How wind works: pressure, rotation, friction and height

The Sun heats the Earth unevenly. Warm air expands and cold air contracts, so the weight of the column of air above any point — the surface pressure — differs from place to place. Air accelerates from higher pressure towards lower pressure, and the force that drives it is the pressure gradient force.

On a weather chart the gradient is drawn as isobars, lines of equal pressure usually every 4 hPa. The closer the isobars, the steeper the gradient and the stronger the wind. A gradient of a few hPa across Ireland gives a light day; 20 hPa or more across the country, as happens when a deep Atlantic depression passes to the north-west, gives a gale.

Two consequences matter on the ground:

• Wind strength is set by the gradient, not by the absolute pressure. A low of 990 hPa with slack isobars can be calmer than a 1010 hPa day with a tight squeeze between systems.
• Gradients change through the day and night as systems move. That is why the same place can swing from calm to strong inside a few hours, and why the forecast is hourly rather than daily.

If the Earth did not rotate, air would flow straight across the isobars into the low and fill it within hours. Because the Earth turns, a moving parcel of air is deflected — to the right in the Northern Hemisphere — by the Coriolis effect. The deflection grows with speed and with latitude, and at Ireland's latitude (51–55° N) it is strong.

Above the friction of the surface, roughly 1 km up, the pressure gradient force and the Coriolis deflection come close to balance. The result is the geostrophic wind, which blows almost along the isobars rather than across them. Around a low in the Northern Hemisphere that means anticlockwise circulation; around a high, clockwise.

This gives a rule sailors and pilots have used for well over a century, Buys Ballot's law: in the Northern Hemisphere, stand with your back to the wind and the low pressure is on your left. In Ireland, with the usual westerly or south-westerly airflow, that puts the depression somewhere to the north or north-west — which is exactly where the Atlantic lows tend to track.

Direction on The Wind Agent is always the meteorological convention: the direction the wind blows from, in degrees true. A south-westerly is FROM 225° and it blows towards the north-east. The instrument prints both, because confusing them has caused real accidents.

The lowest kilometre or so of the atmosphere is the atmospheric boundary layer. Here the surface drags on the air: grass, hedgerows, forests, buildings and waves all extract momentum. Friction has two effects.

1. It slows the wind. The surface wind is a fraction of the geostrophic wind above. Commonly cited figures are roughly 60–80% over open sea and 40–60% over land, less again in towns and forests. These are rules of thumb, not constants; they change with stability and time of day.
2. It turns the wind towards low pressure. Slower air feels less Coriolis deflection, so the pressure gradient wins a little and the surface wind crosses the isobars into the low. The cross-isobar angle is commonly cited as about 10–20° over the sea and 25–40° over land.

So the 10 m wind usually backs (turns anticlockwise) relative to the wind aloft, and going up from the surface the wind veers (turns clockwise). That twist with height is visible in the model data and is one reason the direction at a turbine hub or a crane jib can differ from the direction on the flag.

Roughness is described by a roughness length, a few millimetres over calm water, a few centimetres over short grass and up to a metre or more over towns. The rougher the surface, the deeper and stronger the slowing.

Because friction acts at the surface, wind speed generally increases with height through the boundary layer. This change is wind shear. It is why a crane driver at 80 m and a groundworker at 2 m are not in the same wind, and why a wind turbine hub is placed as high as is economic.

The simplest description is the power law:

s(h) = s₁ · (h / z₁)^α

where s₁ is the speed at a reference height z₁ and α is the shear exponent. A value of 1/7 (about 0.14) is commonly cited for open, neutral conditions; values near 0.1 are typical over open sea, and 0.25 or more over rough ground or in a stable night-time atmosphere.

The Wind Agent does not apply a single exponent. It reads the model's own wind at 10, 80, 120 and 180 m, interpolates between the levels that bracket your working height, and only falls back to an exponent when you ask for a height outside that range — in which case the number is labelled extrapolated.

A reported wind speed is an average — a 10-minute mean in the international (WMO) convention. Around that mean the air is turbulent, and the short peaks are gusts, usually defined as the highest 3-second average in the period.

Turbulence has two sources:

• Mechanical turbulence, from air tumbling over rough ground, buildings, trees and hills. It grows with wind speed and roughness.
• Convective turbulence, from heated air rising off the ground on sunny days or in showery airmasses. It brings strong momentum down from aloft in bursts, which is why a showery north-westerly after a cold front is gustier than a steady south-westerly ahead of it.

