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Reading the SkyMountain Weather for Paragliding, Hang Gliding and Hiking

Part II · The large scale · Chapter 3

Chapter 3

General circulation: why the air goes round

Why do lows turn anticlockwise, and anticyclones the other way? Why does a low make cloud, and an anticyclone the lid that caps your ceiling? The answer is built from three bricks — pressure, the Earth’s rotation and friction — and everything else follows from them.

22 min read · 11 figures · 4 animated illustrations

The essentials in two minutes

The wind has only one engine: differences in pressure. Air would like to flow straight from high to low pressure, the way water runs down a slope. But it flows over a rotating Earth and, seen from this rotating ground, every movement appears deflected: in our hemisphere, always to the right. This is the merry-go-round effect: throw a ball from a spinning merry-go-round and it seems to curve — it is you turning beneath it.

The outcome of this tug-of-war: away from the ground, the wind does not run down the pressure slope, it runs along it, with low pressure on its left. Close the pressure lines into circles: around a hollow (a low), “low pressure on the left” forces the wind to turn anticlockwise; around a hump (an anticyclone), clockwise. A practical corollary: with your back to the wind, the low is on your left.

Near the ground, friction slows the wind and makes it spill over towards low pressure. So air converges at the foot of lows and, unable to pile up, it rises: cloud, rain. It diverges at the foot of anticyclones and is replaced by air that sinks, is compressed, warms and dries out: fine weather, and the subsidence inversion that caps the ceilings. The same friction explains why the wind at launch is weaker than the wind on the chart and turned relative to it. On the scale of the planet, these mechanisms organise three great loops per hemisphere and a river of fast air high up, the jet stream, whose meanders give birth to our frontal systems.

Read on

The whole chapter is open to subscribers, with its animated figures: €3 a month or €25 a year. The book as an ePub, €6.99, comes with a month’s subscription. Contents:

  1. 3.1The engine: the pressure force
  2. 3.2Coriolis without the mystery
  3. 3.3Geostrophic balance: the wind follows the isobars
  4. 3.4Which way lows and anticyclones turn
  5. 3.5Friction: the wind cuts across the isobars
  6. 3.6Chart wind, launch wind
  7. 3.7Low: the air rises. Anticyclone: the air sinks
  8. 3.8The planetary machine: cells and the jet stream
  9. 3.9Reading the wind on an isobaric chart
  10. 3.10Quick reference and summary

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Chapter glossary#

Isobar
Line of equal pressure (reduced to sea level) on a chart. The equivalent of a contour line: close together means a steep “slope” and a strong wind.
Pressure force (pressure gradient force)
The engine of the wind: it pushes air at right angles to the isobars, from high to low pressure, all the more strongly the closer together they are.
Coriolis force
Apparent force due to the rotation of the Earth. Deflects every movement to the right in the northern hemisphere, in proportion to its speed; zero at the equator. Never creates or accelerates motion.
Geostrophic wind
Wind in balance between pressure and Coriolis: parallel to the isobars, with low pressure on the left in the northern hemisphere. A good approximation above the friction layer.
Buys Ballot’s law
In the northern hemisphere, with your back to the wind, low pressure is on the left, a little ahead.
Boundary layer (friction layer)
The lowest layers, about 500 to 1,500 m deep, where friction with the ground slows and deflects the wind; this is where most of our flying takes place.
Convergence / divergence
Horizontal accumulation (or removal) of air, balanced vertically: convergence at the surface forces ascent, divergence at the surface draws down subsidence.
Subsidence
Slow, widespread sinking of air, typical of anticyclones: compression, warming, drying; it produces the subsidence inversion.
Hadley cell
Circulation loop between the equator, where the air rises, and 30° of latitude, where it sinks again; responsible for the subtropical anticyclones and the deserts.
Jet stream
Ribbon of very strong winds (150 to 350 km/h) at high altitude. The polar jet stream, at around 9 to 12 km, on the boundary between polar and tropical air, is the one that steers our frontal systems: its waves (troughs, ridges) create and guide the lows. The subtropical jet, near 30° of latitude, blows higher.
Trough / ridge
Dip (towards the south) and bulge (towards the north) of a wave in the upper flow. Lows deepen ahead of troughs; ridges bring fine weather.