Meteorology and Oceanography: Synoptic Systems and Routeing
Pressure systems and fronts
A candidate must read a synoptic chart fluently: isobars, spacing and geostrophic wind, the difference between a cold and warm front, and the structure of a frontal depression through its life cycle. Know where the warm sector lies and why the wind veers after a cold front passes.
Local and hazardous weather
Understand tropical revolving storms: their formation regions, track behaviour, the dangerous and navigable semicircles, and the recommended evasion manoeuvre in each hemisphere. Know the signs of a squall line, katabatic winds, sea breeze and fog formation by advection and radiation.
Waves, swell and currents
Know how wind speed, fetch and duration determine sea state, and why swell persists after the wind drops. Understand the relationship between wave steepness and breaking, and the effect of current against wind in producing dangerous seas.
Routeing and weather avoidance
Be able to describe least-time routeing, the use of weather facsimile and digital forecasts, and the value of a ship's own observations in verifying forecasts. Understand the limitations of model forecasts in data-sparse regions.
Traps
- Assuming the forecast position of a tropical storm is a certainty rather than a probability cone.
- Confusing the dangerous semicircle in one hemisphere with the other.
- Forgetting that current against wind steepens seas.
- Treating a single observation as a trend.
Rule of thumb
In the northern hemisphere, a vessel unable to avoid a tropical storm should keep the wind on the starboard bow and make maximum progress away from the storm centre; in the southern hemisphere the mirror applies.