Weather
London:
London is a year-round tourist center, with few of its attractions closing or significantly reducing their opening hours in winter. Your best chance of good weather is, of course, at the height of summer in July and August, but there's certainly no guarantee of sun even in those months - when tourist season is at its peak.
A typical London-area weather forecast for a summer day predicts "scattered clouds with sunny periods and showers, possibly heavy at times." Summer temperatures seldom rise above 78°F (25°C), nor do they drop below 35°F (2°C) in winter. London, being in one of the mildest parts of the country, is often quite pleasant in the spring and fall. Yes, it rains, but you'll rarely get a true downpour. Rains are heaviest in November, when the city averages 2 1/2 inches.
Paris:
The weather in Paris can be somewhat fickle. Rain is more common than snow throughout the winter. In recent years, Paris has had about 15 snow days a year, while there are only a few oppressively hot days (over 86°F, or 30°C) in summer. Other than occasional winds and rain, Paris offers some of the most pleasant weather of any capital in Europe, with an average temperature of 53°F (12°C).
As the old song says, Paris is at its best in springtime, even if it is sometimes a little wet. May, June, September and October are also terrific times to visit Paris. The weather's more pleasant than during the heat of summer, the French should be working, and everything is open and geared up for tourists. In addition, you'll avoid the peak crowds.
In winter, Paris has all sorts of cultural events going on, while in summer the weather is warm–occasionally even sizzling. In August, Parisians flee for the beaches to the west and south and many restaurateurs lock up and leave town too. But this is changing rapidly and you'll find considerably more places open in summer than even a decade ago. Things can get a bit hectic around Bastille Day and towards the end of the year so reservations at this time are a good idea.
Rome:
It's warm all over Italy in summer; it can be very hot in Rome. The high temperatures begin in Rome in May, often lasting until sometime in October. In Rome the weather is warm year-round, averaging 50°F (10°C) in winter. For the most part, it's drier in Italy than in North America, so high temperatures don't seem as bad because the humidity is much lower. In Rome, temperatures can stay in the 90s, but nights are usually cooler and much more comfortable. The average high temperatures in central Italy and Rome are 82°F (28°C) in June, 87°F (31°C) in July, and 86°F (30°C) in August; the average lows are 63°F (17°C) in June and 67°F (19°C) in July and August.
April to June and late September to October are the best months for traveling in Italy -- temperatures are usually mild and the crowds aren't quite so intense. Starting in mid-June, the summer rush really picks up, and from July to mid-September the country teems with visitors. From late October to Easter, most attractions go on shorter winter hours or are closed for renovation. Many hotels and restaurants take a month or two off between November and February, spa and beach destinations become padlocked ghost towns, and it can get much colder than you'd expect (it might even snow).
