Meteorology

A fall of 5 hPa in 3 hours is a rapid pressure fall — well above the 3 hPa/3 h threshold that indicates a rapidly deepening depression. This is a serious deterioration warning. Immediately: call the Master and brief on the barograph trace; obtain the latest weather forecast (NAVTEX, Weatherfax, VHF MSI broadcast); compare observed conditions against the current forecast — if conditions are worse than forecast, the depression may be deeper or faster than predicted; plot the probable position and track of the low using Buys Ballot's Law from the current wind direction; review the passage plan and identify ports of refuge; secure all deck items and begin heavy weather preparations; post a dedicated lookout; switch to manual steering; reduce speed as appropriate. Log all observations and actions with times.

This is a classic TRS approach scenario. The Caribbean in October is peak hurricane season. All signs point to a tropical revolving storm: long swell from an unusual direction indicates distant storm; pressure at 1000 hPa and falling rapidly is critical — normal tropical pressure is 1012–1014 hPa, we are already well below that; increasing cirrus is the characteristic first cloud sign; and crucially, the wind is steadily BACKING (N to NW) in the Northern Hemisphere, which by Buys Ballot's Law means the low centre is to my left — and a backing wind in the NH means I am in the NAVIGABLE semicircle, the left-hand side of the storm track (a steadily VEERING wind would mean the dangerous right-hand semicircle). Action: call the Master immediately; broadcast a weather observation via Navtex/SafetyNet if so ordered; obtain the latest tropical cyclone advisory from the relevant RSMC (NHC for Atlantic/Caribbean); determine the storm's position, intensity, and projected track; then the navigable-semicircle manoeuvre: put the wind on the starboard quarter, make maximum speed, and ride the circulation away from the centre and clear of the track. Do not heave to. Make maximum speed and continue to monitor — and if the wind ever begins to veer instead, reassess at once: that is the dangerous-semicircle signature, and the manoeuvre changes to wind on the starboard bow.

Buys Ballot's Law states: in the Northern Hemisphere, stand with your back to the wind and the centre of lowest pressure is to your left and slightly ahead. In the Southern Hemisphere, the low is to your right. It works because of the Coriolis effect — air flowing toward low pressure is deflected to the right in the NH, creating the anticlockwise rotation of a depression. Practical application: if the wind is from the SW (blowing from SW to NE), stand with your back to the SW (facing NE), and the low is to your left — so bearing roughly NW of you. This is immediate, requires no instruments, and can tell you which side of a depression you are on. Combined with the barograph and the cloud sequence, it lets you build a mental picture of the synoptic situation without waiting for a forecast. Particularly valuable when assessing TRS approach — wind direction and its rate of change tell you your position relative to the storm track.

The conditions describe classic advection fog. Warm, moist SW'ly air (14°C) is moving over cold Channel water (8°C). The air will cool toward the sea surface temperature, potentially below its dew point, and dense sea fog will form. The English Channel in spring is one of the world's highest-density fog areas. Actions: check the wet and dry bulb readings — calculate the dew point. If the air temperature is approaching the sea surface temperature and the dew point spread is small (less than 2°C), fog onset is imminent. Immediately reduce to a safe speed. Switch on radar and begin logging all radar contacts. Switch navigation lights to display. Prepare to sound fog signals. Inform engine room to stand by. Alert the lookout to the fog risk and ensure he is positioned to give the earliest possible warning. Check the forecast for the area — if advection fog is forecast, plan an alternative route or consider anchoring to wait for it to clear. Call the Master.

The primary official source is the NAVTEX receiver — automatic continuous broadcasts on 518 kHz providing gale warnings, 24-hour forecasts, and met advisories. I check it at the start of every watch and log what I find. For ocean passages in Sea Areas A3/A4 beyond NAVTEX range, SafetyNet via the INMARSAT terminal provides equivalent content. For synoptic charts I use HF Weatherfax — scheduled broadcasts of surface analysis and prognosis charts that show pressure systems, fronts, and forecast tracks; the schedule is in ALRS Vol. 3. In coastal waters I monitor VHF Ch 16 for Coastguard MSI announcements and then shift to the working channel for the inshore forecast. I also read the barograph — the trace of the last 24 hours tells me what the pressure has been doing, which I cross-reference against the forecast. GRIB data from routing software is useful planning guidance but I treat it as supplementary, not definitive. Whatever I obtain, I record the source, time, and key content in the deck log, and if observed conditions diverge from the forecast I call the Master.