Tidal Accuracy: The Surprising Truth Behind Surf Forecasting

When planning a surf session, it's essential to have accurate information about the tides. A good forecast can make all the difference between catching waves and getting riptides. But what factors affect tidal accuracy? In this post, we'll explore the key variables that influence the accuracy of tide forecasts.

Example Scenario: A Perfect Tidal Window

Let's consider a scenario where you're planning to surf at Bondi Beach in Australia during peak season. On a typical day, the tide rises to 1.5 meters (4.9 feet) and sets to -0.8 meters (2.6 feet). The forecast predicts a strong high tide with waves around 3-4 feet (0.9-1.2 meters).

As you paddle out, the water level stabilizes at 1.45 meters (4.7 feet), just below the predicted height. You catch a few decent waves, but the riptides start to appear as soon as you're halfway out.

Tidal Datum & Reference Levels: What's the Deal?

So, what is tidal datum and reference levels? These are the key components that determine the accuracy of tide forecasts.

• Tidal Datum: This is the official mean sea level (MSL) reference point used to calculate tides. It's set by the World Meteorological Organization (WMO).
• Reference Levels: These are the specific tidal heights and directions at fixed points on the coastline, such as ports or piers. Reference levels help to account for variations in coastal geometry.

In our example scenario, the tidal datum is MSL, while the reference levels are set at Bondi Beach. The actual tide level at this point is 1.45 meters (4.7 feet), which matches the forecasted height of -0.8 meters (2.6 feet). However, if we were to use a different reference level, say the MSL + 10 cm (3.9 in) datum, the predicted tide level would be higher, around 1.55 meters (5.1 feet).

Factors Affecting Tidal Accuracy

Several factors can impact tidal accuracy:

• Coastal geometry: The shape and features of the coastline, such as bays, harbors, or rocky outcrops, can cause tides to vary significantly.
• Water depth: Shallower waters tend to have more variable tides due to the increased drag from the water surface.
• Wind and atmospheric pressure: Changes in wind speed and atmospheric pressure can affect tidal currents and wave patterns.
• Earth's elliptical orbit: The Earth's slightly ellipsoidal shape affects the gravitational force on the oceans, leading to variations in tides.

Conclusion

Tidal accuracy is crucial for surf forecasting. By understanding the factors that influence tidal behavior, we can improve our forecasts and avoid getting caught in riptides or missing out on big waves altogether. Remember, a good tide forecast starts with accurate data and careful consideration of these variables. Happy surfing! Tidal Accuracy: The Surprising Truth Behind Surf Forecasting

Example Scenario: A Perfect Tidal Window

• Tidal rise: 1.5 meters (4.9 feet)
• Tidal set: -0.8 meters (2.6 feet)
• Forecasted wave height: 3-4 feet (0.9-1.2 meters)

Tidal Datum & Reference Levels: What's the Deal?

• Tidal datum: Official mean sea level (MSL) reference point used to calculate tides.
• Reference levels: Specific tidal heights and directions at fixed points on the coastline, such as ports or piers.

In our example scenario, the actual water level at paddle out is 1.45 meters (4.7 feet), which matches the forecasted height of -0.8 meters (2.6 feet). However, if we were to use a different reference level, say the MSL + 10 cm (3.9 in) datum, the predicted tide level would be higher, around 1.55 meters (5.1 feet).

Factors Affecting Tidal Accuracy

• Coastal geometry
• Water depth
• Wind and atmospheric pressure
• Earth's elliptical orbit

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