The climate in Bhutan is extremely varied. This variation in the climatic conditions and average temperature can be attributed to two main factors, the vast differences in altitude present in the country and the influence of the north Indian monsoons.
The Indian summer monsoon lasts from late-June through late-September and is mostly confined to the southern border region of Bhutan. It brings heavy rain and high humidity, to the southern region. These rains bring between 60 and 90 percent of the western region’s rainfall.
Annual precipitation ranges widely in various parts of the country. In the northern border region to Tibet gets about 40 millimeters of precipitation a year which is primarily snow. In the temperate central regions, a yearly average of around 1,000 millimeters is more common, and 7,800 millimeters per year has been registered at some locations in the humid, subtropical south, ensuring the thick tropical forest, or savanna.
Thimphu experiences dry winter months (December through February) and almost no precipitation until March, when rainfall averages 20 millimeters a month and increases steadily thereafter to a high of 220 millimeters in August for a total annual rainfall of nearly 650 millimeters.
Bhutan’s generally dry spring starts in early March and lasts until mid-April. Summer weather commences in mid-April with occasional showers and continues to late June. The heavier summer rains last from late June through late September which are more monsoonal along the southwest border.
Autumn, from late September or early October to late November, follows the rainy season. It is characterized by bright, sunny days and some early snowfalls at higher elevations.
From late November until March, winter sets in, with frost throughout much of the country and snowfall common above elevations of 3,000 meters. The winter northeast monsoon brings gale-force winds at the highest altitudes through high mountain passes, giving Bhutan its name – Drukyul, which in the Dzongkha language mean Land of the Thunder Dragon.