Whilst the forecasts are generally very accurate, they are not 100% accurate, 100% of the time.   Just like your luck, the weather can change in an instant! So, when necessary, check with other sources, such the BOM forecast, your mates, and of course, what’s actually happening outside!

When the weather is not as the forecast predicted, the tendency is to blame the forecast.  We'd like to suggest that the forecast is accurate as it possible can be, it's the weather that changed! 

Let's think about how a forecast is generated.

The first step is to collect real observations from around the globe so that you can feed past weather and current weather into the computer model that generates the forecast.

Weather observations are sourced from many places, such as the daily release of a weather balloon which measures the air pressure a various altitudes.  (There's more to forecasting, than just sea level)

Other sources include ships which are part of a world data collection system that take weather observations such as temperature, air pressure, wind direction and speed and radio them in to a central collection point.  There are also weather buoys that freely float the currents of the ocean and automatically radio their observations in.   Similar concept on solid ground - land based weather stations automatically collect weather data and radio the information in - no human help required.

There will always be areas of the atmosphere where no local observations have been collected.   These areas are calculated using sophisticated estimation systems to use surrounding observations to calculate what's going on in the area without any observations.

The computer model then uses historical data, current data and local knowledge to predict what the weather is most likely to do next - a forecast.

The forecasts are regenerated every 6 hours, and the graphs are updated here moments later. The further out the forecast is (e.g. 7 days) the less chance it has of being accurate due to the chaotic nature of weather.

How to read the Wind & Wave Forecast graph

Unlike the live graphs which represent part of a single day, the forecast graphs cover 7 days.

Each "forecast" arrow is three hours apart from the previous arrow, representing a forecast wind speed/direction and swell every 3 hours of the day for 7 days in advance.  

How to read the Wave Direction & Peak Period Forecast graph

Wave direction is the direction the swell is travelling. As every spot where waves break has a unique coastline, it represents a general indicator of the swell direction prior to being influenced by things such as groins, reefs, points and bays etc.

Peak Period is an estimate of the average time (in seconds) between peaks. As a general indicator, the longer the period, the more powerful the swell. For example, 9-12 seconds may indicate wind waves while powerful swells have a period longer than 15 seconds. Bigger periods will also indicate faster moving swell. For example, a 2 metre swell @ 22 seconds can travel at over twice the speed of a 2 metre swell @ 9 seconds.

Just like the other graphs, the centre of the arrow indicates the point where the value should be read from. Each increment at the base of the graph is a 4 hour period, with the longer bar on the "day" axis indicating midnight.

• What causes a Tide?

  Gravity! Water stays on our planet due to gravity. As the Moon orbits the Earth, it pulls the sea water towards itself. The Sun provides a similar effect, and as our distance from the Sun changes during the year, this also affects the tides.
  
  Sophisticated algorithms are used to compute the tide levels based on the rotation of the Earth, and the position of the Sun and the Moon, and our distance to the Sun.
  
  Predicting tides also relies on knowledge of "harmonics". Hundreds of tide monitoring stations (which are usually mounted on a jetty or other stable structure) measure the real water level. If a massive body of water is pulled one way, then the other, and all the while bouncing of sea walls and beaches, you can imagine how it can affect the calculations. So, from these real readings, the harmonics can be deduced which are then fed into the tide equations to assist in predicting tides.
  
  Tsunami's, strong wind, swell and storm surges will all affect the level of water experienced at your local spot.
  
  

• What use is Tide information!?

  Waves!
  
  The tide graphs are handy for surfers, windsurfers and kitesurfers - anybody who likes to hit a wave now and again. Tides can have a huge effect on the swell.
  
  A wave is caused by the shallowing of the ocean floor as a force of water approaches it. The only way "out" for the energy in the water is up .. thus a wave is created. Thats why waves don't generally break in the open ocean, but do tend to break over shallow areas of water such as reefs and beaches. Reef breaks can be some of the most powerful waves due to the sudden change of depth.
  
  This is where the tide comes into it...if there's a 2 metre swell and there's going to be a 1 metre high tide, then that's going to take some of sting out of the swell, because there may be an additional 1 metre of water depth. (Remembering that the reef/beach may not necessarily be at "mean sea level")
  
  However, if it's low tide (say 10cm), then that 2 metre swell is going to be a lot more exciting!
  
  Fishing!
  
  You can't mention tides without mentioning fishing! Many fish become active on a change of tide .. the changing currents can stir fish into life meaning they'll jump on your hook more readily. Because high tide adds extra depth, it can change which areas fish can feed in - fish can head into areas that were inaccessible on a low tide.
  
  

• The tide graphs are not for navigation

  Using the seabreeze tide graphs to work out if your yacht can make it up the river where you need "at least 78 centimetres of clearance or it will run aground" is not recommended. For navigation issues, please use the official guides at Bureau Of Meteorology, or for Western Australia, the Coastal Data Centre
  
  

• The "Live" tide graph is not from real observations

  Thinking of heading out to go fishing or hit the surf and want to know what level the tide is up to?
  
  The tide graph at the bottom of each page is plotted from the tide predictions and is there as a convenience so you can see a close-up view of the tidal activity - it is not taken from real tide readings in the ocean.
  



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