Objectives
- Explain what causes the tides and why they come about twice a day in many places
- Distinguish spring tides from neap tides
- Read a simple tide table and estimate the times of the tides on a following day
02 Watch
The pull of the Moon
5 min · Video in production
- 1
The Earth and Moon seen from space, with two bulges of water pointing towards and away from the Moon.
- 2
A time-lapse of a harbour rising and falling, with boats grounded at low tide.
- 3
Animation of the Sun, Moon and Earth in line at full and new moon, then at right angles at the quarters.
- 4
A tide table with the high and low times highlighted, and a diver checking it on a boat.
03 Learn
What causes the tide?
The Moon pulls on the water of the Earth. The side facing the Moon is pulled towards it, and the opposite side is pulled away less strongly, which leaves a bulge there as well. As the Earth turns, a coast passes through both bulges and experiences two high tides and two low tides in a day in many places. The Sun has a similar effect, but weaker.
The Moon takes about 24 hours and 50 minutes to return to the same place in the sky. This is why a tide is about 50 minutes later each day, and why the time between two high tides is about 12 hours and 25 minutes.
- High water
- The highest level reached in a tidal cycle.
- Low water
- The lowest level reached in a tidal cycle.
- Tidal range
- The difference in height between high and low water.
- Flood tide
- The period when the water level is rising.
- Ebb tide
- The period when the water level is falling.
- Slack water
- The short period around high or low water, or at a change in direction, when the current is weak.
Springs and neaps
When the Sun, Moon and Earth line up, at new moon and full moon, the pull of the two combines. The tidal range is then at its largest. These are spring tides, and the name has nothing to do with the season. When the Moon is at a right angle to the Sun, at the first and last quarters, the pulls partly cancel, and the range is smallest. These are neap tides.
| Tide | When | Range | Tidal streams |
|---|---|---|---|
| Spring | Around new and full moon | Largest | Strongest |
| Neap | Around first and last quarter | Smallest | Weakest |
Predict first
At full moon, which kind of tide would you expect?
Not every coast is the same
The shape of the coast and the sea floor changes the tides a great deal. Some places have a tidal range of many metres, while others, such as parts of the Mediterranean, have a range of only a few centimetres. Some coasts have one high tide a day (diurnal), some have two (semi-diurnal) and others have a mixture, with two highs of different heights.
Reading a tide table
A tide table gives the time and height of each high and low water for a place. It is predicted from astronomy, so weather can change what actually happens. Strong winds and changes in air pressure can raise or lower the water level from the prediction.
- 1.Find the right port or the nearest reference port for your dive site.
- 2.Look up the date, and note the times and heights of high and low water.
- 3.Work out where the dive falls in the cycle: early flood, mid-tide or near slack.
- 4.Check for corrections for your site, as local times can differ from the reference port.
- 5.Compare the tidal range with the day before and after, to see if it is springs or neaps.
The rule of twelfths
The water does not rise at a steady rate. As a rough guide, it rises one twelfth of the range in the first hour after low water, two twelfths in the second, three twelfths in the third, three in the fourth, two in the fifth and one in the sixth. So the rise is fastest in the middle of the tide, which is also when the tidal streams are usually strongest.
Take a guess first
High water today is at 06:00. Roughly what time is high water tomorrow morning?
04 Explore
Try it yourself
Match each tide term with its meaning.
Match each term with its meaning.
- Spring tide
- Neap tide
- Flood
- Ebb
- Slack water
05 Apply
Scenario
A diver plans a dive on a site with a strong tidal stream. The tide table shows low water at 10:00 and high water at 16:15. The guide says the site is best dived at slack water, but the diver assumes that slack is exactly at low water, 10:00.
What should the diver do?
06 Check
07 Summary
Key points
- The Moon and Sun cause the tides. Most coasts have two high tides a day, about 12 hours 25 minutes apart.
- Springs, with the largest range and strongest streams, happen around new and full moon. Neaps happen at the quarters.
- Tide times move about 50 minutes later each day.
- Tide height and tidal stream are different. Check local stream information for the site.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.