Afghanistan has a continental climate with extreme variations. Summers are scorching in the lowlands (40C+ in Jalalabad and Kandahar) and pleasant in the mountains. Winters bring heavy snow to the Hindu Kush, closing mountain passes and making travel in the highlands impossible.
Spring (April-May) is the best time. Temperatures are mild, the landscape turns green, and mountain passes begin to open. The almond blossoms around Jalalabad are famous.
Autumn (September-October) is equally good. Clear skies, warm days and cool nights. The harvest season means fresh fruit everywhere -- Afghanistan's pomegranates, grapes and melons are superb.
Summer (June-August) is brutally hot in the south and east, though the highlands around Bamian and the Wakhan Corridor remain comfortable. Dust storms are common in the lowlands.
Winter (November-March) is harsh. The Salang Pass closes regularly, cutting Kabul off from the north. Temperatures in Kabul drop well below freezing. The central highlands are snowbound. Only the lowlands around Jalalabad remain mild.
Ramadan affects travel significantly -- restaurants close during daylight hours, and the pace of life slows. The dates shift annually according to the Islamic calendar.