Tides, Currents and Meteorology
Core concepts
Understand the causes of tides (moon and sun gravitational pull), spring and neap tides, and the rule of twelfths for estimating tide height between high and low water. Know how to use tide tables: standard port, secondary port differences, and the difference between height and stream (current) tables. Learn the terminology: set (direction toward which the current flows), drift (speed), ebb and flood, and the effect of wind against tide in producing steep seas. For meteorology, know the Beaufort scale, the meaning of a sea breeze and land breeze, the passage of a frontal system (warm front, cold front, squall lines), and the localised effects of topography and coastal convergence.
Traps
A frequent error is confusing the direction a current is flowing toward (set) with where it comes from. Also watch for the difference between tidal height and tidal stream predictions - they are separate tables. Questions on the rule of twelfths often test the cumulative fractions, not just the first hour. Remember that spring tides occur at new and full moon, not at the quarters.
Worked example
Range 3.6 m. Rule of twelfths over 6 hours: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12. After 3 hours the rise is (1+2+3)/12 = 6/12 = half the range = 1.8 m. So height = LW height + 1.8 m.
Rule of thumb
'First hour one, second two, third three, fourth three, fifth two, sixth one.' Half the rise occurs at half the time.