The Buys-Ballot Rule

The Buys-Ballot rule is a simple meteorological rule of thumb that lets a pilot or observer estimate the location of high and low-pressure areas from the wind direction.

Named after: Christophorus Henricus Diedericus Buys Ballot (1817–1890), Dutch meteorologist, formulated in 1857.

The rule — Northern Hemisphere

"Stand with your back to the wind. Low pressure lies on your left, high pressure on your right."

In other words:

Wind blows around the low counter-clockwise.
The low centre lies 90° to the left of the wind direction.

The rule — Southern Hemisphere

"Stand with your back to the wind. Low pressure lies on your right, high pressure on your left."

In the Southern Hemisphere the Coriolis force is reversed; hence the mirrored rule.

Scientific basis

The rule is a direct consequence of the geostrophic balance (see Geostrophic wind):

Air flows from high to low pressure (pressure-gradient force).
In the Northern Hemisphere the Coriolis force deflects the motion to the right.
In balance the wind blows parallel to the isobars, with the low on the left.

Near the ground the balance is disturbed by friction, so the wind "veers" slightly toward the low (10–30°). Buys-Ballot still holds approximately.

Practical use

Historically important to sailors (before METAR availability) to assess position relative to a storm system — still a useful didactic concept today.

Flight planning

Example — eastbound flight:

Wind from 270° (west) → back to wind, looking east.
Left (north) = low pressure.
Right (south) = high pressure.

Implication:

To avoid headwind: a southerly route over high pressure brings lighter winds; a northerly route over low pressure brings stronger winds and frontal systems.

Ground-based forecasting

Example in Germany on a typical day:

Wind from south-west (220°) → back to wind, looking north-east.
Left (north-west) = low pressure — front approaching from the west.
Right (south-east) = high pressure.

Implication: weather will deteriorate (front coming through); after frontal passage weather improves.

Limitations

Surface wind deviates from the Buys-Ballot rule due to friction — it blows roughly 30° toward the low instead of 90°. At 2 000 ft AGL it is again more accurate.
Very weak pressure gradients produce light winds dominated by local effects (heat differences, mountains).
In mountains the rule is disrupted by valley and Föhn winds.
At the Coriolis null line (equator) the rule does not apply.

Practical PPL exercise

Task: you are flying westbound (270°). Wind is from 350° at 20 kt. Where is the low?

Solution:

Stand back-to-wind so the wind blows on your back → you face 170° (south-south-east).
Left (east) = low.
Right (west) = high.

Implication for your westbound flight:

You are flying west (270°) → the high lies ahead → likely better weather.
The low lies east (behind and beside) → probably frontal weather there.

Buys-Ballot with a wind shift

Wind veers from SW to NW (clockwise, "veering"):

Low passing → typical post-frontal pattern with better visibility, cooler air, showery weather.
Classic cold-front passage.

Wind backs from NW to SW (counter-clockwise, "backing"):

Low approaching → warm front incoming.
Visibility deteriorates, ceiling drops.

Consistency with Coriolis and global circulation

Buys-Ballot is the direct consequence of the mid-latitude westerly drift:

Northern Hemisphere: lows move west to east, with polar air on the north (low = left for eastbound observers).

Buys-Ballot's Law