The ratio of gust to mean speed is the gust factor. Over open sea it is commonly cited near 1.2–1.3; over rough land 1.5–2.0 is common, and in convective showers or in the lee of hills it can be higher still. A forecast mean of 20 mph with a gust factor of 1.6 means gusts near 32 mph — and gusts are what move suspended loads, flip drones and lift a kite off the beach.

Gusts are also the noisiest thing a model forecasts. The model does not resolve individual eddies; its gust is a statistical estimate derived from the mean wind and the turbulence it diagnoses. That is one reason the instrument shows the ensemble spread rather than a single gust number with false precision.

On a sunny day the ground heats the air above it, the boundary layer becomes unstable and deep, and turbulence mixes fast-moving air from aloft down to the surface. Near-surface winds pick up from late morning and usually peak in mid-afternoon.

At night, especially under clear skies with light gradients, the ground cools by radiation and a shallow layer of cold, dense air forms. This stable layer suppresses mixing. The surface wind drops, sometimes to calm, while the air a few hundred metres up is decoupled from the ground and can actually speed up — sometimes forming a low-level jet with its core between roughly 100 and 500 m.

The practical effect is counter-intuitive and important:

• At 10 m, evenings and nights are often calmer than afternoons.
• At 80–180 m, the same night can be windier than the afternoon, and the shear between the ground and the hub or jib is at its largest.

On cloudy, windy days with a strong gradient, the cycle is weak or absent because mechanical mixing keeps the layer stirred. Over the open sea it is also weak, because the water temperature barely changes between day and night. Coasts sit between the two and add the sea breeze, which is covered in its own article.

Most of Ireland's strong wind arrives with Atlantic depressions — low-pressure systems that form along the polar front and travel north-eastwards, steered by the jet stream. Their winds follow a recognisable sequence as they pass to the north-west of the country:

1. Ahead of the warm front, the wind backs to the south or south-east and freshens as the pressure falls. Cloud thickens and rain arrives.
2. In the warm sector, the wind is south-westerly, often strong and relatively steady, in mild, moist air.
3. At the cold front, the wind veers sharply — commonly to the west or north-west — and there can be a squall, a sudden short-lived jump in speed with heavy rain.
4. Behind the cold front, the air is colder and showery. Mean winds may ease, but the gust factor rises as convection drags momentum down.

The tightest gradients are usually on the southern flank of the low, which is why a depression passing between Iceland and Scotland can still give Ireland a gale. Rapidly deepening lows — a fall of 24 hPa or more in 24 hours, sometimes called explosive cyclogenesis — produce the most damaging events, and the strongest of them are given names by Met Éireann and its partner services.

A numerical weather prediction model solves the equations for pressure, momentum, temperature and moisture on a three-dimensional grid. Global models such as ECMWF IFS, NOAA GFS and DWD ICON cover the whole planet with grid spacings of roughly 9 to 25 km; regional models go finer. The model then parameterises what the grid cannot resolve — turbulence, the friction of the surface, convection — and outputs wind at fixed heights above ground.

This matters for reading the numbers:

• Each grid point represents an area, not a point. A headland, a valley or a harbour wall smaller than the grid is smoothed away. The model wind is the wind the grid cell would have if it were uniformly rough.
• Different models disagree. They use different grids, physics and starting data. The instrument shows each model as its own line and never averages them into one comforting number.
• Every forecast has a lead-time cost. Errors grow with time. An ensemble — many runs from slightly different starting points — measures how fast. The fraction of members above your limit is the exceedance probability, the number the instrument is built around.

Observations close the loop. A METAR from the nearest airport or a reading from a marine buoy shows what the wind actually did at a known height, so you can see whether the model is running high or low today.

Put together, the four facts give a short checklist for reading any wind number:

1. What is driving it? Look at the gradient and the pressure tendency. Tightening isobars mean rising wind; a frontal passage means a direction change.
2. Which way is it blowing? Check the FROM direction against your site: what is upwind of you — open sea, a hill, a forest, a building?
3. At what height? The 10 m number is the standard, but it is not your jib, your hub or your drone ceiling. Read the speed at your working height and the shear above and below it.
4. How steady? Compare gust with mean. A high gust factor means the decision is about the peaks, not the average.
5. How sure? Look at the ensemble spread and the model agreement before acting on a single line.

None of this replaces your own procedure, your equipment limits or what you see on site. It is the background that makes the instrument's numbers interpretable — and arguable afterwards, which is the point of keeping a record.