Meteorology and Oceanography
Meteorology and Oceanography examines whether you can read a synoptic chart, anticipate weather before it arrives, and manoeuvre to avoid the worst of it - including the one system that can sink a ship in hours.
Pressure, wind and Buys Ballot
Wind follows the pressure gradient, deflected to the right in the Northern Hemisphere and to the left in the Southern. Buys Ballot's law: with your back to the wind in the Northern Hemisphere, low pressure lies on your left; in the Southern Hemisphere, on your right. Geostrophic wind runs parallel to straight isobars, and surface friction turns it roughly 10–20° toward the low. Close isobars mean strong wind - as a working figure, 1 hPa per 60 nm in low latitudes corresponds broadly to gale force.
Tropical revolving storms
Structure: eye, eye wall and spiral rainbands, with an overall diameter often 200–500 nm and an eye of perhaps 10–30 nm. In the Northern Hemisphere the dangerous semicircle is the right-hand half relative to the track, where the rotational wind is reinforced by the storm's forward motion and tends to set a vessel toward the track ahead of the centre. The left-hand half is the navigable semicircle.
Avoidance
- Northern Hemisphere, dangerous semicircle: bring the wind on the starboard quarter and proceed at best speed.
- Northern Hemisphere, navigable semicircle: bring the wind on the starboard bow and proceed at best speed.
- Southern Hemisphere: substitute port for starboard throughout. Never attempt to cross ahead of a storm tracking toward you, and treat this traditional advice with caution near the centre or during recurvature.
Waves, swell and currents
Fetch, strength and duration build sea height; swell outruns the wind that raised it. Significant wave height is the mean of the highest third of waves. Shallow water steepens and refracts waves, and currents such as the Gulf Stream or Agulhas meeting a contrary sea produce short, steep, dangerous waves.
Worked example
Northern Hemisphere, gale with wind NE, pressure falling, and the wind veering (shifting clockwise) hour by hour. Veering places you in the dangerous semicircle. A NE wind puts the centre to the south-east, left of the downwind direction, which is consistent. Action: bring the wind on the starboard quarter and make best speed, so the storm passes astern rather than across you.
Traps
Applying Northern Hemisphere rules in the south; relying on the veer rule close to the centre; forgetting storm forward speed when estimating true wind; and confusing swell direction with wind direction when choosing a comfortable heading.