Meteorology and Oceanography
Weather questions on this certificate are practical: read a synoptic chart, apply Buys Ballot, decide which semicircle of a tropical revolving storm you are in and how to evade, and know the local winds and sea conditions of the Eastern Mediterranean and Red Sea.
Pressure, wind and charts
Isobar spacing gives pressure gradient and therefore wind strength; Coriolis turns the wind to the right in the northern hemisphere and friction backs it over the sea by roughly 10°–20°. Buys Ballot's law: in the northern hemisphere, stand with your back to the wind and low pressure is on your left (southern hemisphere: on your right). Note that this applies to the true wind, not the apparent wind shown by the anemometer.
Tropical revolving storms
Warning signs: a swell from an unusual direction, falling pressure, cirrus then cumulonimbus, and a wind that veers or backs. In the northern hemisphere the dangerous semicircle is to the right of the track, where the wind and the storm's advance combine. Evasion: in the dangerous semicircle bring the wind on the starboard bow and make good way; in the navigable semicircle bring the wind on the starboard quarter and run.
Local weather for Egyptian waters
- Khamsin: spring southerly or south-westerly wind over Egypt and the Red Sea, hot, dry and dust-laden, reducing visibility and raising sand haze.
- Shamal: strong north-westerly wind in the Gulf and Gulf of Aden.
- Meltemi: summer northerly in the Aegean and Eastern Mediterranean.
- Mediterranean winter lows bring gales and heavy swell; sea fog is common in winter with warm, moist air over a cool sea.
- Red Sea tides are small (often under 1 m); currents reverse seasonally and are influenced by wind.
Worked example - which semicircle?
A TRS centre bears 120°T at 180 nm and is moving 300°T at 12 kn. The vessel's bearing from the centre is the reciprocal, 300°T - the same as the track, so the vessel lies dead ahead of the centre on the track line, with a no-change nearest approach in about 15 hours. In the northern hemisphere the dangerous semicircle is to the right of the track (bearing 030°T from the centre), so a vessel directly ahead is in the path of the storm and must be treated as exposed; alteration to starboard to put the wind on the starboard bow and open the range is the usual response.
Traps that cost marks
- Applying Buys Ballot with the apparent wind.
- Reversing the dangerous and navigable semicircles for the southern hemisphere.
- Confusing swell direction with local wind sea.
- Reading chart datum as mean sea level when computing under-keel clearance.