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Highs, Lows & Fronts: The Engines of Your Weather
💡 Highs mean settled weather and lows mean wind and rain — and a falling barometer with tightening isobars is your early warning that a depression's fronts are on the way.
Weather at sea is driven by air pressure — the weight of the atmosphere, measured in millibars (mb), with ~1013 mb as average. A high-pressure system (an anticyclone) is descending, drying air that usually brings settled skies and light-to-moderate winds. A low-pressure system (a depression) is rising air that pulls in cloud, rain, and the strongest winds a coastal sailor is likely to meet. The tighter the isobars (lines of equal pressure) on a chart, the stronger the wind — think of them like contour lines on a hill: steep spacing means a fast slide.
Air doesn't flow straight from high to low; the Earth's rotation deflects it (the Coriolis effect). In the Northern Hemisphere this makes wind circulate clockwise around a high and anticlockwise around a low. Hence Buys Ballot's law: stand with your back to the wind and the low pressure is on your left (Northern Hemisphere) — a quick way to locate the bad weather without a chart.
Depressions carry fronts — boundaries between warm and cold air masses. A warm front (warm air riding over cold) brings gradually lowering cloud, steady rain, and a wind that backs then veers. Behind it, a cold front (dense cold air undercutting warm) arrives faster with a sharp squall line, heavy showers, a sudden wind veer, and then clearing, gusty air. The classic clue: a falling barometer means a low and its fronts are approaching; a rising barometer signals the high rebuilding and improving conditions.