The Vosges rain shadow: how dry Alsace really is, and why

How the Vosges keep the rain off the vineyards

The Alsace vineyards follow the eastern foothills of the Vosges, between the mountains and the Rhine plain. The dominant winds blow from the west and bring oceanic air. These westerlies discharge their rain on the western slopes of the Vosges and then descend, dried, over the vineyards: the mountains shelter Alsace from oceanic influences.

Meteorologists call the result a rain shadow: an area of reduced precipitation on the lee side of a mountain barrier, where the descending air warms and clouds dissipate. In Alsace the barrier is the Vosges, and the lee side is the east, where the vines grow.

The climate of the vineyard is continental, sunny, warm and dry, with one of the lowest rainfalls in France: 500 to 600 mm of water a year (CIVA). The block diagram below follows the air from the western slopes to the vineyards and shows the rift valley beneath them.

How dry is dry? Colmar, Strasbourg and Paris compared

Climate normals give a firmer measure than adjectives. Météo-France publishes 30-year averages for its stations. For 1991–2020, the normal annual precipitation is 595.0 mm at Colmar-Meyenheim, 635.7 mm at Strasbourg-Entzheim and 634.3 mm at Paris-Montsouris.

Colmar-Meyenheim is the driest of the three and falls within the 500 to 600 mm range that CIVA gives for the vineyard. Yet its gap with Paris is 39.3 mm, about 6 per cent of the Paris total. The claim, sometimes repeated, that Alsace receives less than half the rain of Paris does not match these normals. Strasbourg-Entzheim even receives slightly more than Paris-Montsouris.

A normal describes the one place where the rain gauge stands, not every slope around it. These values are best read as an order of magnitude for the area.

When the difference appears: the cold half of the year

The monthly normals show when Colmar is drier. From November to April, Colmar-Meyenheim receives less rain than Paris-Montsouris in every month; from May to October, it receives more in every month. Added up, Colmar collects 227.3 mm from November to April against 296.7 mm in Paris, and 367.7 mm from May to October against 337.6 mm.

February is the driest month at both stations (30.6 mm at Colmar, 41.8 mm at Paris) and May the wettest (69.8 mm and 69.0 mm). Colmar's lower annual total therefore comes from the cold half of the year; its wetter warm half offsets part of that deficit.

This matters when reading claims about ripening. Rainfall alone does not show that autumn is drier in Colmar than in Paris: the September and October normals are 51.3 mm and 56.8 mm at Colmar, against 44.7 mm and 55.2 mm at Paris. The amount of rain says nothing by itself about sunshine, humidity or disease pressure, which need their own measurements.

The rift beneath the vineyard

The relief that creates the rain shadow is also the edge of a rift. In the Upper Rhine Graben the centre has sunk to form the Rhine plain, while its shoulders, the Vosges to the west and the Black Forest to the east, stand raised relative to it (Planet-Terre, ENS de Lyon). The vineyards grow on the sub-Vosges hills, a field of fractures between the Vosges fault to the west and the Rhine fault to the east.

This shaped the soils. About 150 million years ago the sea covered what is now the Rhine rift valley, and sedimentary rocks such as sandstone, limestone and marl settled on the granite basement. On the Alsace side three units stand out: the Vosges mountains (granite and sandstone, sometimes shale), the sub-Vosges hills with their great diversity of soils, and the Rhine alluvial plain (marl and alluvium). Four fracture fields, Saverne, Ribeauvillé, Rouffach-Guebwiller and Thann, break up the wine-growing area further.

The result is a mosaic. Most wine villages stand on four or five different formations, often in very small adjoining parcels, across a vineyard of about 15,500 hectares. Much of the diversity of Alsace wines comes from this patchwork of soils (CIVA).

Two grands crus, two rocks: Brand and Rangen

Rock, aspect and altitude combine on a single site. The Brand at Turckheim rests on granite, a rock of quartz, feldspar and mica that gives a sandy, coarse soil. It faces south-south-east and climbs to 380 metres. This subsoil warms the root zone, which favours good ripening and the breakdown of malic acid (CIVA).

The Rangen, at Thann and Vieux-Thann, is a steep, south-facing slope on volcanic and sedimentary rocks. The two sites illustrate the mosaic described above: within the same rain-shadow climate, granite at Turckheim, volcanic and sedimentary rocks at Thann.

Exercise 1: the rain shadow in numbers

Question
Explain how the Vosges create a rain shadow over the Alsace vineyards, then use the Météo-France normals for 1991–2020 to assess the claim that Alsace receives less than half the annual rainfall of Paris.

Model answer
The dominant westerly winds bring oceanic air towards the Vosges. They discharge their rain on the western slopes and descend, dried and warmed, over the vineyards on the eastern foothills, an area of reduced precipitation on the lee side of the barrier. The vineyard's rainfall is 500 to 600 mm a year (CIVA). The normals give 595.0 mm at Colmar-Meyenheim against 634.3 mm at Paris-Montsouris: a gap of 39.3 mm, or about 6 per cent. The claim is therefore wrong: Colmar is drier than Paris, but not by half. The monthly normals show that the difference arises from November to April.

Mark scheme (10 marks)
- Westerly winds, rain on the western slopes, dried air descending to the east (3 marks)
- Correct use of the 1991–2020 normals for Colmar-Meyenheim and Paris-Montsouris (4 marks)
- A justified conclusion that the claim is wrong, noting that the difference lies in the cold half of the year (3 marks)

Exercise 2: reading the soil mosaic

Question
Describe how the Upper Rhine Graben produced the soil mosaic of the Alsace vineyard, and contrast the rocks of the Brand and Rangen grands crus.

Model answer
The Rhine plain is the sunken centre of a rift whose raised shoulders are the Vosges and the Black Forest. The vineyards lie on the sub-Vosges hills, a field of fractures between the Vosges fault and the Rhine fault, where sedimentary rocks (sandstone, limestone, marl) laid down on the granite basement are cut up by faults. Four fracture fields divide the area further, so that most villages stand on four or five formations. The Brand at Turckheim is granite of quartz, feldspar and mica giving a sandy, coarse soil, facing south-south-east and rising to 380 metres; the Rangen at Thann and Vieux-Thann is a steep south-facing slope on volcanic and sedimentary rocks.

Mark scheme (10 marks)
- The rift: sunken plain, raised Vosges and Black Forest, the two bounding faults (4 marks)
- The mosaic: sedimentary cover over granite, fracture fields, four or five formations per village (3 marks)
- An accurate contrast of Brand and Rangen (rock, aspect) (3 marks)