Climate zones and USDA zones in France

Map of climates in France + USDA zones

USDA hardiness zones

The USDA zone concept is American, it was created by the U.S. Department of Agriculture (hence the USDA designation). It is a geographical division into 26 isothermal zones based on the average of minimum temperatures over the last 20 years. Concretely, these indicators translate into the “probable minimum temperature” your region will experience in winter, which is fundamental when discussing hardiness!

This way of approaching hardiness zones is used in many countries, including France. USDA zones have the advantage of being fairly precise and widely used worldwide. The scale comprises 26 levels but in France only 10 are used, from zone 6a to zone 10b.

Summary table :

Type 1: mountain climates - USDA zone 6a

Mountain climates are quite diverse and found across almost all of France: Pyrenees, Massif Central, Alps, Jura, Morvan, Ardennes but also, at lower altitudes, short stems of bulb to the east of Champagne and part of Lorraine and Franche-Comté…

In these geographic areas, one finds:

• a high number of days and a high cumulative precipitation (rain, snow…)
• an average temperature below 9.4°C with more than 25 days during which minimum temperature was below -5°C and fewer than 4 days with a maximum above 30°C.
• Interannual variability of July precipitation and of winter and summer temperatures is maximal.

→ This climate corresponds to USDA zone 6a (observed minimum temperatures: -23.3 to -20.6°C)

Type 2: semi-continental climate and climate of montane margins - USDA Zone 6b

Climate type 2 is an intermediate climate between montane climates and climate types 3, 4 and 8. The area concerned by this climate includes montane fringes and extends over large areas in Burgundy, Lorraine and Alsace.

In these areas:

• temperatures are less cold than in mountain areas (but, at equal altitude, colder than elsewhere),
• precipitation is slightly lower and less frequent,
• there is a low ratio between autumn and summer precipitation

→ This climate corresponds to USDA zone 6b (observed minimum temperatures: -20.6 to -17.8°C)

Type 3: degraded oceanic climate of the central and northern plains - USDA zone 7a

Climate type 3 covers the whole Paris Basin with an extension to the south (middle Loire Valley, northern Massif Central and Saône Valley).

The climate remains oceanic but with marked contrasts:

• temperatures are intermediate (around 11°C mean annual temperature, with between 8 and 14 days with temperatures below −5°C).
• precipitation is low (less than 700 mm annual total), especially in summer, but rain falls on average on 12 days in January and on 8 days in July, values averaged for France as a whole.
• interannual variability of precipitation is minimal while that of temperature is high.

→ This climate corresponds to USDA zone 7a (observed minimum temperatures: −17.8 to −15°C)

Type 4: Altered oceanic climate - USDA zone 7b

Climate type 4, altered oceanic climate appears as a transition between true oceanic (type 5) and degraded oceanic (type 3). Between Nord-Pas-de-Calais and Normandy, it forms a narrow band while to the west this transition widens to reach more than 150 km. This climate also affects the south-west of the Massif Central, from Dordogne to Aveyron, and north of the Pyrenees.

Characteristics :

• Mean annual temperature is fairly high (12.5°C) with a low number of cold days (between 4 and 8 per year) and a sustained number of hot days (between 15 and 23 per year).
• Annual thermal amplitude (July–January) is close to the minimum and interannual variability is average.
• Total annual precipitation (800–900 mm) falls mainly in winter; summers are fairly dry.

→ This climate corresponds to USDA zone 7b (observed minimum temperatures: -15 to -12.2°C)

Type 5: Oceanic climate - USDA zones 8a and 8b

Oceanic climate occupies a thin strip along the North Sea and the whole of Normandy, Brittany, Vendée and Charentes. A reduced oceanic area occupies the west of the heathlands and Pyrénées-Atlantiques.

In these areas :

• temperatures are moderate and very homothermal: annual amplitude (less than 13°C difference between July and January), number of cold days (fewer than 4) and hot days (fewer than 4) and interannual variability are minimal.
• Annual precipitation is abundant (a little over 1000 mm) and frequent in winter (more than 13 days in January).
• summer is also rainy (8-9 days in July) but totals are low.
• Finally, oceanic climate is characterised by strong interannual variability of winter precipitation.

→ This climate corresponds to USDA zones 8a and 8b (observed minimum temperatures: -12.2 °C to -6.7 °C)

Type 6: Altered Mediterranean climate - USDA zone 9a

Altered Mediterranean climate extends mainly over the southern Alps and Pre-Alps, encompassing most of the two departments Alpes-de-Haute-Provence and Drôme. Small remnants are also identifiable on the left bank of the Rhône, at the level of Ardèche and in the form of a narrow strip to the west, between Pyrénées-Orientales and Hérault.

The characteristics :

• annual mean temperature is high, with a small number of cold days and hot days numbering between 15 and 23 per year.
• low interannual variability of July minimum temperatures: summer is consistently hot from year to year.
• mean annual precipitation total (800–950 mm) but not distributed homogeneously.
• autumn and winter, wet and highly variable from year to year, are opposite to summer, dry and stable relative to the 1971–2000 norm.

→ This climate corresponds to USDA zone 9a (observed minimum temperatures: -6.7 to -3.9°C)

Type 7: Climate of South West Basin - USDA zone 9b

This type of climate applies to a geographically disparate area, straddling several regions (Aquitaine, Languedoc) and centred on the middle Garonne basin, referred to for convenience as “South-West Basin”.

It is characterised by:

• a high annual mean temperature (above 13°C) and a high number (> 23) of hot days, while days with frost below -5°C are rare.
• a high annual temperature range (15 to 16°C) and low interannual variability of winter and summer temperatures.
• precipitation is scarce in annual total (less than 800 mm) and in winter, but is somewhat higher during summer. It is more frequent in winter (9-11 days) than in summer (fewer than 6 days). This distribution indicates precipitation intensity is low in winter (oceanic rainfall) and higher in summer (thunderstorm disturbances from Spain or Bay of Biscay). Interannual variability of precipitation is moderate.

→ This climate corresponds to USDA zone 9b (observed minimum temperatures: -3.9 to -1.1°C)

Type 8: True Mediterranean climate - USDA zones 10a and 10b

This climate occupies a strip roughly 100 kilometres wide around the Mediterranean Sea, from the Pyrenees to the Var. Beyond that, in the Maritime Alps, the Mediterranean climate narrows so much that it ends up appearing only sporadically within alpine valleys.

The climatic characters are very sharply defined, more so than in any of the seven previous climates.

• Annual temperatures are high, cold days are extremely rare and hot days frequent.
• The interannual amplitude is large (> 17°C between July and January) while these characters are very stable from one year to the next.
• The very high ratio between autumn precipitation and summer precipitation (> 6) is the main character of this climate. The annual precipitation total is low, with an arid summer but a fairly wet winter despite a low number of rainy days. These characters are also stable from year to year.

→ This climate corresponds to USDA zones 10a and 10b (observed minimum temperatures: -1°C to +4.4°C)

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