Beijing is renowned for its pleasant springs (65-75 F/18-24 C) and brisk falls (50s F/10-15 C). Summers are hot, with temperatures ranging 85-95 F/30-35 C. Persistent smog can hamper views on particularly hot days. Winters can be extremely cold and dry, especially when Siberian winds blow in from the north; expect frequent temperatures below freezing.
Beijing's rainy season is June-July. Be prepared for dust and sand storms at almost any time, but especially in early spring. September-October—except for the first week of October, which is a national, week-long holiday—is the best time to visit Beijing, hands-down, as blue skies, crisp air and fewer crowds permeate the city. Early spring is another good time to visit, when the winds blow over the city and the weather is still cool.