Map from Google - Bhutan

In Bhutan, the climate is influenced by altitude: it's subtropical in the lowlands, and becomes progressively colder as you climb the slopes of the Himalayas. In addition, the climate is influenced by the summer monsoon, which brings rains and widespread cloudiness for almost half a year.

In the thin flat southern region, in cities like Gelephu and Phuentsholing, the climate is subtropical, with a very mild winter, when the night minima are around 10 °C in January, and the maxima around 23/24 °C; a hot spring from March to May, with afternoon thunderstorms becoming more frequent; and a monsoon season from June to mid-October, with abundant rainfall, sometimes torrential, enhanced by the proximity of the mountains. In winter, sometimes it can get cold at night, while in April and May highs can reach 40 °C. The total annual rainfall can exceed 3,000 millimetres. Here, the best time goes from November to February.
Here are the average temperatures of Gelephu.
Gelephu average temperatures
Gelephu J F M A M J J A S O N D
Min (°C) 10 12 15 20 22 24 25 25 24 21 15 11
Max (°C) 23 25 29 32 32 31 31 31 31 30 27 25

In hilly and mountainous areas up to 1,500 metres, the climate becomes progressively milder, with relatively cold and dry winters, and warm and rainy summers, during which the monsoon rains occur in the southern side, while in some inland valleys the summer rains, although present, become less intense. For example, in Punakha, located at 1,300 metres, the average goes from 11.5 °C in January, to 24 °C in July, so the summer is hot and humid despite the altitude.
Punakha average temperatures
Punakha J F M A M J J A S O N D
Min (°C) 5 8 11 14 17 20 20 20 20 16 10 6
Max (°C) 17 19 22 25 27 29 28 28 27 26 23 20

Above 2,000 metres, winter begins to be cold, with nights usually below freezing. In the capital, Thimphu, located in a valley, at an altitude varying between 2,200 and 2,600 metres, the average temperature goes from 5 °C in January to 20 °C in the period from June to August. Winter is dry and sunny, but sometimes snow can fall. In summer, the monsoon rains are plentiful, so much so that 1,400 mm of rain fall in a year, of which more than 250 mm per month from June to August, and 370 mm in July. In the nearby city of Paro, a similar climate is found.
Thimphu average temperatures
Thimphu J F M A M J J A S O N D
Min (°C) -3 1 4 7 13 15 15 16 15 10 5 -1
Max (°C) 12 14 16 20 23 24 25 25 23 22 18 15

In some inland valleys like that of Jakar, at 2,500 metres, the monsoon penetrates with more difficulty, so that the annual precipitation is smaller, and stops to 750 mm per year, but still with a winter minimum and a summer maximum.
Above 2,800 metres, the daily average temperature in January drops around freezing, and even summer nights begin to be cold.
Glaciers and snowfields begin at high altitudes, above 5,000 metres. In the northernmost areas, the summer rains are not very heavy, but the sky in summer is often cloudy, and you can find mists, or rather clouds lying over the mountain slopes.
The highest peak of Bhutan is Gangkhar Puensum, 7,570 metres high.

Temple in Bhutan

When to go

The best times to visit the mountainous part of Bhutan are spring and autumn, and particularly March and April, and from mid-October to mid-November. During the summer, the rains can make the roads muddy, so it becomes difficult to travel, and besides the clouds can hide the mountain peaks. In winter, the weather is generally good, but it's cold in the mountains, besides snow can fall at high altitudes, and it can also block some passes.

What to pack

In winter: for the plain, spring/autumn clothes, light for the day, a jacket and a sweater for the evening. For Punakha and around 1,500 metres, spring/autumn clothes for the day, warm for the evening, jacket, hat, scarf, sunglasses, sunscreen. For Thimphu and above 2,000 metres, warm clothes for the evening, but during the day a fleece and a light jacket may be sufficient; hiking shoes. At high altitudes, cold weather clothing, synthetic thermal long underwear, fleece, parka, wind jacket, warm boots.

In summer: for the plain, lightweight clothing of natural fibres, light raincoat or umbrella. For Punakha and around 1,500 metres, light clothing, a sweatshirt for the evening, raincoat or umbrella. For Thimphu and above 2,000 metres, spring/autumn clothes, light for the day, sweater and jacket for the evening. At higher altitudes, warm jacket, down jacket, hat, gloves, scarf. For overnight stays outdoors, sleeping bag, warm jacket.