Wind and weather in the Kali Gandaki valley

The Kali Gandaki is older than the mountains around it. As the Himalaya rose, the river kept cutting down, and today it flows south from the Tibetan side of the range straight between two peaks over 8,000 m, Dhaulagiri to the west and Annapurna I to the east. Measured from those summits to the river, the gorge is more than 5,000 m deep, and it is often called one of the deepest in the world.

That geography shapes everything about the weather. South of the main range, around Tatopani and Ghasa, monsoon clouds pile up against the mountains and drop heavy rain. North of it, around Jomsom, Kagbeni and Lo Manthang, the same clouds have already lost most of their moisture. In a few hours' drive you move from banana trees and bamboo to a high desert of bare cliffs and irrigated fields.

Anyone who spends a day in Jomsom learns the rhythm. Mornings are still and clear. Somewhere between nine and eleven o'clock, a breeze starts from the south. By noon it is a strong, steady wind blowing up the valley, carrying dust and grit, and it often keeps going until after sunset before dying away at night.

The cause is the sun. Through the morning, the dry slopes and plateau north of the main range heat up quickly, and the warm air over them rises. Air is drawn in from the south to replace it, and the only low route through the mountains is the Kali Gandaki gorge. Squeezed between Dhaulagiri and Annapurna, the flow speeds up, a little like water through a narrow pipe. Meteorologists have studied it for decades as a textbook example of a strong daily valley wind. At night the pattern weakens or reverses, and the valley falls calm again.

The wind is not dangerous in itself, but it shapes every plan in Mustang. Small planes cannot land safely at Jomsom once it rises, so all flights from Pokhara operate early in the morning, and even a short delay can mean cancellation for the day. Walkers in the valley set off soon after sunrise and try to finish by early afternoon, when walking into the wind becomes tiring and the dust stings.

The wind also makes Mustang feel colder than the thermometer says. A sunny 15 °C afternoon in Kagbeni can feel close to freezing in the open. Plan clothing for wind, not just for temperature.

• Book the first flight of the day to Jomsom where possible
• Keep at least one spare day for flight cancellations
• Start walking or driving by 7 or 8 am
• Carry a windproof jacket, a buff or scarf, and sunglasses
• Use lip balm and moisturiser: the air is very dry
• Change camera lenses indoors or in a vehicle

Pokhara, on the southern side of the Annapurnas, is one of the wettest places in Nepal, with more than 3,000 mm of rain in a typical year, most of it from June to September. Jomsom, less than 80 km to the north-west as the crow flies, gets only a small fraction of that, usually a few hundred millimetres a year at most. Lo Manthang is drier still. Fields survive only where channels bring snowmelt from the mountains, which is why every village in Mustang is a green patch in a brown landscape.

The rain shadow is why Upper Mustang and Dolpo are among the few Nepali treks that work in the monsoon. Skies are often cloudy and there may be light rain, but trails stay mostly dry. The catch is getting there: the road through the wet hills south of Ghasa suffers landslides, and cloud in the gorge cancels flights.

You can watch the rain shadow happen on the drive north from Beni. Around Tatopani the hillsides are thick with forest and the air is humid. By Ghasa pine takes over, by Lete and Kalopani the trees thin, and by Tukuche and Marpha you are among bare slopes and irrigated orchards.

The table gives a broad picture. Temperatures vary with altitude: Lo Manthang, about 1,100 m higher than Jomsom, is several degrees colder at every season. Conditions differ from year to year, so check current status before you travel.

Winter in Mustang is cold rather than snowy. Jomsom usually sees clear, bright days and frosty nights from December to February, with a few snowfalls a season. Muktinath and the passes above it get more snow, and the Thorong La, the high pass on the Annapurna Circuit to the east, can close for days after a storm. In Upper Mustang, temperatures fall far below freezing at night, water pipes freeze and many households close up and move south to Pokhara or Kathmandu until spring.

That seasonal migration is part of life in Lo. If you visit in late March or early April, you may see families returning with loaded jeeps and horses, reopening houses, cleaning irrigation channels and preparing fields for ploughing.

The same weather affects different journeys in different ways. A pilgrim flying in for two nights at Muktinath worries most about flights and the cold dawn bath. A trekker heading for Lo Manthang cares about afternoon wind on the passes and frosty nights in simple lodges. Someone driving the road from Pokhara is watching for landslides south of Ghasa. When we plan a trip with you, we start with the month and work backwards from these risks.

As a rule of thumb, the shorter and more flight-dependent your trip, the more it matters to travel in the clear months of October, November, April and May. Longer treks and road trips can absorb a delay or two, and they open up the monsoon months, when Mustang is green, quiet and far emptier than the rest of the country.

• Short flying trips: choose October to May and keep a full spare day
• Upper Mustang treks: April to October, including the monsoon months
• Road trips from Pokhara: October to May, avoiding the landslide season
• Winter visits: Lower Mustang only, with warm clothing and flexible plans
• Photography: late September to November for the clearest air

The Kali Gandaki is not only a route for people. In autumn, flocks of demoiselle cranes migrating south from Central Asia toward India use the valley as a passage through the Himalaya, and on clear October days you may see long V-shaped skeins high over Jomsom or Kalopani. Birds of prey also follow the valley. Local people have watched the cranes for generations, and their calls carry far in the still morning air before the wind starts.

Whatever month you come, let the valley set the timetable. Be up early, be out in the calm, and when the wind arrives, find a warm lodge kitchen, order a pot of tea and watch the prayer flags stream north.