Why look at the weather before setting out
The sea changes with the weather, and a good part of the scares are avoided by reading the forecast well and understanding what you see in the sky and on the barometer. This block is not eliminating, but it is one of the most practical: it helps you decide whether to go out, when and where.
Pressure and isobars
Atmospheric pressure is measured in hectopascals (hPa) or millibars (mb) —they are equivalent—, and its reference value at sea level is 1013 hPa. On weather maps, the isobars are the lines joining points of equal pressure. The key reading: the closer together the isobars, the stronger the wind; far apart, light winds.
Lows and highs
- Low (low pressure): the air converges and rises; it brings unstable weather —clouds, rain and wind—. In the northern hemisphere it rotates anticlockwise (against the clock). It is usually mobile and moves from west to east.
- High (high pressure): the air sinks and diverges; it brings stable, dry weather. In the northern hemisphere it rotates clockwise. Stationary highs, with their stable air, can favour the formation of fog: good weather is not always good visibility.
Buys-Ballot's law
A practical rule for locating the low without a map: in the northern hemisphere, if you stand with your back to the wind, the low pressure is on your left (somewhat forward because of friction) and the high on your right. In the southern hemisphere it is the other way round.
The wind and the coastal breezes
The wind is named by the direction it comes from: a NW wind comes from the north-west. (Currents, by contrast, are named by where they go.)
On the coast, in calm weather, two opposite breezes appear:
- Sea breeze: by day. The land heats up faster than the sea, the air rises over the land and draws in the air from the sea; it blows from the sea towards the land.
- Land breeze: by night. The land cools faster, the cold air flows towards the sea; it blows from the land towards the sea.
The fronts
A front is the zone where two air masses of different temperature collide:
- Cold front: the cold air advances and sharply lifts the warm air → clouds of vertical development and showers. It usually moves faster than the warm front.
- Warm front: the warm air advances over the cold → gentler, more widespread rain.
- Occluded front: the cold front, being faster, catches up with the warm one and lifts the warm air off the ground; typical of the mature phase of a low.
Measuring the wind and the sea: Beaufort and Douglas
- The Beaufort scale measures the force of the wind, from 0 (calm) to 12 (hurricane), by its effects on the sea and on land.
- The Douglas scale describes the state of the sea (wave height), from 0 to 9.
Do not confuse them: Beaufort = wind; Douglas = sea.
Wind sea and swell
- Wind sea: the waves generated by the wind blowing at that moment and place; short and disordered.
- Swell (long sea): the waves that persist after the wind has dropped or that arrive from a distant area; long and regular, and sometimes a forewarning of a storm that has not yet arrived.
Fog
It reduces visibility and forces you to take extra care. The two most common: radiation fog (clear, calm nights, typical of a high) and advection fog (moist air passing over colder water). Remember: good anticyclonic weather does not rule out fog.
The weather bulletins
Before setting out, check the forecast: bulletins and warnings of coastal phenomena from AEMET, as well as information from the harbourmasters' offices and notices to mariners by radio (channel 16 and working channels). A recent bulletin is worth more than any general rule.
How to study it
- Fix the golden pair: low = anticlockwise and bad weather; high = clockwise and good weather (northern hemisphere).
- Link isobars close together → strong wind.
- Remember the breezes by their timing: sea breeze by day (from the sea), land breeze by night (from the land).
- And do not confuse Beaufort (wind) with Douglas (sea).