The influence of the monsoon
To know the best season to visit the Himalaya, you must understand the general characteristics of the climate of Southeast Asia.
Southeast Asia is under the influence of the monsoon (deriving from the hindi word "mausaam").
The monsoon is a large mass of humid air moving from the Indian ocean towards the Asian continent in summer. It is characterized by high humidity and heavy rain.
Arriving on the Himalayan foothills, the air must rise which causes huge rainfalls that may accumulate up to 10 metres of water in 3 months.
The monsoon reaches the Northwestern Himalayas by the end of June and leaves by mid-September.
As it progresses northward, it passes over higher ranges. This dries out the air and it runs out of water (foehn effect). In short, the monsoon flows through the foothills of the Himalayas, but doesn't pass beyond the highest passes and mountains.
Thus, we can divide the Himalayas into a region that receives the monsoon rains and the one that doesn't.
The Green Himalayas
The "Green Himalayas" is characterized by a temperate climate and a dense forest cover.
The Ladakh and Lahul & Spiti located behind the border of the monsoon rains are part of the desert Himalayas, characterized by an arid climate and sparse vegetation.
You can very clearly see this border on this satellite photo.
best season to visit green Himalayas spreads from April to June and from October to November. From July to September, monsoon makes this part of the Himalayas inadequate for tourism, while from December to March, cold and snow make its visit a bit complicated.
The high passes leading to the desert part of the Himalayas are under snow from November to May.
Ladakh, and Lahul & Spiti are accessible from June to October. Leh, capital of Ladakh, is accessible all year by air but the surrounding places are cut off. So Himalayan Ecotourism organizes tours to Ladakh and Lahaul Spiti only when it is accessible by road. Ladakh also has permanent frost during winter.
The period from June to October is good season for visiting this part of the Himalayas because the monsoon rains don't arrive there and therefore the weather is usually clear.