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Reading the SkyMountain Weather for Paragliding, Hang Gliding and Hiking

Part II · The large scale · Chapter 5

Chapter 5

Reading weather charts

You know what a low is, how it turns and what its fronts will put you through. This chapter teaches you to see all of that at a glance on two documents: the surface chart, which shows the actors, and the upper-air chart at 500 hPa, which directs them. It ends with a method in five steps and with the four typical situations that make the weather in the Alps.

26 min read · 10 figures · 3 animated illustrations

The essentials in two minutes

A surface weather chart is an IGN map of pressure (an IGN map is the French national topographic map): the isobars are contour lines. Where they are tight, they signal a steep slope, and therefore a strong wind; and you have known since chapter 3 which way the air flows along them. Add the fronts — semicircles for the warm front, triangles for the cold, always planted on the side the front is moving towards — and the chart tells the story of chapter 4 in a single drawing: who is coming, from where, and at roughly what time.

But the surface shows only the actors. The director lives at around 5,500 m, on the “500 hPa” chart: a river of westerly air that meanders around the globe. Each meander towards the south, the trough, brings cold air and deepens the lows just ahead of it; each meander towards the north, the ridge, sets up fine weather. Sometimes a pocket of cold air breaks away and wanders on its own: the cut-off low, a specialist in unforeseen thunderstorms under an innocent-looking barometer.

The expert move is to overlay the two levels. An anticyclone supported by a ridge means solid fine weather; the same anticyclone caught by the forward side of a trough means doomed fine weather. Five steps, ten minutes, one written sentence: you have your 48-hour trend, and you know when not to believe the weather icons. Finally, four situations come back again and again — westerly flow, bise (a cold, dry north-easterly wind), southerly flow with foehn, slack pressure gradient — and each has well-known consequences at launch and on the trail.

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. 5.1The surface chart: an IGN map of pressure
  2. 5.2Front symbols
  3. 5.3The shapes of the pressure landscape
  4. 5.4Reading a complete chart
  5. 5.5Aloft: the 500 hPa chart
  6. 5.6Troughs, ridges, cut-off lows
  7. 5.7Making the two levels talk
  8. 5.8Temperature charts: T850, thickness, T500
  9. 5.9Where to find the charts
  10. 5.10The method in five steps
  11. 5.11Typical situations in France and the Alps
  12. 5.12Key figures

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Chapter glossary#

Isobar
Line joining points of the same pressure reduced to sea level. The wind follows it, low pressure on the left; tight isobars signal a strong wind.
Geopotential height
Altitude, in metres or decametres, at which a given pressure is found (for example 500 hPa). High: warm air; low: cold air.
Height contour
Line of equal geopotential height on an upper-air chart. Read like an isobar: the upper wind follows it, low values on the left.
Trough
At the surface: a V-shaped valley of low pressure. Aloft: a southward wave in the flow, cold air. Its forward (eastern) side deepens surface lows.
Ridge (of high pressure)
At the surface: a U-shaped crest of high pressure. Aloft: a northward wave, warm air, subsidence; fine weather, and a heatwave if it becomes blocked.
Cut-off low
Pocket of cold air aloft detached from the main flow, spinning on itself, moving slowly and erratically. Scattered thunderstorms, unreliable forecasts, little signature at the surface.
Frontolysis, frontogenesis
Weakening or strengthening of a front. A front in frontolysis often arrives “empty”: a veil with no rain.
ANASYG, PRESYG
Météo-France’s graphical synoptic analysis and forecast: enhanced frontal charts (activity, jets, areas of significant weather).
Slack pressure gradient
Extensive area of almost uniform pressure: no synoptic wind, breezes in control, heat thunderstorms possible in summer.
Retour d’est (easterly return flow)
Moist easterly flow pushed against the French–Italian border by a Mediterranean low (Gulf of Genoa): cloud and precipitation held on the border ridges, fine weather further west. The corresponding easterly wind is also called the “lombarde”.
T850, T500
Temperature at 850 hPa (around 1,500 m) and at 500 hPa (around 5,500 m). The first describes the air mass and gives the freezing level; the difference between them indicates the instability.