This is probably the most commonly asked question we get at The Mountain Company. There is no definitive answer as there are pros and cons for each month and the answer, of course, depends on your objective and interests. However, there are reasonably predictable patterns to the weather in the Himalayas and Karakoram which can help you determine the best month to time your visit.
In the Himalayas there are two main trekking seasons, pre-monsoon & post-monsoon.
The pre-monsoon season starts around the end of February and continues until the end of May. In late February it is still the tail end of winter so temperatures are colder especially at higher altitudes. Early March is a good time to visit places at a lower elevation (between 800m and 2000m) , such as the Annapurna middle hills north of Pokhara, because it is pleasant walking conditions in cool temperatures
As one gets into April and May it will be very hot & sweaty at these lower altitudes and the views will be poor with a haze blocking out the Himalayan peaks. The haze comes from dust & pollution from Northern India as well as smoke from villages in the Himalayas.
The advantage of trekking in the Annapurnas during March & April is that the rhododendrons will be in bloom (and other places of same similar aspect & elevation). In mid March the rhodendrons flower lower down and will ripple upwards in altitude as the season progresses.
If you are looking to trek high passes over 5000m like Cho La, Larkya La or Dhampus Pass it is better to leave it until April & May. If you attempt passes in March you will normally find that they are still blocked with winter snows, whereas by April and into May much of this snow has melted.
For example Roland crossed the Larkya La on the Manaslu Circuit on April 19 this year. This is a good time to cross during the pre-monsoon season because by that stage alot of the snow has melted and crucially the ground warmed by the sun means any fresh snow does not stay too long. We experienced quite heavy snowfall when we were at Samdo (3,850m), however after a day most of this had melted away.
After returning to Kathmandu from Manaslu Circuit, Roland then set off on April 26 with another group to lead the Dhaulagiri Circuit . We crossed the French and Dhampus pass around mid May which again worked well with limited snow on these passes. Dhaulagiri Circuit is better later in the season into May because the temperatures are warmer especially for the night in Hidden Valley at 5100m!
A typical characteristic of pre-monsoon weather is to get clear blue skies in the morning with clouding building during the day. Often there is precipitation in the afternoon and then the skies clear off again in the evenings.
One of the main advantages of trekking in the pre-monsoon season is that there are fewer other trekkers around compared to October & November. Importantly, this also means that it is easier & cheaper to get flights to Kathmandu at this time of year.
October is the most popular trekking month in the Nepal Himalayas. This is because the weather is generally stable with clear blue skies and, unlike the pre-monsoon season, it is often clear for the whole day with no afternoon cloud blocking the sun.
The beginning of the post-monsoon trekking season starts at a different date each year as it depends on when the monsoon finishes. Sometimes the stable weather can start in early to mid September whereas other years it can go into October. In October 2007 there was still heavy rain into the first week of October causing the flights to Lukla to be cancelled for 5 days!
By mid October the weather should have settled, this is the best time to view the Himalayan peaks and for photography because the monsoon has cleared the dust and other impurities from the air. As the season progresses into November the weather becomes more stable and predictable with most days having sun and blue skies. The temperature begins to cool off especially in the evenings once the sun has gone down, for this reason you should make sure you have a warm sleeping bag!
It is also possible to trek during December although of course the temperatures are colder at night. This is one of my favourite times of year to visit more popular places like the Khumbu because there are far fewer people around compared to October and November. It often feels warmer during the day because one feels the sun's heat whereas in pre-monsoon the sun is usually blocked by cloud later in the day.
It is possible to get snowfall at this time of year and if it does snow it is likely to settle at a relatively low altitude and could stay on the frozen ground for a long time. However, over the last few years there have been dry winters in the Himalayas so has been a great time to trek.
For most of the Himalayas this is not a good time to visit due to the heavy rains associated with the monsoon. The timing and severity of the monsoon depends year on year, normally the rains start in July and carry on through to early September. The heavy rain causes landslides on many trekking routes, avalanches on the high peaks and lower down there is likely to be leeches.
However, there are a few areas located in the Trans Himalaya which are in the rain shadow of the main himalayan range. In Nepal, it is possible to trek in Upper Mustang; in India Ladakh and in Pakistan Baltistan and K2.
For most people it is too cold to trek at this time of year although it is definitely still possible if you are well prepared. There is a trek called Zanskar river trek (Chadar) in Ladakh that is only possible at this time of year! During the winter the Zanskar region of Ladakh is cut off from the outside world and can only be visited in by following the old trade route across the frozen Zanskar river.