Systems and the synoptic picture
In temperate latitudes the weather is governed by depressions tracking eastward with their warm and cold fronts, separated by ridges of higher pressure. Wind blows nearly along the isobars, anticlockwise around a low in the northern hemisphere, and harder where the isobars are close. The Chief Mate builds the picture from the forecasts, NAVTEX, weatherfax or GRIB data and the synoptic chart, and reads the trend of the barometer to anticipate change before it arrives.
| Feature | Typical weather |
|---|---|
| Warm front approaching | Lowering cloud, continuous rain, wind backing then veering |
| Cold front passing | Squally showers, a sharp veer, rising pressure, clearing |
| Tight isobars | Strong winds |
| Ridge / high | Lighter winds, settled, risk of fog |
| Reading the synoptic picture. |
Ocean routeing
On ocean passages the mate understands the global wind belts, the ocean currents, and the seasons and regions of the tropical revolving storms, and helps plan seasonal routeing that catches favourable winds and currents while avoiding the storm regions in their season. Routeing charts and professional routeing advice inform the plan, but the sea state — driven by wind strength, duration, fetch and any wind-against-current or wind-against-tide — is watched continuously and reported to the Master.
TIP — Watch the barometer's trend A steadily falling glass warns of a deepening low and rising wind before the wind arrives. The trend matters more than the value, and a rapid fall deserves the Master's attention.
WARNING — Plan ocean passages around the storm season The first rule of ocean routeing is to keep the ship out of tropical-storm regions in their season. Seasonal, climatological planning is what avoids the worst danger the ocean holds.