Meteorology: Pressure Systems, Fronts and Forecasts
What the examiner expects
You must understand how pressure systems drive weather. Know the characteristics of a depression, an anticyclone, a ridge and a col. Be able to describe the passage of a frontal system: warm front approach brings cirrus, then altostratus, then lowering stratus with continuous rain; the warm sector gives drizzle and poor visibility; the cold front brings squally showers, a wind veer and a sharp temperature drop. Understand Buys Ballot's law: in the Northern Hemisphere, with your back to the wind, low pressure lies to your left. Know the Beaufort scale descriptors and the difference between swell and sea.
Traps in questions
Candidates often confuse veering and backing: veering means the wind shifts clockwise, backing anticlockwise. Another trap is the direction of circulation around a Northern Hemisphere low, which is anticlockwise, and around a high, which is clockwise. Questions may ask where fog is most likely: radiation fog forms over land on clear calm nights, advection fog forms when warm moist air moves over a cold surface at sea. Also watch for the difference between a gale warning and a storm warning in the shipping forecast, and for the meaning of 'imminent' versus 'soon'.
Worked rule of thumb
To locate a low from wind alone in the Northern Hemisphere, stand with your back to the wind and extend your left arm about 90 to 110 degrees; the low centre lies in that direction. Example: wind from the north, back to the wind facing south, left arm points east, so the low lies to the east. Use this to anticipate the wind veer as the system tracks. For swell, remember it can travel far beyond the generating area and arrive with a long period, which affects rolling and cargo lashings.