The Causes of Wind - ATPL 050 — Meteorology

Air does not move because it wants to; it is pushed. This lesson builds the push from first principles: how wind is defined and measured, the pressure gradient force that starts it, the Coriolis force that refuses to let it run straight, the geostrophic balance the two settle into, and the friction near the ground that spoils that balance in exam-quotable ways. It ends with convergence and divergence — the bookkeeping that connects wind to rising air, and rising air to weather.

Measuring the wind

Wind is the horizontal movement of air relative to the Earth's surface, and the surface wind is that movement measured close to the ground, by convention with an anemometer at a height of 10 m. A wind is always named by the direction it blows from, and the units split by purpose:

Direction — in degrees true in written reports, and in degrees magnetic when passed by the tower, because the pilot compares the tower's wind with a magnetic runway heading.
Speed — in knots (kt), with metres per second (m/s) in use in parts of the world.

The reported wind is never an instantaneous gust but an average: averaged over 2 minutes for local routine and special reports and for ATS (air traffic services) units, and over 10 minutes for the METAR (aerodrome routine meteorological report) and the SPECI (aerodrome special meteorological report). The tower's two-minute wind is the fresher number; the METAR's ten-minute wind is the steadier one.

The pressure gradient force

Every wind starts with a pressure difference. The horizontal pressure gradient is the change of pressure per unit of horizontal distance: on the chart, how tightly the isobars are packed. The gradient gives rise to the pressure gradient force (PGF), which acts perpendicular to the isobars, from high pressure toward low, and is stronger where the gradient is steeper: tightly packed isobars mean a strong force and a strong wind, widely spaced isobars a weak one. If the pressure gradient force acted alone, air would simply pour straight across the isobars into every low until it filled.

Coriolis and the geostrophic wind

Air never pours straight in, because the Earth rotates underneath it. The Coriolis force acts on every moving parcel, at right angles to the wind: to the right in the northern hemisphere, to the left in the southern. It grows with wind speed, changes direction but never speed, and weakens toward the equator, where it vanishes.

Now let the two forces play out. Air starts moving down the gradient, straight toward the low; Coriolis deflects it; the deflection grows as the parcel accelerates, until the flow runs parallel to the isobars with the pressure gradient force and the Coriolis force in exact opposition. That balanced flow is the geostrophic wind — the wind of straight isobars, no friction, and enough latitude for Coriolis to work. Looking downwind, the low pressure lies on the left in the northern hemisphere and on the right in the southern: the rule that lets a chart's isobars be read as arrows. On upper-air charts, contour lines called isohypses play the isobars' part, and the same rules apply.

Latitude sets the exchange rate. For the same isobar spacing, the geostrophic wind speed varies inversely with the sine of the latitude: Coriolis is weaker at low latitudes, so a weaker force balances the same pressure gradient only at a higher speed — the same spacing drives a stronger wind nearer the equator. Within about 15 degrees of the equator the Coriolis force is too feeble for the balance at all, and the geostrophic model is not used there.

Curved isobars: the gradient wind

Real isobars curve, and curved flow needs a spare force pointing toward the centre of the turn. Around a low, the pressure gradient force must exceed Coriolis to bend the flow inward, and the balance settles at a speed below geostrophic. Around a high the roles reverse: Coriolis must exceed the pressure gradient force, which it can only do at a speed above geostrophic. This corrected flow is the gradient wind, and it carries a counter-intuitive exam favourite: for the same isobar spacing, the wind around a high is stronger than the wind around a low.

The friction layer

Within roughly the lowest kilometre the ground drags on the flow, and the drag unbalances the geostrophic pair. Friction slows the wind; a slower wind has less Coriolis; and the now-stronger pressure gradient force pulls the flow partly across the isobars: the wind in the friction layer blows across the isobars toward the low pressure. Compared with the geostrophic wind, the surface wind in the northern hemisphere is backed (rotated anticlockwise) and slowed; in the southern hemisphere it is veered and slowed. The exam values are fixed:

Over water — smooth surface, little drag: about 70 per cent of the geostrophic speed, crossing the isobars at about 10 degrees.
Over land — rough surface, strong drag: about 50 per cent of the geostrophic speed, crossing at about 30 degrees.

The friction layer breathes with the day. Sunshine stirs convection, convection couples the surface to the faster air above, and the surface wind strengthens and veers by day (northern hemisphere) as momentum is mixed downward; after sunset the coupling dies, and the surface wind slackens and backs through the night. How deep the friction layer reaches is set by three things: the terrain, the wind speed and the stability; rougher ground, stronger wind and less stable air all deepen it. And the chart shorthand survives friction: isobar spacing still reads as wind speed — packed isobars, strong surface wind, crossed at the angle the surface dictates.

Convergence and divergence

Convergence is more air flowing into a region than out of it; divergence is the opposite. Both happen wherever streams of air crowd together, spread apart, or change speed along their path: air slowing down piles up and converges, air accelerating stretches out and diverges. And because air cannot pile up indefinitely, the column must answer vertically: convergence at the surface forces air to rise; divergence at the surface draws air down.

That vertical answer ties the surface chart to the upper air. A surface low has friction-layer winds spiralling in across the isobars (convergence), so its air rises, cools, and builds cloud and weather, with divergence aloft carrying the air away; a low deepens while the divergence aloft removes more air than the surface convergence supplies. A surface high is the mirror: convergence aloft, sinking air warming on the way down, divergence at the surface, and with the descent comes the fair, stable weather of the anticyclone. The pressure-systems module builds both characters out in full.

What the exam asks

The friction-layer percentages and angles, the averaging times and the hemisphere rules are hard recall; the force logic (why the balance forms, why curvature changes the speed, why friction turns the wind) is where the understanding marks live. The overview, following the learning objectives:

Define wind and surface wind, state the units and reference of reported directions, and give the 2- and 10-minute averaging rules

Define the horizontal pressure gradient and describe how the pressure gradient force acts on it

Explain the Coriolis force, the development of the geostrophic wind, its flow relative to the isobars in both hemispheres, and the effect of latitude

Explain the gradient wind and how it differs from geostrophic in cyclonic and anticyclonic flow

Describe the friction-layer wind: the 70%/10° and 50%/30° rules, the diurnal variation, the factors setting the layer's depth, and the isobar-wind relationship

Describe convergence and divergence and relate them to pressure systems, vertical motion and cloud