Map from Google - France

Albeit in a temperate zone, in France there are different climates: oceanic in the west and northern coast, transitional between oceanic and continental in Paris and the central regions, continental in the eastern regions, and Mediterranean in southern coastal regions. In addition, there are of course the mountainous regions, with a colder climate.

On the Atlantic coast, the climate is generally cool and wet, rainy and windy all year round, but especially in autumn and winter.
In the north, the coasts of the English Channel are generally colder than the western Atlantic coasts. In these areas (Upper and Lower Normandy, see Mont Saint-Michel), the weather is dull and wet, cool but not cold in winter (except when it's reached by the currents from Eastern Europe), and cool in summer: the average temperature in Dunkirk goes from 5 °C (41 °F) in January, to 18.5 °C (65.5 °F) in August.
Further west, in Brittany, the climate is even more influenced by the Atlantic Ocean: so in Brest the average temperature increases in January and February to 6.5 °C (43.5 °F), while in July and August it doesn't go above 17 °C (63 °F). The islands of Brittany (Bréhat, Belle Île, Ouessant) have a very mild climate: the temperature almost never drops below freezing (0 °C or 32 °F), so that palm trees and tropical species can grow; in summer, however, it never touches 30 °C (86 °F).
Here are the average temperatures of Brest.
Average temperatures - Brest
BrestJanFebMarAprMayJunJulAugSepOctNovDec
BrestJFMAMJJASOND
Min (°C)4456911131311975
Max (°C)9911131618202119151210
Min (°F)393941434852555552484541
Max (°F)484852556164687066595450

The rains are frequent throughout the year, and abundant from September to March, but frosts and snowfalls, at least, are rare.
Here is the average precipitation in Brest.
Average precipitation - Brest
BrestJanFebMarAprMayJunJulAugSepOctNovDecYear
BrestJFMAMJJASONDY
Prec. (mm)1401151008075555060901201201401145
Prec.(in)5.54.53.93.132.222.43.54.74.75.545.1
Days181515121199911151617156

The temperature of the ocean is very cool still in summer, and is not the best for swimming: it reaches 17 °C (63 °F) in August in Brest, and 18 °C (64 °F) in Saint Malo.
Sea temperature - Brest
BrestJanFebMarAprMayJunJulAugSepOctNovDec
BrestJFMAMJJASOND
Sea (°C)111010111214171717151312
Sea (°F)525050525457636363595554

Moving south along the Atlantic coast, summer becomes gradually milder: in Nantes the average temperature in July reaches 18.5 °C (65.5 °F). Further south, summer is quite warm, since the south-central area of France is often protected by the Azores Anticyclone. Hence, in Bordeaux, Aquitaine, the average temperature goes from 6.5 °C (43.5 °F) in January, to 21.5 °C (70.5 °F) in July and August. During summer this area can experience cool and rainy days caused by the Atlantic fronts, but also hot and sunny days, with temperatures around 33/34 °C (91/93 °F) and above. In autumn and winter, as well as rains, there may be strong winds, but this region is also exposed to the cold wind from the north-east, the (Bise), so it is more prone to frost than Brittany.
Average temperatures - Bordeaux
BordeauxJanFebMarAprMayJunJulAugSepOctNovDec
BordeauxJFMAMJJASOND
Min (°C)335711141616131064
Max (°C)101215172124272724191411
Min (°F)373741455257616155504339
Max (°F)505459637075818175665752

The sea temperature in Aquitaine becomes a bit milder, so that it reaches 21 °C (70 °F) in August. In the southernmost part of the coast (see Biarritz), it reaches 21 °C (70 °F) in July and 22 °C (72 °F) in August.
Sea temperature - Bordeaux
BordeauxJanFebMarAprMayJunJulAugSepOctNovDec
BordeauxJFMAMJJASOND
Sea (°C)121111121417202119171513
Sea (°F)545252545763687066635955

Paris

The region of Paris is located between the cool area of the Atlantic coasts, and the more continental areas on the border with Germany. The average temperatures are quite cool: in January the daily average is 4.5 °C (40 °F), while in July it's 20.5 °C (69 °F). However, Paris is located in a continental area, that can be heated in summer by the Azores High, which brings subtropical air masses, therefore it can be hot in summer, with temperatures about 32/33 °C (90/91 °F) or above; in addition, during winter the city is exposed to cold spells from Eastern Europe, so that there may be some icy or snowy days, with night temperatures about -6/-8 °C (18/21 °F), or even below.
Anyway, usually the westerlies prevail, so the weather is unstable, with many wet and rainy days, cool in summer and mild in winter. The most intense Atlantic low pressure areas, can bring strong winds from the ocean, especially in winter.
Average temperatures - Paris
ParisJanFebMarAprMayJunJulAugSepOctNovDec
ParisJFMAMJJASOND
Min (°C)22571013151512953
Max (°C)781216202325252116118
Min (°F)363641455055595954484137
Max (°F)454654616873777770615246

Rainfall in Paris is lower than in the Atlantic coast, since it amounts to less than 640 mm (25 in) per year; however, there is no really dry season. Here is the average precipitation.
Average precipitation - Paris
ParisJanFebMarAprMayJunJulAugSepOctNovDecYear
ParisJFMAMJJASONDY
Prec. (mm)504050506550605550605060640
Prec.(in)21.6222.622.42.222.422.425.2
Days10910910987891011110

In Le Mans, which hosts the famous car race of the "24 Hours", the climate is similar to that of Paris.

