Autumn, October and November: the clear months
The monsoon withdraws through late September and takes the haze with it. What follows is the reason Nepal's trekking season exists. Air quality is at its annual best, rainfall is close to nil, and the Himalayan skyline from the Chandragiri deck is visible on most mornings, running from Annapurna in the west through Manaslu, Ganesh Himal and Langtang to Dorje Lakpa. Daytime temperatures of 23 to 26 degrees on the valley floor translate to something like 16 to 19 degrees at the summit, which is comfortable in a fleece.
The cost is company. October carries Dashain, the largest festival of the Nepali year, and November carries Tihar and the Thankot Mahalaxmi jatra, the street festival built around a chariot procession. Domestic visitor numbers at Chandragiri peak in this window. Weekends and public holidays produce real queues at the base station. Go on a weekday, and go early.
Winter, December to February: the sharpest views and the cold
Cold dry air is transparent air. The single clearest views of the Himalaya from Chandragiri come on winter mornings, and this is the only season in which Everest is realistically visible from the ridge, as a small distant tooth on the eastern horizon rather than the dominant feature people expect. Valley fog is common at dawn in December and January, which produces the specific pleasure of standing in sunlight at 2,551 m looking down onto a sea of cloud with Kathmandu somewhere underneath it.
Plan for cold. Overnight lows on the valley floor sit around 3 to 5 degrees in January, and the summit runs 6 to 8 degrees below that with wind across the open ground by the temple. Visitors regularly arrive in a t-shirt after a warm morning in the city and turn straight round. Shivaratri, the major night in the Mahadev calendar and a significant occasion at Bhaleshwor Mahadev, usually falls in late February or early March.
Spring, March to May: rhododendron, then haze
March and early April are the best walking weeks of the year. The Chandragiri forest carries rhododendron, Nepal's national flower, through this window, and the trail from Thankot up to the ridge is at its most rewarding: warm enough to be pleasant, cool enough to climb 1,000 m without suffering. Mid-April brings the Nepali new year and Matatirtha Aunsi, the new-moon day on which Nepalis honour their mothers, observed at the Matatirtha pond just north of Thankot.
Against that, visibility degrades steadily. Dust, agricultural burning and pre-monsoon heat build a haze layer over the valley that thickens through April and dominates May. By May the mountains are a morning-only proposition at best, afternoon thunderstorms are frequent, and the valley floor is running at 29 degrees. May is the month we would most readily skip.
Monsoon, June to September: green, wet and cloud-bound
Be honest with yourself about what you are buying. From mid-June to early September the Chandragiri ridge is frequently inside cloud, and the mountain view that justifies a foreign visitor cable car ticket may simply not exist on the day you go. July is the wettest month of the year. Trails are slick, leeches are active on the forest sections between Thankot and the ridge, and the Rajpath west of Nagdhunga carries a real landslide risk after heavy rain.
There is another side. The terraces below the hill are their most vivid green of the year, the trails are empty, domestic tourism drops away and the cable car queue disappears. Clearances after heavy rain can be spectacular and brief. If you are already in Kathmandu during monsoon, watch the sky: if you can see the surrounding hills clearly from the Ring Road in the morning, the trip is on. If you cannot, spend the money elsewhere.