Wind, pressure, and whether you leave the harbour

Station plots on a weather chart

A station circle encodes cloud cover (how much of the circle is filled), a wind barb (from where the wind blows, with feathers for speed in knots), and often a pressure value. One SailPrep diagram is read as northwest 20 knots with 50% cloud.

A weather vane on deck is the analogue instrument: it points into the wind. Charts talk in true direction; the boat feels apparent wind — do not mix those two in an answer.

Wind rose

A wind rose summarises how often the wind blows from each compass point at a place, sometimes with speed bands. It is climatology, not today’s forecast — useful for choosing an anchorage or a season, not for a 12-hour passage plan.

Highs, lows, and isobars

Isobars are lines of equal pressure in hectopascals. Tight isobars mean stronger geostrophic wind. In the northern hemisphere wind blows anticlockwise around a low and clockwise around a high, slightly across the isobars toward low pressure at the surface.

A developing low with a warm and cold front is a passage-maker: rain on the warm front, a veer and harder rain on the cold front, then clearer polar air. Clouds are classed by height — high, middle, low, and clouds of vertical development (cumulonimbus).

Sea breeze and Beaufort

A coastal breeze reverses twice a day: by day from sea to land (sea breeze) as the shore heats; by night from land to sea (land breeze). It can add to or cancel the gradient wind — the reason a forecast “10 knots” feels like 18 in a sunny afternoon bay.

Wind force in exams is described on the Beaufort scale (points, not just knots). Know the sea state that goes with force 4–6 as a skipper; force 8 is a gale. Combine Beaufort with fetch, depth, and tidal stream before you decide to leave.

Exam habits

• Barb points to where the wind is going? No — meteorologists draw the barb from the direction the wind comes from.
• Filled station circle = more cloud, not more wind.
• Low = counterclockwise (NH), bad weather more likely. High = clockwise, often fair but fog possible.
• Wind rose ≠ forecast. Isobar chart = synoptic now.

Related guides

• IALA buoys and marks
• Sailing knots
• Navigation lights
• COLREGs quick revision