Meteorology and Oceanography: Weather Systems, Forecasts and Their Effect on the Vessel
Domain 3 tests how pressure systems, fronts and sea state affect passage decisions, and whether you can interpret a forecast and barometer correctly.
Pressure and wind
Isobars show the pressure gradient: close isobars mean strong wind. In the Northern Hemisphere, wind blows counterclockwise around a low and clockwise around a high, crossing isobars toward low pressure at about 10–20° over the sea. Buys Ballot's law: in the Northern Hemisphere, with your back to the wind, low pressure is on your left. A falling barometer usually signals an approaching low or deteriorating weather; a rapid fall warns of strong wind.
Systems and hazards
Fronts bring wind shifts, rain and visibility changes. A cold-front passage gives a sharp veer in the Northern Hemisphere, squalls and then clearing. Warm fronts give prolonged rain and poor visibility. Tropical revolving storms have a dangerous and a navigable semicircle; in the Northern Hemisphere the dangerous semicircle is on the right of the storm track when facing the direction of movement. Fog forms when air reaches its dew point; reduce speed and use radar.
Worked example
Wind is from 315°T in the Northern Hemisphere. With your back to the wind you face 135°T; low pressure is on your left, toward 045°T. The depression centre therefore bears northeast of you.
Traps
Applying Buys Ballot the wrong way round; confusing veering with backing; treating a forecast as a nowcast; ignoring swell and sea state in under-keel clearance; assuming the strongest winds are at the storm centre; using mean wind and ignoring squall gusts.