Module 13 - Meteorology
Buys Ballot’s Law
What you will learn
Use Buys Ballot’s law as a coarse northern-hemisphere orientation clue while respecting apparent-wind, local-effect and system-position limits.
With your back to the true surface wind in the northern hemisphere, lower pressure lies broadly to the left. The rule does not locate a low precisely.
Buys Ballot’s law states that, in the northern hemisphere, a person standing with their back to the wind has lower pressure on the left and higher pressure on the right. It reflects the broad circulation around pressure systems.
On a moving sailing yacht, the apparent wind can differ substantially from the true surface wind. Headlands, valleys, squalls and sea breezes can also alter the local direction. Use the best available true-wind estimate and treat the result as coarse orientation.
The law does not tell you the low’s distance, depth, track, fronts or arrival time. Combine it with a calibrated barometer, current forecast, valid synoptic chart and observations before drawing a passage conclusion.
Use the law as one cross-check
The yacht is stopped in sheltered water. The true surface wind is southwest, pressure is falling and the synopsis places a low west of Scotland moving northeast.
1. Orient to true wind
Stand mentally with your back to the southwest wind.
2. Place lower pressure broadly
Lower pressure lies to the left, towards the southeast side of that wind orientation.
3. Compare
Check whether that broad relationship is consistent with the synopsis and chart.
4. Keep the limits
Do not infer exact centre, distance, movement or front timing from the rule.
Sense check: If apparent wind or strong local funnelling is used, the inferred pressure direction can be misleading.
Common mistake or limitation
- Using apparent wind from a moving yacht as though it were undisturbed true surface wind.
- Treating ‘left’ as an exact bearing and distance to the low centre.
- Ignoring the southern-hemisphere reversal or local effects around land and squalls.
Recap
- In the northern hemisphere, back to the true surface wind puts lower pressure broadly to the left.
- The rule gives orientation, not distance, track or timing.
- Verify it with forecast, chart, pressure tendency and observations.
Optional quick check
Section 7 of 13
What can Buys Ballot’s law support most reliably?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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