Pressure, wind & the Coriolis force

Why the wind refuses to blow from high to low

Air is pushed from high pressure towards low — that force is real and it never stops. Yet the wind on every weather chart you will ever see runs along the isobars, not across them. The reason is that the Earth turns underneath the moving air. Switch the rotation off on the bench and watch the wind fall straight down the gradient; switch it on and watch it swing until the deflection exactly balances the push.

Earth's rotation: ON ✓Round a lowRound a highStraight isobars

f = 2Ω sin φ. Slide the latitude to zero and watch it vanish: on the equator the ground does not turn about the local vertical at all, so there is no deflection, no geostrophic balance, and — the fact that matters most in this subject — no tropical revolving storm can form within about 5° of the line. At 45° the parameter is 1.031e-4 s⁻¹ and a parcel left to itself would circle in 16.9 hours.

Why a low blows harder than a high with the same isobars

Add the curvature and the balance changes. Going round a LOW the centrifugal effect acts outwards, with Coriolis and against the gradient, so less wind is needed to balance: the real wind is sub-geostrophic. Round a HIGH it acts inwards, against Coriolis, so the wind must be stronger: super-geostrophic. And there is a hard ceiling on the high — squeeze the isobars too tight and no balance exists at all, which is exactly why you never meet a small, violent anticyclone, only small violent depressions.

Try it: set the curvature radius small and the isobars tight, then switch to "round a high" — the engine refuses, with the reason, instead of inventing a number.

Buys Ballot — the law you can use on the bridge wing

Stand with your back to the wind: in the northern hemisphere the low is on your LEFT(on your right in the southern). It follows directly from the balance you just built, and it means any officer who can feel the wind can point at the depression without a chart.

That inward component is what fills a depression eventually — and, in a tropical storm, what feeds the eyewall. Over land friction trebles the angle to about 30°.

The barometer — reading it properly

A barometer reads the pressure where it is, not at sea level, and the atmosphere loses about 1 hPa for every 8.5 m you climb. Correct for the instrument's index error, add the height correction, and only then compare with the chart or the normal for the place. Then watch the tendency — a fall of 3 hPa in three hours is the classical warning signal.