Beirut has a Mediterranean climate with hot, dry summers and mild, wet winters.
Spring (March-May) and autumn (September-November) are the most pleasant times to visit. Temperatures range from 18-25°C, the humidity is manageable, and you can comfortably walk around the city and take day trips without baking in the heat.
Summer (June-September) is hot and humid, with August temperatures reaching 30°C or higher and very little breeze. This is peak season for nightlife and beach clubs, and when the Lebanese diaspora returns for holidays. If you can handle the heat, the city is at its most lively.
Winter (December-February) brings cool temperatures (averaging 10-15°C), frequent rain and grey skies. It rarely freezes at sea level, but it is not beach weather. The upside is fewer crowds and lower prices.
Almost all of Beirut's rainfall -- about 860 mm annually -- falls between November and March. Summers are bone dry. Snow is very rare at sea level, though you can see it on the mountains behind the city.
The ideal time for a first visit is April-May or September-October: warm, dry, and with the city in full swing.