Wind & pressure forces

What does a wind of 270 degrees mean?

Read the origin, reference, units and averaging period together.

Direction comes from upstream

Wind is air motion relative to the Earth's surface; meteorological wind normally describes the horizontal component. Wind direction names the direction from which it blows: a 270° wind comes from the west and travels towards the east. Surface wind is the representative wind measured near the surface, conventionally at 10 m above the ground in an appropriately exposed location. It is not automatically identical to the instantaneous wind at a cockpit, on a sheltered apron or above a ridge.

Meteorological reports use direction in degrees true. The examination convention distinguishes this from wind given by a control tower in degrees magnetic for runway-related use; consult the local AIP where regional arrangements differ. Speed is commonly given in knots, with metres per second used in some states or products. One knot is approximately 0.5144 m/s, so 10 m/s is about 19.4 kt. A direction comparison is meaningless if one value is true and the other magnetic without allowing for variation.

An average is not a gust

The reported mean wind comes from anemometer measurements. The specified averaging period is two minutes for local routine and special reports and the wind used by ATS units, and ten minutes for METAR and SPECI. Significant discontinuities and reporting rules affect the data used; the observing lesson examines the actual code groups. A gust or lull describes a short-duration departure from the mean, so a ten-minute mean of 15 kt does not promise a steady 15 kt throughout an approach.

Which way does the pressure-gradient force act?

Distinguish the force direction from the final wind direction.

A horizontal difference creates a force

The horizontal pressure gradient is the change in pressure per unit horizontal distance at a common height. Its mathematical gradient points towards increasing pressure. The pressure-gradient force on the air points the opposite way, from higher towards lower pressure, perpendicular to the isobars. Closer isobars at the same chart scale and interval indicate a stronger gradient, and therefore a greater pressure-gradient force per unit mass for the same density.

If air initially at rest experiences an unbalanced horizontal pressure-gradient force, it starts accelerating towards lower pressure. It does not need an existing wind for this force to act. The eventual wind can cross, follow or curve along isobars depending on Coriolis, friction and acceleration. Saying that the wind always blows straight from H to L confuses one force with the outcome of all forces.

On an upper constant-pressure chart, the corresponding pattern is represented by height contours, or isohypses, rather than surface pressure isobars. A stronger horizontal height gradient is similarly associated with stronger pressure-gradient forcing at a geometric level. Keep the chart's coordinate system clear before using its contour spacing to assess winds.

How can a force towards low pressure produce wind along the isobars?

Build the geostrophic balance in the free atmosphere.

Coriolis changes direction

In an Earth-fixed reference frame, Coriolis is an apparent acceleration acting perpendicular to the horizontal wind: to its right in the Northern Hemisphere and to its left in the Southern Hemisphere. Its magnitude increases with wind speed and with the sine of latitude, reaching zero at the equator and its maximum latitude factor at the poles. It does not initiate motion in air at rest and, being perpendicular to velocity, does not directly do work on the horizontal flow.

Away from the equator, air accelerated towards lower pressure is deflected as its speed grows. In the ideal steady, straight, frictionless balance, Coriolis opposes the pressure-gradient force. The resulting geostrophic wind runs parallel to straight isobars, or to the corresponding upper-air height contours. Looking downstream, lower pressure lies to the wind's left in the Northern Hemisphere and to its right in the Southern Hemisphere. A westerly wind in the north therefore has lower pressure on its northern side in this idealised case.

Geostrophic balance assumes horizontal acceleration and friction are negligible. It is a useful approximation for large-scale flow above the friction layer, not for every turn around a small low, a thunderstorm outflow or a sea breeze. At the equator, where the Coriolis parameter vanishes, this balance is not a valid way to infer an infinitely large wind.

When does the geostrophic speed comparison change?

Hold the pressure gradient and density fixed before comparing balances.

Latitude changes the speed needed for balance

For the same horizontal pressure-gradient force and air density, a larger Coriolis parameter needs a lower wind speed to balance it. Thus the geostrophic speed is lower at higher absolute latitude, all else equal. Moving towards the equator reduces the Coriolis parameter and the computed geostrophic speed would increase, but the approximation becomes inappropriate sufficiently near the equator. Actual latitude comparisons must not silently change the pressure gradient or density as well.

Curvature requires inward acceleration

Gradient wind follows curved contours in an ideal frictionless balance. Because its path curves, the net horizontal force must supply centripetal acceleration towards the centre of curvature. Around a low, the pressure-gradient force is directed inward while Coriolis is outward. The pressure-gradient force must exceed Coriolis to leave a net inward acceleration, so cyclonic gradient wind is slower than the geostrophic wind for the same pressure gradient, latitude and density.

Around a high, the pressure-gradient force is outward and Coriolis is inward. Coriolis must be larger to leave the required inward acceleration. The anticyclonic gradient wind is therefore faster than the equivalent geostrophic wind. These speed comparisons apply to both hemispheres; the rotation directions reverse. They describe a force balance, not a universal claim that every real high has stronger wind than every real low.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.