Pressure, Wind and the Coriolis Effect

Highs, Lows and air pressure

In short: air has weight. Where it piles up you get a High; where there is less of it you get a Low.

Read first: this is the first topic of the Meteorology area — start here. Next: Humidity, Dew Point and Fog, then Depressions, Fronts and Synoptic Charts.

• High (anticyclone) = a mound of air. Low (depression) = a dip in the air.
• As a rough guide, pressure above about 1025 mb is called high and below about 1000 mb is called low.
• In practice a High or a Low is always relative: a 1008 mb Low can sit between two 1020 mb Highs. Read the pattern on the chart, not just the number.
• Air is a fluid. A pile of air tends to fall and spread out under gravity, so air is pushed out of Highs and into Lows. This push is the start of every wind.

Normal values and records

• The record lows at sea level are all in the eyes of hurricanes (tropical revolving storms).
• 1 mb (millibar) = 1 hPa (hectopascal). Forecasts and barometers use either unit.

Pressure tendency: when the barometer warns you

In short: the rate of change matters more than the reading. A fast fall means wind is coming.

• A fall of more than 2 mb in 3 hours is significant: strong winds are probably on the way.
• It is even more serious if the fall continues for two 3-hour periods in a row.
• In severe storms and hurricanes the pressure can drop as fast as 10 mb in 3 hours.
• Normal values for your area: average monthly pressures, with the extreme Highs and Lows, are listed in Sailing Directions (the US equivalent is the Coast Pilots). Compare your corrected reading with them.
• On the bridge: log the barometer every watch and look at the barograph trace. Report a fast fall to the Master at once, check the latest forecast and start heavy-weather preparations early.

The Coriolis effect

In short: the Earth turns under the moving air, so the air’s path looks bent. It bends right in the NH and left in the SH.

• The push from High to Low is the same in every horizontal direction. But we measure the air’s movement against the rotating, round Earth beneath it. The result is the Coriolis effect.
• Anything moving in a straight line above a rotating surface traces a curved path on that surface.
• It acts on everything that moves over the Earth: winds, ocean currents, even ballistic missiles.
  • Northern Hemisphere (NH): moving things curve to the right.
  • Southern Hemisphere (SH): moving things curve to the left.
• The effect is zero at the Equator and grows towards the poles. This is why tropical revolving storms never form on, or cross, the Equator.
• Exam wording: it is an apparent effect, not a real force. Say “Coriolis effect”. Some textbooks and websites still write “force” for the same thing: in the oral, use “effect”.
• See it: Coriolis Carousel: Demo (Physics Demos). A ball rolled in a straight line across a turning carousel looks curved to the people riding it. The riders are like us on the turning Earth; the ball is like the moving air.

How pressure, Coriolis and friction make the wind

In short: pressure starts the wind, Coriolis turns it along the isobars, friction slows it near the sea and turns it in towards the Low.

Wind round Highs and Lows

• Air starts to move from High to Low. Coriolis turns it until the push towards the Low and the Coriolis turn balance. The air then flows along the isobars.
• NH: wind blows anticlockwise round a Low and clockwise round a High.
• SH: the other way round — clockwise round a Low, anticlockwise round a High.
• Buys Ballot’s law: in the NH, stand with your back to the wind and the Low is on your left. In the SH it is on your right.
• Isobar spacing: the closer the isobars, the steeper the pressure change and the stronger the wind.

Geostrophic wind and friction

• The geostrophic wind is the wind above the friction layer (roughly the lowest 500–1000 m), where surface friction no longer acts. It blows parallel to the isobars.
• At sea level, friction with the surface slows the wind. The weaker wind is turned less by Coriolis, so it blows slightly across the isobars, in towards the Low.
• The surface wind is weaker than the geostrophic wind, because of friction.
• Friction is greater over land than over the sea, so over land the wind is slower and crosses the isobars at a larger angle.
• On the bridge: the geostrophic wind scale on a synoptic chart gives the wind above the friction layer. The wind on deck is weaker, and turned in towards the Low.

Backing and veering

In short: veer = clockwise, back = anticlockwise. Same words, same meaning, in both hemispheres.

• A wind is named by the direction it comes from: a westerly blows from the west.
• Veering: the wind changes direction clockwise, e.g. S → SW → W.
• Backing: the wind changes direction anticlockwise, e.g. W → SW → S.
• The meaning is the same in the Northern and Southern Hemispheres. What changes between hemispheres is how a passing Low makes the wind shift, not the words.
• On the bridge: log wind direction every watch. A steady veer or back, together with the barometer, tells you where a Low is going relative to the ship.

The Beaufort scale and sea state

In short: the Beaufort scale (Force 0–12) links wind speed to what the sea looks like.

• It is an empirical scale: it was built from observed sea conditions. So you can read the wind from the sea, and the sea from the wind.
• The table of wind speed against sea state is in NP100, The Mariner’s Handbook and on the Met Office website.

Figures: Met Office Beaufort wind force scale.

• “Hurricane force wind” = Force 12, 64 kn and over (Met Office Beaufort scale). It describes wind strength only.
• It is not the same as a hurricane, which is a tropical revolving storm (TRS). A deep temperate Low can give hurricane force winds; a TRS has its own structure and rules (see the TRS topics).
• Wave heights in the table are for the open sea, well away from land, after the wind has blown for some time.