MetMary
01 Aviation meteorology

Buys Ballot’s Law

← LOW
Northern hemisphere
Left hand
Right hand
Wind
L · low, a valley
H · high, a hill
wind flow
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Note

The rule

Northern hemisphere, wind at your back → low on your left. Southern → on your right.

The terrain

Height is pressure, so L is a valley and H a hill. Under the gradient alone the air would roll straight into the valley. Coriolis turns it until it runs almost along the contours — which is why the flow circles the valley instead of filling it.

Keys

move the low · H hemisphere · P panel.