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True Wind

A comprehensive guide covering definitions, vector math, and practical applications of true wind in maritime navigation and meteorology. Learn how to calculate apparent wind, adjust course over ground to math coordinates, and determine true wind speed and direction. Discover the impact of errors, turning ships, and correct averaging methods. Access open-source calculation codes and a full manuscript for in-depth understanding. For more information and to provide feedback, visit the provided links. Questions can also be directed to the experts listed.

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True Wind

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  1. True Wind

  2. Suspect True Direction Data

  3. Suspect True Speed Data

  4. Coordinate Systems

  5. Definitions • True Wind (T) – Wind vector with • speed and direction referenced to fixed Earth and • direction from which the wind blows referenced to true north (not magnetic north). • Apparent Wind (A) – Wind vector with • speed referenced to the vessel and • direction from which the wind blows referenced to true north. • Platform-Relative Wind (P)– Wind vector with • speed and direction referenced to the vessel and • direction from which the wind blows referenced to the bow.

  6. Definitions • Course Over Ground (C) • Vector motion of the vessel with a speed and direction to which the vessel is traveling referenced to true north. • Heading (hθ) • The direction to which the bow of the vessel is pointing referenced to true north.

  7. Definitions • Zero-line-reference (Rθ) • The angle between the zero line on the wind vane (anemometer) and the bow of the vessel. • Measured clockwise from the bow.

  8. Impact of 180˚ Error in Rθ

  9. 1D Example of True Wind Vector Math • Vessel is stationary on a day with true wind blowing directly over the bow.

  10. 1D Example of True Wind Vector Math • Vessel moves forward inducing additional wind relative to the vessel.

  11. 1D Example of True Wind Vector Math • A = T + M • T = A – M • M = –C • T = A – (–C) = A + C

  12. Calculation Details • Calculate the apparent wind in math coordinates. A’θ = 270˚ – (hθ+Rθ+Pθ) |A| = |P| • Adjust the course over ground to math coordinates. C’θ = 90˚ – Cθ

  13. Calculation Details • Calculate the components of the true wind vector. Tu = T’u = |A| cos(A’ θ) + |C| cos(C’ θ) Tv = T’v = |A| sin(A’ θ) + |C| sin(C’ θ) • Calculate the true wind speed and direction from which the wind is blowing. |T| = (Tu2 + Tv2)0.5 T θ = 270˚ – arctan(Tv/Tu) • NOTE: Arctan function must have a range of -180˚ to +180˚.

  14. Averaging Errors

  15. Turning Ship Alters Apparent Wind

  16. Incorrect Averaging Method

  17. Correct Averaging Method

  18. For More Information • http://samos.coaps.fsu.edu/html/tools_truewinds.php • Full Manuscript from Journal of Atmospheric and Oceanic Technology “Establishing more truth in true winds” • Open source true wind calculation codes in C, FORTRAN, IDL, and MatLab

  19. Questions?

  20. Wrap-Up • Thank you for participating! • Please take a moment to provide feedback on the course at http://www.surveymonkey.com/s/SH3RBQR • Course content and presentations will be available at http://samos.coaps.fsu.edu/html/mt_shortcourses/shortcourse_Inmartech2014.php • Questions can be sent to Shawn Smith: smith@coaps.fsu.edu Daniel Wolfe: Daniel.Wolfe@noaa.gov Jeremy Rolph: rolph@coaps.fsu.edu

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