Meteorology, Tides and Hydrology
Weather fundamentals
Understand pressure systems, fronts, and how they drive wind and visibility. Know the difference between a depression and an anticyclone, the passage of a cold front versus a warm front, and the signs of approaching bad weather. Learn the Beaufort scale and how to estimate wind speed from sea state. Understand fog formation, radiation fog versus advection fog, and how to interpret a forecast and its validity period.
Tides and hydrology
Know the causes of tides, spring and neap cycles, and how to read a tide table. Understand chart datum, height of tide, and the rule of twelfths for estimating tidal height over a period. In inland waterways, understand how rainfall upstream, weirs, sluices and locks affect water level and current. Learn the terms for flood and ebb in estuaries and how salinity and current affect navigation.
Traps
- Confusing chart datum with mean sea level.
- Applying the rule of twelfths to a non-sinusoidal tidal curve.
- Assuming a forecast is valid beyond its stated period.
- Ignoring upstream rainfall when judging inland water levels.
Worked rule of thumb
Rule of twelfths: in a six-hour tide, the rise is 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range per hour. If the range is 3 metres, the first hour adds 0.25 m and the third adds 0.75 m. Always check whether the local curve is sinusoidal before using it.