Meteorology
This domain tests the practical reading of weather: pressure systems and their motion, the wind they generate, the visibility they bring, and how to keep a ship out of a tropical revolving storm. The questions are applied - a barometer reading, a wind direction and a cloud type are the inputs.
Pressure and wind
- Isobars close together mean a strong pressure gradient and strong wind; the spacing matters more than the absolute pressure.
- Buys Ballot's law: in the northern hemisphere, with your back to the wind, low pressure lies to your left (southern hemisphere: to your right).
- A falling barometer means an approaching depression or a deepening one; a rapid fall of 3 hPa or more in an hour is a storm warning. A steady rise after a front has passed signals improvement.
Systems, fronts and visibility
- Warm front: cirrus, then thickening altostratus, then steady rain, moderate visibility. Cold front: cumulonimbus, heavy squally showers, good visibility between showers, a sharp veer of wind.
- Fog: radiation fog over land on calm clear nights, advection fog over cold water when warm moist air moves across it, frontal and steam fog near ice.
- Beaufort scale, cloud classification and swell direction are all fair game as identifiers.
Tropical revolving storms
- They form over warm water (above roughly 26–27 °C) between about 5° and 20° of latitude, and are absent within about 5° of the equator.
- In the northern hemisphere the dangerous semicircle is the right-hand one relative to the track, where the wind veers and the storm approaches; the navigable semicircle is on the left, where the wind backs.
Worked example
In the northern hemisphere the centre of a tropical storm bears 130°T from the ship, moving WNW, and the wind is NE and veering. Facing the wind (looking NE), the low is 90–110° to the left, confirming a centre to the south-east. Because the storm track carries it WNW and the wind is veering, the ship is in the right-hand, dangerous semicircle, and the storm is likely to pass ahead. Action: put the wind on the starboard bow and make maximum speed on a course that increases the distance from the track - never run with the wind on the starboard quarter into the advancing centre.
Traps that cost marks
- Reversing Buys Ballot between hemispheres, or using isobar spacing backwards.
- Treating a veering wind as improvement: in the northern hemisphere veering usually means the dangerous semicircle.
- Confusing advection fog (needs a temperature difference between air and sea) with radiation fog (needs clear skies and light wind).
- Ignoring swell direction, which often gives the storm's bearing before the wind does.