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Wind Factors For Diving

Wind Factors For Diving
Ideal wind conditions for diving | © Phil Watson

Whether your experience is good or bad when diving or snorkelling is often determined before you even enter the water. No matter how familiar you are with a dive site, you never enter the water without thoroughly assessing the conditions present.

Study The Weather

Base your decision to go out into the water on the knowledge of what weather conditions you, and your dive or snorkelling buddies, can handle.

Marine weather forecasts and warnings produced by the Bureau of Meteorology are available on all media. It is vitally important to be aware of the current weather conditions in the area you plan to dive or snorkel in, and also how conditions will develop over the course of your outing — and a bit longer, just in case.

Visit www.bom.gov.au/marine for the latest marine weather charts, satellite and radar images as well as warnings and forecasts for the next four days. This site also provides links to tidal information, sunset and sunrise times as well as full schedules for all radio and phone services. Before heading out, run through the five vital weather safety checks to be prepared.

Five Vital Weather Safety Checks
Five Vital Weather Safety Checks

Forecasts may be wrong so check the weather observations close to the time of your dive.

Weather Patterns

Predicting weather is enormously complex and — as anyone knows who watches TV weather reports regularly — not entirely accurate. Plus those general weather reports are for those on land. While that's about all that most people need to know about weather, it's not enough for divers, because weather patterns in coastal regions often differ from forecasts for inland areas. This is why a marine-oriented forecast is vital when you're planning a dive along the coast. Then you need to know how the weather typically changes during the course of a day.

For example, calm wind conditions in the early morning hours is a typical summertime weather pattern. But, as the sun heats land masses much faster than water, the land quickly warms and forms an upward convection current. This pulls cooler air in from the sea and creates a sea breeze. The wind usually intensifies until late afternoon, when it begins to dissipate. Night-time often brings very little or no wind. Using this knowledge, you can normally assume that morning will bring the calmest wind conditions in coastal regions, with the wind building gradually throughout the day.

However, this general pattern is not an absolute rule. Weather is influenced by many different factors and can behave differently from what's been described. So regardless of your knowledge of how weather happens, there's still no substitute for an in-depth marine forecast of the area where you're planning your dive.

Understanding Wind Forecasts

Understanding marine wind forecasts is a key skill for divers and snorkellers. The principles are quite simple and can generally be applied to all dive sites.

Wind Direction — This refers to where the wind is coming from. For example, a northerly wind is coming from the north, and therefore blowing towards the south.

Wind Strength — Marine forecasts give wind strength in knots (nautical miles per hour), whereas land forecasts give wind strength in kilometres per hour, or kph. Very roughly, 1 knot is equal to 2 kph. So a 10 knot wind is approximately 20 kph (actually 18.5 kph to be more precise). Be sure that you are reading the correct units when checking the forecast.

For diving purposes wind strength can roughly be categorised as follows:

  • Light winds
    5 kn (9.26 kpm) – 10 kn (19 kpm)
  • Moderate winds
    10 kn (19 kpm) – 15 kn (28 kpm)
  • Moderate to strong winds
    15 kn (28 kpm) – 20 kn (37 kpm)
  • Strong to very strong winds
    20 kn (37 kpm) – 30 kn (56 kpm)

The Bureau of Meteorology (BoM) issues a:

  • Strong wind warning
    For winds averaging more than 25 kn (46 kpm) and up to 33 kn (61 kpm)
  • Gale warning
    For winds averaging 34 kn (63 kpm) and up to 47 kn (87 kpm)
  • Storm warning
    For winds averaging 48 kn (89 kpm) or more

You don't want to be diving or snorkelling when any of the BoM warnings apply.

Know What The Forecast Is Telling You

The wind can change direction and strength very quickly. It's important to understand the key terms when reading a weather report.

