Meteorology and Oceanography
Pressure, wind and weather signs
This domain tests interpretation of pressure systems, fronts, fog, sea state and tropical revolving storms, and the action an OOW should recommend.
- Buys Ballot in the Northern Hemisphere: stand with back to wind; low pressure is to your left and slightly forward.
- Falling pressure, increasing wind and a veering wind often indicate an approaching frontal system; check barometer tendency and trend.
- Fog: reduce speed, use sound signals, post lookout, and plot positions at shorter intervals.
- TRS: in the Northern Hemisphere the dangerous semicircle is to the right of the track; in the Southern Hemisphere it is to the left.
- Swell and current can set a vessel off track even with a steady course; compare ground and water track.
Worked example
In the Northern Hemisphere the wind is NE true. Stand with your back to the wind: you face SW. The low centre lies to your left, therefore to the SE of the vessel. The expected circulation is counter-clockwise around the low, which is consistent with NE wind at a position NW of the centre. The OOW should plot the bearing and monitor pressure and wind direction to see whether the low is approaching.
Traps
Applying Buys Ballot in the wrong hemisphere. Reading the barometer without correcting for height. Assuming fog clears before reducing speed. Confusing the dangerous semicircle with the navigable semicircle.