Nepal splits cleanly into five climatic zones stacked by altitude, and the monsoon governs everything. The monsoon arrives from the southeast in mid-June, drenches the country through September, and retreats by early October. This pattern dictates the entire trekking calendar and explains why October and November concentrate half the year's visitors into eight weeks.
October and November deliver what trekkers actually want — stable weather, clear mountain views, moderate temperatures at elevation, and trails dry enough to walk without constant mud. Daytime temperatures in Kathmandu sit around 25°C, dropping to 10°C at night. In Pokhara the pattern holds similar. Above 3000 meters, days remain warm in sun but nights drop below freezing. The Annapurna and Everest regions see their clearest skies of the year during this window. Teahouses fill, permits peak, and flights to Lukla book out weeks ahead. The shoulder weeks in late September and early December offer nearly identical conditions with fewer people, though September trails can still be muddy from late monsoon and December mornings turn colder.
December through February constitutes winter. Kathmandu stays dry and pleasant during the day — 18°C to 20°C — but nights drop to 2°C or 3°C, and most buildings lack heating. Pokhara runs a few degrees warmer. Higher elevations become genuinely cold. Passes above 5000 meters like Thorong La close intermittently due to snow. The Khumbu still functions for trekking, but temperatures at Everest Base Camp drop to minus 20°C at night. The advantage: flawless visibility, empty trails, teahouses eager for business. Chitwan National Park becomes comfortable during winter — daytime temperatures around 23°C, no rain, animals concentrated around water sources as vegetation thins.
March and April form the spring season. Temperatures rise — Kathmandu reaches 28°C by April, Pokhara similar. Rhododendron forests across the middle hills explode into bloom, concentrated between 2000 and 3500 meters, peaking late March through mid-April. The downside arrives as heat builds: dust and pollution from India combine with local sources to create a persistent haze across the Terai and up into the Kathmandu Valley. Mountain views from lower elevations grow murky. By late April, afternoon thunderstorms begin signaling the approaching monsoon.
The monsoon itself, June through September, drops 80 percent of Nepal's annual rainfall. Kathmandu receives around 1400 millimeters total each year, with July alone accounting for 300 to 400 millimeters. Trails turn to mud, leeches emerge in forested areas below 2500 meters, and afternoon cloudbursts become routine. Two regions escape: Mustang and Dolpo sit in the rain shadow north of the main Himalayan spine and remain dry even through monsoon months, though reaching them requires long approaches or expensive flights. Chitwan floods partially and closes some areas, though the park stays open and wildlife viewing continues on elephant back and in canoes.