  • Wind speed over the water is given in knots — 1 knot is 1.852 kph. Wind speeds mentioned in forecasts refer to the average wind over a 10 minute period at a height of 10 m (33 ft).
  • Gusts are increases in wind speed lasting for just a few seconds. They typically range 30–40 per cent greater than the average wind speed.
  • Squalls are a sudden large increase in wind speed (usually accompanied by a change in wind direction) that lasts several minutes and then suddenly dies.

Basic Principles

When wind blows across water it creates waves. The size of the waves depends on how strong the wind is and how far it is blowing across the water (this is called 'fetch'). Strong winds blowing over a large distance of water will create larger waves than a light wind blowing over a short distance of water.

Waves and Wind
Waves and Wind

If the water is protected from the wind by a land mass then the likelihood of waves being produced is reduced. An area sheltered from the wind is referred to as a 'lee'. This principle is much the same as on land where standing in a strong wind will be unpleasant, but getting behind a wall in the lee of the wind will protect you from the wind.

Onshore Winds Versus Offshore Winds

When the wind is blowing towards the shore this is referred to as an onshore wind. When the wind is blowing onshore it tends to create chop or waves as it pushes the water towards the shore.

When the wind is blowing away from the shore this is called an offshore wind. In this case the water close to shore is protected from the wind by the land and so it is unlikely that waves will be created. Offshore winds are highly desirable for diving.

Onshore vs Offshore Winds
Offshore winds are favoured by divers

Predicting Offshore Winds

Check the weather forecast for the wind direction. See Weather, Tides, Conditions etc., for an overview forecast, plus links to other resources for more detailed weather forecasts.

Have a look at a map of the area you plan to dive or snorkel. The dive site pages on The Scuba Doctor website all have a Goggle Map of the dive site location. Zoom out and go full screen to get a better overview. See Melbourne Dive Sites and Melbourne Snorkelling Sites.

From the map you want to determine what direction the dive site faces. You want the wind to be from the opposite direction to the way the dive site faces. Thus, on a shore dive you want the wind at your back as you enter the water.

As an example, consider a dive site at the northern zone of Port Phillip, such as the Jawbone Marine Sanctuary in the Williamstown area. Lets use the site The Jawbone, Williamstown as an example. This dive site faces towards the south, and ideally, we want the wind to be blowing in the opposite direction to the way the dive site faces, as this will give us offshore winds. A north wind would be ideal as it will be offshore at this site and should produce calm conditions. However, if we wanted to dive at Portsea Pier on that same day, the northerly wind would be blowing directly onshore as this site faces to the north. As a result, if the wind is moderate or strong, it would create choppy seas.

Wave Heights on Port Phillip
Wave Heights on Port Phillip | Source: Marine Safety Victoria

When offshore winds (blowing from shore) are forecast, even a marine weather report can be less than accurate at times. Strong offshore winds can result in marine advisories because these winds can create dangerous sea conditions. But since it's blowing from shore, the wind doesn't have time to affect the seas close to shore. This results in calm seas close to the beach, even though seas may be raging just a bit further out. Because many prime dive sites are close to shore, you can often experienced excellent diving conditions while TV and radio weather reports are warning people not to venture out. This points out the need to have not only accurate information, but a lot of local knowledge as well when assessing diving conditions.

Port Phillip

With regard to Port Phillip, there is a general principle that for the:

  • Northern end of Port Phillip north winds are ideal,
  • Eastern side of Port Phillip easterly winds are ideal,
  • Southern end of Port Phillip south winds are ideal, and
  • Western side of Port Phillip westerly winds are ideal.

There can be local exceptions to this however.

Port Phillip Map
Port Phillip Map | Source: Google Maps

Back Beaches and Bass Strait

The general principles for Port Phillip don't necessarily apply outside of Port Phillip, in Bass Strait. For example, Portsea Back Beach is 'down south' from Melbourne but it faces south (the opposite to Portsea Pier which faces north). So, northerly winds are ideal at Portsea Back Beach, whereas they would be undesirable at Portsea Pier.

In the ocean areas outside of Port Phillip, swell can be generated from large distances away so localised predictions may not be reliable. See Diving the Back Beaches for more information.

