Weather guide
Lows, highs and fronts
Before the hour-by-hour detail, there's the "big picture": synoptic weather. It explains where the wind comes from and what's on the way.
Pressure: low and high
Atmospheric pressure is read on isobar charts (lines of equal pressure). Two players:
- Low (L): low pressure, often unsettled weather, strong wind. In the northern hemisphere the wind circulates around it, counter-clockwise.
- High (H): high pressure, generally calm, fair weather, light wind. The wind circulates clockwise.
Tightly packed isobars = strong wind. Around a low, the wind turns counter-clockwise (northern hemisphere).
The isobar rule: the tighter the lines, the stronger the wind. Widely spaced isobars = a slack pressure pattern, light wind, and room for thermal breezes.
Fronts
A low drags fronts along with it, boundaries between air masses:
- Warm front: steady rain ahead of it, veering wind, a building sea.
- Cold front: squalls, a sharp wind shift and gusts as it passes, then clearing showery skies.
Example
Cold front forecast for 4 pm: before, SW 20 knots and a low overcast; as it passes, a squall and a shift to NW; after, NW 25 knots gusting with sunny spells. You anticipate the shift so you're not caught out.
Why it matters on board
Identifying the general situation means understanding why the model forecasts a given wind — and therefore judging how reliable it is. A deep, fast-moving low is more uncertain than a stable high: keep more margin.