Meteorology and Weather Interpretation
Pressure, wind and fronts
A skipper must read a synoptic chart and a barometer. Isobars close together mean strong wind; the closer the spacing, the stronger the gradient. In the Southern Hemisphere, wind circulates clockwise around a low and anticlockwise around a high, and is deflected across the isobars towards low pressure (Buys Ballot: with your back to the wind, low pressure is on your right). Know the passage of a cold front: a wind shift (often to the south-west), a sharp temperature drop, squally rain, then clearing with a rising barometer. A warm front brings a gradual fall, thickening cloud and steady rain.
Local South African weather
Know the Agulhas Current (warm, fast, southward along the east coast) and how a south-westerly wind against a north-east-flowing current builds steep, dangerous seas. Recognise the coastal low and the South African cut-off low, which bring heavy rain and strong winds, and the summer south-easter (the "Cape Doctor") with its tablecloth cloud over Table Mountain. Sea breeze develops by day as land heats; land breeze at night.
Interpreting forecasts
Understand the difference between wind speed (knots) and gust; between a gale warning (34–47 kn) and a strong wind warning. Note that forecasts give significant wave height - the average of the highest third of waves - so individual waves can be considerably higher.
Rule of thumb
A fall of 1 hPa per hour, sustained, signals an approaching depression and worsening weather; a rise of 1 hPa per hour signals improvement. Trap: questions often ask for wind direction as the direction the wind blows from, not towards.