SEASONS: Paris has a four-season year. Spring (mid-March through mid-May) brings blossoms in the parks, and an alternation between warm spells and wintry weather. Summer (mid-May through mid-September) features full foliage on the trees and long warm days. Autumn (mid-September through mid-November) finds the leaves turning color and dropping, and the first frosts. Winter (mid-November through mid-March) brings short cold days, snow and fog, and bare branches in the parks.
SUNNIEST MONTHS: May through August. This is when you can expect to get the sunniest weather of the year. In general, 40% of the year's daylight hours will be sunny--from a high of 50% in August, to a low of 19% in December. September will get the most clear days--about one day a week. December and January are quite cloudy. During these months, two or three days each week will see no sunshine at all.
WARMEST MONTHS: Late June through the end of August is the warmest time of year. At this time, afternoons are usually mild to warm, and only rarely hot. The short nights tend to be cool to mild, and only infrequently chilly. The hottest temperature of the year will be around 93░, and will probably occur during a July hot spell.
Throughout this warm season, only a few nights will be hot enough that you will sleep better with some sort of room-cooling. Usually, an open window is sufficient--especially with some cross-ventilation.
COOLEST MONTHS: The middle of November through the end of March usually brings the worst cold weather. At this time of year, you can expect frosty mornings on two or three days out of the week. Afternoon temperatures will remain below freezing on only six or seven days during the entire winter--days when Arctic air holds the entire continent in its ice grip. The coldest temperature of the year will be around 17░, and will probably occur during one of these Arctic outbreaks, anytime from December through February.
Snow is fairly common during the winter months, but rarely stays on the ground for long in the city proper. You're far more likely to find it in the suburbs and surrounding countryside. Even during the coldest months, rain is more likely in Paris than snow.
DRIEST MONTHS: June through September, the summer, brings your best chances of staying dry. For the year as a whole, about four days a week will get no measurable precipitation; that is, they will get less than a hundredth of an inch. Glorious June will have 62% such days, whereas gloomy December will get only 47%. A "dry day" in the table, however, is one with less than a tenth of an inch--a more useful measure. It takes at least that much to wet the ground under the trees.
THINGS TO KNOW: The early morning fogs and mists which characterize the cooler months are most common along the Seine and in the lower-lying neighborhoods. They are most common under clear nighttime skies, with very light winds. With no wind at all, you get dews or hoarfrosts instead.
These descriptions (and the table) apply best to the city center, which will often have a distinct "urban heat island" effect. Under windless and rainless conditions (or near), the city center will be many degrees warmer than the outlying suburbs. This is especially noticeable at night and in the winter. Precipitation of any kind diminishes this effect, and a strong wind will eliminate it completely.