Near the ground the wind is not quite geostrophic. Friction with trees, buildings and waves slows it, and that small change explains why lows bring clouds and highs bring clear skies, connecting to what lifts the air.
Key idea
In the lowest kilometer, friction slows the wind, so the Coriolis force weakens and the wind crosses the isobars at an angle toward low pressure. In the Northern Hemisphere air spirals counterclockwise and inward around a low and clockwise and outward around a high; in the Southern Hemisphere the turning is reversed. Air spiraling into a low rises, building clouds; air spiraling out of a high sinks, clearing the sky.
Why it works
The Coriolis force grows with wind speed. When friction slows the wind, the Coriolis force shrinks, and the pressure gradient force wins a little, pulling the wind across the isobars toward the low at an angle, often 10° to 45°, more over rough land than over smooth water. Around a low, that means air converging toward the center. It can't pile up there or go down into the ground, so it rises, cools and forms clouds and rain. Around a high, air diverges at the surface, and air from above sinks to replace it, warming as it sinks and drying the sky.
Worked example: Turning around a low
Which way does surface air circle a low-pressure center in Australia?
- Australia is in the Southern Hemisphere, where moving air turns left.
- Air heading in toward the low is turned left, which sets up a clockwise spin.
- Clockwise and inward: the reverse of a Northern Hemisphere low.
Answer: Clockwise, spiraling inward
Worked example: Find the low
You are in Ohio with a steady wind blowing from the south. Roughly where is the nearest low?
- Buys Ballot's law: in the Northern Hemisphere, stand with your back to the wind and the low is on your left (a little ahead).
- Back to a south wind, you face north; your left is west.
- The low lies to the west, a little to the north: weather in the west is coming your way.
Answer: To the west (a little northwest)
Common mistake
Mixing up the turning directions. In the Northern Hemisphere: counterclockwise into a low, clockwise out of a high. In the Southern Hemisphere: clockwise into a low, counterclockwise out of a high. Check with the rule that moving air turns right in the north: air heading inward toward a low, turned right, ends up going around counterclockwise.
| Northern Hemisphere | Southern Hemisphere | Weather | |
|---|---|---|---|
| Low | counterclockwise, inward | clockwise, inward | rising air: clouds, rain |
| High | clockwise, outward | counterclockwise, outward | sinking air: clear, dry |