North-east of Paris, in the region on the border with Belgium (Nord-Pas de Calais), the climate, apart from the coastal area of which we have already spoken, is similar to that of Paris but a bit cooler: in Lille, the average temperature goes from 2.5 °C (36.5 °F) in January, to 18 °C (64 °F) in July.

The eastern regions, on the border with Germany, have a more continental climate. In Nancy, Lorraine, the average goes from 1 °C (34 °F) in January, to 18 °C (64 °F) in July. Alsace, lying on the Rhine corridor, has a "German" climate, characterized by cold and gray winters, accompanied by frequent snowfalls, although generally not abundant, and warm summers, sometimes hot, with some chance of thunderstorms. Even in Strasbourg, the average temperature in January is 1.5 °C (34.5 °F), while that of July is 19.5 °C (67 °F). Further south, in Mulhouse, which is sheltered from the mild westerlies by the Vosges, the average in January is around freezing and snowfalls are frequent.
Average temperatures - Strasbourg
StrasbourgJanFebMarAprMayJunJulAugSepOctNovDec
StrasbourgJFMAMJJASOND
Min (°C)-2-12591214131072-1
Max (°C)46111519232524211585
Min (°F)283036414854575550453630
Max (°F)394352596673777570594641

Moving south, in east-central France, the climate is moderately continental, in cities like Dijon, Burgundy, and Lyon, Rhone-Alpes, where the average goes from 3 °C (37.5 °F) in January to 21 °C (70 °F) July. In these region, cold spells with snowfalls are possible in winter, as well as heat waves in summer.
Average temperatures - Lyon
LyonJanFebMarAprMayJunJulAugSepOctNovDec
LyonJFMAMJJASOND
Min (°C)01361013161512842
Max (°C)681215202427272217107
Min (°F)323437435055615954463936
Max (°F)434654596875818172635045

In the mountainous regions (Alps, Pyrenees, Jura, Massif Central) the climate varies mainly according to altitude: it becomes obviously colder with increasing altitude. In summer, there are frequent thunderstorms in the afternoon. In Chamonix (Haute-Savoie, north of Mont Blanc), the average temperature in January and July are respectively -2 °C (28 °F) and 16 °C (61 °F); in Lus la Croix-Haute, in the Isère Department (western side), they are respectively -1 °C (30 °F) and 16.5 °C (61.5 °F).
Average temperatures - Chamonix
ChamonixJanFebMarAprMayJunJulAugSepOctNovDec
ChamonixJFMAMJJASOND
Min (°C)-6-5-2258101084-1-4
Max (°C)2381216202222181373
Min (°F)212328364146505046393025
Max (°F)363746546168727264554537

In Chamonix, as usually happens in the mountains, summer is a rainy season, because of afternoon thunderstorms. Here is the average precipitation.
Average precipitation - Chamonix
ChamonixJanFebMarAprMayJunJulAugSepOctNovDecYear
ChamonixJFMAMJJASONDY
Prec. (mm)958585901201301151251001151001101275
Prec.(in)3.73.33.33.54.75.14.54.93.94.53.94.350.2
Days10810101413121210101010128

Lourdes, at the foot of the Pyrenees, in the Hautes-Pyrenees Department, at 400 metres (1,300 feet) above sea level, is a bit cold in winter (it's usually above freezing, but in colder periods it can snow), while summer is warm or even hot during the day, but nights are usually cool.
Average temperatures - Lourdes
LourdesJanFebMarAprMayJunJulAugSepOctNovDec
LourdesJFMAMJJASOND
Min (°C)0135912141411831
Max (°C)9101415192323242218129
Min (°F)323437414854575752463734
Max (°F)485057596673737572645448

The regions of France bordering the Mediterranean Sea (south coast and Corsica) have a Mediterranean climate, with mild, rainy winters and hot, dry summers. It's better, however, to distinguish between the different zones.
The sheltered areas like the Côte d'Azur (see Saint Tropez, Cannes, Nice) have a particularly mild climate, similar to the Italian Riviera. Generally, as is the case in Italy, the rainiest season is autumn, followed by winter.
Average temperatures - Nice
NiceJanFebMarAprMayJunJulAugSepOctNovDec
NiceJFMAMJJASOND
Min (°C)5681014172020171396
Max (°C)131315172024272724211614
Min (°F)414346505763686863554843
Max (°F)555559636875818175706157