Ricketts Point

Given the popularity of diving and snorkelling at the various sites within the Ricketts Point Marine Sanctuary, we'll make particular mention of it. If you look at an overview on a map of Ricketts Point Marine Sanctuary you will see that this area, being on the eastern side of Port Phillip, faces to the west. Applying the general principles already discussed we could predict that easterly winds would be ideal here as they are offshore.

Ricketts Point Marine Sanctuary Map
Ricketts Point Marine Sanctuary Map | Source: Parks Victoria

If we zoom in closer on our map to a more localised view we can also see that areas of Ricketts face south, and so are protected from a north wind. So, our most favourable winds are easterlies and northerlies. Note however that strong northerlies can cause some degree of chop that can get unpleasant the further out you go, especially if there is any westerly component, such as a NW wind. Winds from the west and the south are onshore and undesirable. So, it is best to avoid W, SW and S winds, unless they are very light.

Visibility

The reason we dive and snorkel is to see what's down there, so visibility is an extremely important factor in evaluating a dive site. How calm the sea is or how little current you have to contend with is irrelevant if you can't see your hand in front of your face once you're underwater.

There are a myriad of factors that influence visibility, including weather, seasonal variation, bottom composition, water motion, and even the time of day. All of these affect either how much particulate matter is in the water (turbidity) or the amount of light that can penetrate underwater (absorption).

Severe surface chop or wave action caused by winds increases absorption, thus decreasing the amount of light that can penetrate the water. Bottom composition affects turbidity. As a rule, the finer the bottom substrate, the easier it can be disturbed and become suspended in the water. The easier the substrate can be disrupted, the more likely water motion such as waves, current, or surge will affect visibility.

When winds are very light they may have little influence on a dive site, regardless of the direction. Keep in mind though that many shore dive sites in Port Phillip are quite shallow and it doesn't take much in the way of chop to stir up the bottom and spoil the visibility. If you must dive on a muddy or silty bottom, maintain good buoyancy control and make sure you keep your fins away from the bottom.

Factors Affecting Dive Visibility
Factors affecting dive visibility

Also keep in mind the conditions over the preceding days prior to your dive. If there have been strong winds and rough seas for a day or two before your dive the water may still be dirty until it has had time for the particles and silt to settle. Recent rain may also spoil visibility due to run-off from the rivers, creeks, and rains.

Weather and seasonal variation have a significant impact on visibility. Warm temperatures and increased sunlight during summer can produce conditions which cause explosive plankton blooms. This is one reason that in temperate-water regions like Melbourne, visibility tends to be better in winter than summer.

The time of day you make a dive can also be a significant factor in the quality of underwater visibility. A physical property of light is that when it strikes the surface of the water at an angle of less than 48 degrees, most of it is deflected. The higher the angle, the more light can penetrate. Taking this to a practical level, the highest light penetration, and the best visibility, normally occurs when the sun is at an optimal angle — between 10 a.m. and 2 p.m.

Other Considerations

Learning how to accurately assess diving conditions takes time. While the basics are pretty simple, the intricacies of how environmental conditions interact can be confusing and even misleading. That's why it's best to gain experience under a more seasoned buddy, or by joining a dive or snorkelling club. However, even experienced and highly trained divers aren't immune to mistakes, particularly when they dive an unfamiliar site. Whether you're a novice or highly experienced, seek assistance when you dive a new area for the first time.

Remember, it's not the environment that causes problems, it's the decisions you make in evaluating the environment.

See also, Dive Site Selection.

Useful Downloads

Disclaimer: The information provided here is of a general nature and predictions aren't always correct. It's essential that each individual assesses the conditions at the site and determines if it's suitable for their personal ability and training. Be aware of any predicted weather changes that are imminent. Call off the dive immediately if you uncomfortable with any aspect of the site, conditions, etc.

Prepared by David Reinhard | davidreinhard@bigpond.com
Additions and editing by The Scuba Doctor.

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