France has a wide variety of climates. From the mediterranean one to the continental one, you will encounter here many conditions for your flights. Nothing unusual for european pilots however.
We have, however, a few special features which you shoud be aware of, since they are of particular interest for airmen.
Low pressures on the gulf of Genova and high pressures on the
south west of France and Spain always result in these famous winds,
no matter which part of the year you consider. The Mistral comes
from the North in the Rhone valley. You will feel it on the coast
as East as Cannes and Nice. Montpellier, Nimes, Marseille, Aix
en Provence, Toulon, and further North Avignon, Valence are all
included in the Mistral area. The Tramontane blows from the West
in the Perpignan/Carcassonne/Montpellier/Nimes area.
They are at their strongest over the coast. They usually wipe the sky of any cloud. 50 knots at ground level with gusts are not uncommon for these winds, meaning sometimes rough rides with turbulence, especially if you have to fly over the Alps. Expect orowaves effects. Landing and take off may be tricky.
They can blow for several days. In Montelimar, almost one day out of two, you feel the Mistral.
An other wind, blowing from the South, in the South of France. It brings hot air from Africa and sand from the Sahara. The next day, you'll find your aircraft covered with a thin layer of sand. It blows over the French mediterranean coast.
If you come from an inland country, you might be surprised to discover that airports located near the sea or the ocean can be suddenly unusable because of mist coming from the sea, even if the wheather forecast was good and the sun shines anywhere else. Don't underestimate this phenomenon and be sure to have an alternate airport where to land. This is true even on the mediterranean coast. These mists can be really sudden and beat any wheather forecast.
Crossing the mediterranean sea to go, for exemple, in Corsica, may be tricky in summer. Because of haze, there is often no visible horizon at all; the crossing will have to be done using your instruments. Be prepared for this and have at least some instrument practise. You should know that because of this phenomenon, all the French flying clubs along the mediterranean coast request that their pilots go to Corsica with a FI on a misty day at least once before they are allowed to do it on their own.
Alsace is located along the Rhine valley, with the Vosges mountains West of it, and the Black Forest in Germany (Schwarzwald) East. The valley is really flat and in winter, it is not uncommon, when high pressures are located in the North of France, to have a thin layer of clouds above the valley, with the top of the mountains (which are not very high : 3000ft) emerging from the clouds. This will prevent you from landing or taking off in alsacian airports unless you're instrument rated, even if the wheather is sunny everywhere else in the vicinity. Check the aeronautical forecasts.