An orange windsock extends above a field beside the path to Green Lane Farm.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

East–north plane. force arrows share a scale. wind has a separate scale.

Gold: pressure gradient · Blue: Coriolis · Coral: friction · white: velocity.

How to read these results

Northern hemisphere · eastward 10.00, northward 0.00 m/s. Steady force balance; pale filaments advect with the wind. Flat f-plane experiment; pressure gradient fixed. Tracers wrap at the window edge (150 km per scene unit).

Wind, PGF and Coriolis

Explain how pressure differences start air moving and how Earth’s rotation and surface friction change its direction.

Drag to rotate. Hold Ctrl and scroll to zoom, or pinch on a touch screen.

Why it happens

Air accelerates in response to pressure differences. Earth’s rotation deflects its motion, while surface friction changes both wind speed and the angle at which it crosses pressure contours.

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A windsock responds to moving air. Its open mouth faces into the wind; meteorological wind direction names where the air comes from.

Try a comparison

Add friction and describe the cross-isobar component. Select Release from rest and scrub Process.

Reading the result

The selected pressure field and drag isolate these forces; changing a slider is not a prediction of wind at a real location.

A common misunderstanding

Wind always blows straight from high pressure to low pressure.

The pressure-gradient force accelerates air towards lower pressure, while Earth’s rotation deflects moving air and friction affects flow near the surface. Their combined effects determine wind direction. Winds aloft can run approximately along isobars, while surface winds commonly cross them towards lower pressure.

Cases

Exam language

Coriolis

An apparent acceleration in a rotating reference frame. For horizontal motion on Earth it deflects flow right in the Northern Hemisphere and left in the Southern; that horizontal component vanishes at the equator.

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Aqua MODIS observed Cyclone Evan near Fiji on 16 December 2012. Its cloud bands spiral around the centre; a mathematical logarithmic spiral is an approximation, not a measured exact fit.

PGF

Pressure-gradient force: the force per unit mass associated with a pressure difference, directed from higher towards lower pressure.

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NOAA surface analyses during a late-February 2022 storm. Isobars join equal pressure; front symbols identify analysed boundaries between air masses.

Geostrophic wind

An idealised wind in which the pressure-gradient force is balanced by Coriolis acceleration, so flow runs parallel to straight isobars. It is most useful above the frictional surface layer and away from the equator.

Friction

Drag between moving air and the surface, which slows the wind and changes its direction. Near the ground this allows wind to cross isobars towards lower pressure. Higher in the atmosphere, surface friction is much weaker.

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A field windsock shows near-surface wind direction. Drag from terrain and vegetation slows flow and can turn it across isobars; the pressure gradient itself is not measured by a windsock.

Isobar

A line joining places with the same atmospheric pressure on a weather chart. Closely spaced isobars indicate a large pressure gradient over a given distance. Wind also responds to Earth’s rotation and friction, so it does not simply follow that gradient.

Buys Ballot’s law

With your back to the wind in the Northern Hemisphere, lower pressure generally lies to your left; the relation reverses in the Southern Hemisphere. Surface friction modifies the angle.

Pressure gradient

The change in atmospheric pressure over a given distance. A larger gradient exerts a stronger pressure-gradient force on air. Closely spaced isobars often indicate a strong gradient, but wind also depends on friction and Earth’s rotation.

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