Meteorology
Meteorology on the OOW deck paper tests your ability to read synoptic charts and forecasts, recognise frontal weather, and make a safe avoidance decision for a tropical revolving storm (TRS).
Pressure, wind and synoptic charts
Wind flows from high to low pressure and is deflected to the right in the Northern Hemisphere and to the left in the Southern. Buys Ballot's law: in the Northern Hemisphere, with your back to the wind, low pressure is on your left; in the Southern Hemisphere it is on your right. Closely spaced isobars mean a strong pressure gradient and strong wind. A falling barometer with a veering wind often indicates an approaching warm front; a rapid rise and squally veering indicates a cold front passage.
Fronts and fog
A warm front brings stratiform cloud, continuous rain, poor visibility and a gradual veer. A cold front brings cumuliform cloud, heavy showers, squalls and rapid clearing. Advection fog forms when warm moist air moves over a cold sea surface; it is common over the Grand Banks and can persist for days. Radiation fog forms over land at night under clear skies and light wind. Steam fog forms when very cold air passes over warmer water.
Tropical revolving storms
A TRS needs sea-surface temperature above about 26.5°C, sufficient Coriolis force (not within about 5° of the equator) and latent heat release. In the Northern Hemisphere the dangerous semicircle is the right-hand semicircle relative to the storm track; the navigable semicircle is the left-hand side. In the dangerous semicircle the wind veers; in the navigable semicircle it backs. Avoid the centre and the path ahead of it.
Worked example: semicircle and action
A vessel at 18°N 120°E receives a TRS warning: centre 15°N 118°E, moving northwest at 10 knots. The vessel is northeast of the centre. Which semicircle is she in and what action should she take?
In the Northern Hemisphere the dangerous semicircle lies to the right of the track. A northwest track puts the northeast quadrant in the right-hand, dangerous semicircle. Action: put the wind on the starboard quarter, increase speed and steer to increase distance from the centre; plot the storm and monitor forecasts. If unable to make way, consider heaving to and keep the wind on the starboard quarter.
Traps
- Confusing veering and backing between semicircles.
- Applying Buys Ballot's law in the wrong hemisphere.
- Assuming all TRS have the same size and track.
- Forgetting to allow for storm movement when plotting.
- Treating advection fog as radiation fog and expecting it to clear at sunrise.