Visibility and water clarity
0 of 5 hands-on
01 Introduction
Visibility is how far you can see underwater. It can range from over 40 metres in clear tropical water to less than one metre in a muddy estuary. It affects safety, navigation and enjoyment, and unlike many other conditions it can change from one dive to the next, or even during a dive.
Objectives
- Explain what limits visibility underwater
- Identify the natural and human causes of poor visibility
- Describe how to dive safely and responsibly when visibility is poor
02 Watch
How far can you see?
4 min · Video in production
- 1
A side-by-side comparison of the same reef in clear water and in cloudy water.
- 2
Particles in the water scattering and absorbing light, shown as a beam passing through.
- 3
A diver kicking up silt and the cloud spreading behind them.
- 4
A group in poor visibility holding a line, staying close, and turning the dive on time.
03 Learn
What limits how far you can see?
Water is not perfectly clear. Two things reduce visibility: absorption, in which water takes up light (red first), and scattering, in which particles in the water bounce light in all directions. Particles are the main reason for poor visibility. The more particles, the less you can see.
| Cause | Particles involved | Typical situation |
|---|---|---|
| Plankton blooms | Living plankton | Warm, nutrient-rich water in spring and summer |
| Sediment and runoff | Silt, sand and clay | After heavy rain, near river mouths |
| Waves and surge | Sand and silt stirred from the bottom | Rough weather, shallow sites |
| Tidal streams | Sediment carried by moving water | Estuaries and strong-tide sites |
| Divers | Silt stirred by fins and hands | Silty bottoms, caves and wrecks |
| Thermoclines and haloclines | Layers with different particles | Layered water |
Reading visibility ahead of time
- Recent heavy rain usually means poor visibility near rivers and in sheltered bays.
- Calm weather for several days tends to improve it.
- Visibility is often best around slack water at high tide, when clear water has come in from the open sea. This varies from site to site.
- Spring tides move more water and often stir up more sediment.
- A local dive centre or guide knows how a site behaves, so ask them.
Predict first
You are planning a dive at a bay beside a river mouth, two days after heavy rain. What would you expect?
Visibility and colour
Even in clear water, colour is lost as you go deeper, with red disappearing first. Plankton and silt change the colour of the water as well, giving a green, brown or milky look. Lights and filters can restore the colours close to the subject.
Move the depth slider. At what depth does red begin to disappear?
Colour and depth
Red is gone by about 5 m
Orange by about 10 m
Yellow by about 20 m
Green by about 35 m
Blue lasts the deepest
Approximate depths. Clear water and a high sun let colours last a little deeper.
Measuring visibility
Visibility is normally estimated as the horizontal distance at which you can still see a diver, a line or a large object. Scientists lower a white or black-and-white disc called a Secchi disc and note the depth at which it vanishes. Visibility is different from brightness. Water can be dark but clear, or bright but cloudy.
Take a guess first
In a dive, you can see your buddy's fins clearly at 3 m but not at 6 m. What would you record as the visibility?
Diving in poor visibility
- 1.Decide whether the dive is worth doing at your level of experience. Poor visibility can be stressful.
- 2.Stay closer to your buddy than normal, within touching distance if needed.
- 3.Use a line or a guide rope where available, and agree the signals for lost contact.
- 4.Keep your gauge checks regular and watch your depth carefully.
- 5.Shorten the dive if needed, and ascend promptly if the group cannot stay together.
Looking after visibility
- Stay off the bottom and keep your fins up and away from silt.
- Use a frog kick or a modified flutter kick over silty bottoms.
- Stay behind the leader on a silty site so the group does not fin through the cloud.
- Do not touch or move rocks, which release silt and harm the animals living beneath.
04 Explore
Try it yourself
Match each cause with how it affects visibility.
Match each cause with its effect.
- Heavy rain and runoff
- Plankton bloom
- Fin kicks on a silty bottom
- Calm weather for days
- Waves and surge
05 Apply
Scenario
A group descends into a murky site with about 2 m of visibility. The guide has asked everyone to stay close, but one diver is hanging back, 5 m behind the others, with the group barely visible.
What should the diver do?
06 Check
07 Summary
Key points
- Particles that scatter and absorb light limit visibility.
- Plankton, runoff, waves, tides and divers all add particles.
- In poor visibility, stay closer, use a line, shorten the dive and know the lost-buddy plan.
- Good buoyancy and technique protect the clarity of the water for everyone.
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.