Meteorology and Oceanography: Pressure Systems, Storms and Local Winds
What this domain tests
Practical weather interpretation for route decisions: pressure systems and wind laws, fronts and fog, tropical revolving storms and avoidance, sea and swell, currents, tides, and the regional weather that affects Iranian waters.
Core rules
- Wind flows from high to low pressure, deflected to the right in the Northern Hemisphere; around a Northern Hemisphere low the circulation is anticlockwise, and the wind is roughly 90° clockwise from the bearing to the centre.
- Buys Ballot: with your back to the wind, low pressure bears to the left in the Northern Hemisphere and to the right in the Southern Hemisphere.
- Tropical revolving storms: eye, eye wall, spiral bands; the dangerous semicircle is the right-hand side of the track in the Northern Hemisphere. The 1-2-3 rule gives 100 nm per 24 h, 200 nm per 48 h, 300 nm per 72 h of probable movement.
- Fog: advection fog (warm moist air over a cold sea), radiation fog, frontal fog; the Persian Gulf shamal brings dust, haze and short steep seas.
- Monsoon: south-west monsoon roughly June to September, north-east monsoon November to March; sea state depends on wind speed, fetch and duration.
Worked example
In the Northern Hemisphere the wind is from 045°. Bearing to the centre = 045° + 90° = 135° (south-east), confirmed by a falling barometer of 1.5 hPa per hour. If the system is expected within 24 h, the 1-2-3 rule puts the centre about 100 nm away. With a west-north-westward track, the ship lies on the right-hand side of the track, the dangerous semicircle: the aim is to increase distance from the centre and track, not to cross ahead.
Traps
- Applying Northern Hemisphere rules in the Southern Hemisphere.
- Using apparent wind instead of true wind for Buys Ballot.
- Assuming a storm will follow a straight track.
- Treating a moderate barometer fall as unimportant when the tendency, not the absolute value, is the warning.