You are 200 nm offshore, mid-Atlantic crossing. The 0600 NAVTEX has just printed a gale warning. Your chartplotter is showing a pressure reading of 994 hPa and falling. The skipper asks you to assess the situation and give a routing recommendation by 0800. This is exactly the kind of scenario an examiner will construct.
Sources of Weather Information
At sea you should be using layered sources — no single source is sufficient:
- NAVTEX (518 kHz / 490 kHz): coastal safety messages including gale warnings, strong wind warnings, navigational warnings
- GRIB files (via Iridium, SSB or satellite): gridded numerical forecast data; useful for passage planning but treat as guidance, not gospel
- Weatherfax / HF radio schedules: synoptic charts broadcast by national met services
- VHF Met broadcasts: coastal, near-range (e.g. UK Coastguard)
- Commercial router or professional meteorologist: best practice for offshore passages
Reading a Synoptic Chart
Key features to identify:
- Isobars: lines of equal pressure. Closely spaced = strong pressure gradient = strong winds. The gradient wind relationship means wind roughly follows isobars, deflecting towards low pressure due to Coriolis effect (left in Southern Hemisphere, right in Northern)
- High (anticyclone): outflowing, clockwise in the Northern Hemisphere, generally stable, light winds
- Low (depression): inflowing, anticlockwise in the Northern Hemisphere, associated with frontal systems, deteriorating weather
- Fronts: warm front (red semicircles) — gradual cloud build, steady rain, wind backs then veers on passage; cold front (blue triangles) — sharper onset, squally, wind veers markedly, visibility improves rapidly after passage
- Occluded front: characteristics of both, common in mature depressions
Pressure Tendency and Beaufort
A falling barometer — especially rapid falls of 1 hPa or more per hour — indicates an approaching low. Cross-reference rate of fall with synoptic chart position to estimate timing of deterioration. Know the Beaufort scale to translate forecast wind speeds into expected sea state and vessel behaviour. Gale = Beaufort 8 (34–40 kn).
Buys Ballot's Law
In the Northern Hemisphere: stand with the wind on your face, the centre of low pressure is approximately 90–120° to your right. This allows you to quickly determine depression bearing from observed wind direction and plan to keep the low to one side.
Applying the Forecast — Routing Decision
- Plot the depression track and speed from successive synoptic charts
- Estimate time of arrival of worst conditions at your position
- Identify the navigable semicircle (Northern Hemisphere: left/port side of storm track is less severe)
- Assess vessel capability, crew state, sea room, and port of refuge options
- Make a clear, documented recommendation — altered course, heave-to, divert, or proceed with contingency