Rainfall along the southern coast follows a Mediterranean pattern, with a maximum in autumn and a minimum in summer. Here is the average precipitation in Nice.
Average precipitation - Nice
NiceJanFebMarAprMayJunJulAugSepOctNovDecYear
NiceJFMAMJJASONDY
Prec. (mm)70454070453510207213510595740
Prec.(in)2.81.81.62.81.81.40.40.82.85.34.13.729.1
Days65575422577661

The temperature of the Mediterranean Sea is higher than that of the Atlantic Ocean, especially in the French Riviera (Côte d'Azur), where it reaches 23 °C (73 °F) in August, however, it's a bit lower compared with other Mediterranean countries.
Sea temperature - Nice
NiceJanFebMarAprMayJunJulAugSepOctNovDec
NiceJFMAMJJASOND
Sea (°C)131313141619222321191614
Sea (°F)555555576166727370666157

On the other hand, west of this region there's an annoying wind, called Mistral, which often blows in the Rhone Valley, south of Valence, while the Tramontane blows over Languedoc-Roussillon. These dry and cold winds blow for about one hundred days per year, and can easily reach 80/100 kph. So, cities like Nimes, Arles, Avignon, Marseille have an average winter temperature a bit lower than the Cote d'Azur, around 6/7 °C (43/45 °F), instead of 8/9 °C (46/48 °F), and besides, they are exposed to sudden drops in temperature, and even frosts during the cold season. However, summer is hot and sunny here as well, because cold winds usually give a break in the warm season.
Even the sea temperature, in this part of the coast is slightly cooler than in the French Riviera (about 1/2 degrees Celsius).
The western area of the Mediterranean coast, overlooking the Gulf of Lion, has mild winters, but is also exposed to the north-west winds, which are channeled through the so-called gates of Carcassonne. In Perpignan, the average temperature is 8.5 °C (47.5 °F) in January and 24 °C (75 °F) in July. Further to the north-west and away from the sea, in Toulouse, the temperature is a bit lower, ranging from 6 °C (43 °F) in January to 22 °C (72 °F) in July.
The island of Corsica has a Mediterranean climate on the coasts, with mild and wet winters, and warm or hot and sunny summers, even though it's exposed to the Mistal, which can be strong, especially on the western and southern part of the island. In the interior, there are steep and high mountains, so that the landscape becomes quickly greener, and the weather gets colder with increasing altitude, up to the higher altitudes, where there's a typical mountain climate, and winters are characterized by abundant snowfall.
Here are the average temperatures of Ajaccio, the main city, located on the west coast.
Average temperatures - Ajaccio
AjaccioJanFebMarAprMayJunJulAugSepOctNovDec
AjaccioJFMAMJJASOND
Min (°C)445711141717151285
Max (°C)141415172125282826221715
Min (°F)393941455257636359544641
Max (°F)575759637077828279726359

Corsica is the French region where the sea reaches the highest temperature, ie, 24 °C (75 °F) in August.
Sea temperature - Ajaccio
AjaccioJanFebMarAprMayJunJulAugSepOctNovDec
AjaccioJFMAMJJASOND
Sea (°C)141414141720232423201815
Sea (°F)575757576368737573686459

When to go

The best time to visit Paris goes from late spring (mid-May to late June) to early autumn (September). The mid-summer period (July-August) is a good time, but sometimes it can get hot. The same can be said in general for all the inland areas of France.

To visit Brittany, Normandy and the Atlantic coastal areas, the best time is the summer (June to August).

To visit the Mediterranean regions, the best times are spring and autumn, especially the months of May and June and the first half of September (in the second half of September the temperature is still pleasant, but the rains may become abundant, even with the risk of floods). In mid-summer (July-August), however, the heat is usually bearable, because it's humid but also tempered by breezes, at least along the coast.

For swimming and sunbathing, the Mediterranean Sea (where the best time is July and August) is preferable to the Atlantic Ocean (especially the northern part of the latter, which is cooler), because the temperatures of both the air and the sea are higher, and sunshine is also more frequent.

For skiing, late winter and early spring (February-March) are preferable, because the days are longer than in December and January. In some locations at high altitudes, there are also summer ski resorts.

To visit the mountain regions, summer is a good time, even though there may be some thunderstorms in the afternoon.

What to pack

In winter: warm clothing, sweater, coat, rain jacket or umbrella; scarf, hat and gloves for the colder north-central area and the mountains.

In summer: for Brittany and Normandy, clothes for spring and autumn, t-shirt for hot days, jacket, raincoat or umbrella. For the rest of the country, light clothing, but also long pants, jacket and sweater for the evening and cooler days; raincoat or umbrella. For the mountains, hiking shoes, sunscreen and sunglasses.