Synoptic Charts vs Prognostic Charts
An examiner will test whether you know what you are actually looking at. A synoptic chart (analysis chart) shows the state of the atmosphere at a single moment in time, based on simultaneous observations from multiple stations. A prognostic chart (prog) shows a forecast state at a future time, generated by numerical weather prediction models. Use the analysis to understand now; use the prog to plan ahead. Never treat a prog as ground truth — its accuracy degrades beyond 48–72 hours.
Reading a Synoptic Chart
Isobars are lines of equal mean sea-level pressure. Closely-spaced isobars indicate a steep pressure gradient and strong winds. Wind flows roughly along isobars, deflected by the Coriolis effect: in the Northern Hemisphere, low pressure has anticlockwise circulation (cyclonic); in the Southern Hemisphere the sense reverses. Buys Ballot's Law: stand with the wind on your back in the Northern Hemisphere — low pressure is to your left.
Fronts: a warm front (red semicircles) has a gentler slope — broad cloud shield, persistent rain, gradual pressure fall, wind backing then veering. A cold front (blue triangles) is steeper — squalls, heavy showers, rapid pressure fall then sharp rise, wind veers sharply. An occluded front combines characteristics of both. A stationary front — barbs on both sides — gives prolonged poor weather.
Local Forecasting Techniques
As master you supplement received forecasts with direct observation:
- Barometric trend: a fall of 1 hPa or more per hour is significant; rapid falls indicate approaching low or deepening system.
- Diurnal pressure variation: in tropical regions the atmosphere exhibits a regular semi-diurnal (twice-daily) oscillation — the 2 mb wave — with pressure maxima at approximately 1000 and 2200 local time, and minima at approximately 0400 and 1600 local time. The amplitude is roughly 2 hPa. A fall of more than 2–3 hPa below the expected diurnal curve is the primary barometric warning of an approaching tropical revolving storm and must never be dismissed as part of the normal diurnal cycle.
- Clouds: a sequence of cirrus → cirrostratus → altostratus → nimbostratus indicates an approaching warm front. Towering cumulus / cumulonimbus indicate instability and convective risk.
- Wind and sea state: persistent swell from a direction with no local wind indicates a distant storm generating that swell.
- Sea/land breezes and katabatic/anabatic flows are local effects that can override synoptic-scale forecasts in coastal and mountainous areas.
- Weather reporting (MAREP/ALRS): the master is required to transmit a weather observation report when wind reaches Beaufort force 6 or more, visibility falls below 1 nm, or unexpected swell changes are encountered.
Weather Systems at Command Level
A tropical revolving storm (TRS) requires a different decision framework from a mid-latitude depression. The 1-2-3 rule is used in passage planning when TRS risk exists. It constructs a danger area around the forecast storm centre by adding a forecast-error buffer to the forecast radius of 34-knot winds: 100 nm at 24 hours, 200 nm at 48 hours, and 300 nm at 72 hours. The vessel's planned track must remain outside this combined radius at each time step. In the Northern Hemisphere, the dangerous semicircle is to the right of the storm track; the navigable semicircle to the left — but no semicircle is safe at close range.
At command level, the examiner expects you to explain how you decide — which forecasts you consult, how you cross-check them against your own observations, when you alter plan, and when you seek additional meteorological advice.