Basics of marine weather
Marine weather is the weather at sea: wind, waves, fog, rain, visibility and storms. Sailors must know it because a ship at sea cannot run to shelter quickly.
Wind and air pressure
Air pressure is the weight of the air above us. It is measured in hPa (hectopascal); about 1013 hPa is normal. Warm air rises and makes low pressure. Cold air sinks and makes high pressure. Wind is air moving from high to low pressure. Because the Earth spins, in the northern half the wind turns a little to the right, so air goes around a low pressure area in an anticlockwise direction.
Waves
Wind pushes the water and makes waves. Wave height depends on three things: how strong the wind is, how long it blows, and how far it blows over open water. The Beaufort scale describes wind from 0 (calm) to 12 (hurricane force). Force 6 (about 22 to 27 knots) gives big waves, and force 8 or more is a gale where small boats must stay in port.
Fog and visibility
Fog is a cloud on the sea surface. It forms when warm moist air flows over a cold sea and cools. Fog cuts visibility, so ships slow down, use radar and sound the horn.
Storms
A tropical cyclone (called a typhoon in the north-west Pacific) is a huge spinning storm over warm sea water with strong winds, heavy rain and very high waves. In the middle is the calm eye. Ships use forecasts and keep far away from the storm track.
Marine weather near Japan
Seas near Japan have clear seasons. In winter, cold dry winds blow from the north-west, bringing rough seas on the side facing the continent. In summer warm moist winds blow from the south-east, bringing humid air and, sometimes, fog over the cold current in the north. In spring and autumn low pressure systems pass often, so the weather changes quickly.
Typhoons mostly come from late summer to autumn (August to October is the busiest). They form over warm tropical sea, then often curve north towards the islands. A cold current from the north (Oyashio) and a warm current from the south (Kuroshio) meet off the east coast. Where warm moist air flows over the cold water, sea fog is common in early summer. Fishing crews watch the weather map closely before every trip.
The same ideas help in any sea: a monsoon wind in the Indian Ocean also changes direction with the seasons.
Try it
In the 3D: in the Waves mode, guess the wave height first, then move the wind slider to check. In the Pressure mode, make the gap very small and watch the arrows slow down. At home: blow gently and then hard across a tray of water. Which makes the bigger ripples? That is wind making waves.
Key formulas and definitions
- Wind blows from high pressure to low pressure; a bigger gap gives a stronger wind.
- Normal sea-level pressure is about 1013 hPa.
- 1 knot = 1.852 km/h (1 nautical mile per hour).
- Distance = speed x time (used to see how soon a storm will reach you).
Worked examples
1. A wind blows at 20 knots. What is that in km/h?
20 x 1.852 = 37.04 km/h, about 37 km/h.
2. High pressure is 1024 hPa and low pressure is 996 hPa. What is the pressure gap?
1024 - 996 = 28 hPa. Wind blows from the 1024 hPa area to the 996 hPa area.
3. A typhoon is 480 km away and moves towards you at 24 km/h. How many hours before it arrives?
Time = distance / speed = 480 / 24 = 20 hours. The ship should move away or go to port well before this.
Common mistakes
- Saying wind blows from low to high pressure. It is the other way: high to low.
- Thinking waves only depend on wind speed. Duration and the open-water distance (fetch) also matter.
- Thinking the eye of a typhoon is safe. The eye is calm, but the strongest winds are right around it, and waves stay huge.
- Mixing up knots and km/h. One knot is 1.852 km/h.