Some informations about the climate in Ladakh and Zanskar.
First of all, the climate is dry. July and August are the hottest months with temperatures around 30°C during the day at an altitude of 3,500 m, while summer nights remain rather cool (5° to 10°C). In winter, temperatures can reach -25°C, and snow falls in abundance on the Himalayan foothills from September/October. Then, access to Zanskar by road becomes impossible.
The best period to visit Ladakh and Zanskar extends from May to October.
However, it is still possible to visit Leh or do some small treks during the winter season.
Weather warnings can be found on the Indian Government’s website: http://www.imd.gov.in/Welcome%20To%20IMD/Welcome.php and on its facebook page: https://www.facebook.com/India.Meteorological.Department/
Some regional differences in climate, between Zanskar, a little more watered than Ladakh, and more snowy in winter. Changtang, due to its average altitude, has colder temperatures, making the region particularly inhospitable in winter.