In Bhutan, a small Asian country crossed by the Himalayas, the climate is influenced by altitude: it is subtropical in the lowlands, and becomes progressively colder as you climb the mountains slopes. In addition, the climate is influenced by the summer monsoon, which brings rains and widespread cloudiness for almost six months per year, but especially from June to late September.

In the thin flat southern region, in cities like Gelephu and Phuentsholing, the climate is subtropical. Winter is very mild, but sometimes it can get cold at night. Spring, from March to May, is hot, especially in April and May, when the temperature can reach as high as 40 °C (104 °F); in addition, afternoon thunderstorms become more frequent. From June to mid-October, it's the monsoon season, with abundant rainfall, sometimes torrential, which are enhanced by the proximity to the mountains, and muggy heat. Total annual rainfall can exceed 3,000 millimeters (118 inches). The best time in this area is from November to February.
Here are the average temperatures of Gelephu.
Gelephu - Average temperatures
Min (°C)101215202224252524211511
Max (°C)232529323231313131302725
Min (°F)505459687275777775705952
Max (°F)737784909088888888868177

In hilly and mountainous areas up to 1,500 meters (5,000 feet), the climate becomes progressively milder, with relatively cold, dry winters and warm, rainy summers, with monsoon rains on the southern side, while in some inland valleys, the summer rains are less intense. For example, in Punakha, located at 1,300 meters (4,250 feet), the average temperature ranges from 11 °C (52 °F) in January to 24 °C (75 °F) in July, so the summer is quite hot and humid despite the altitude. Here are the average temperatures.
Punakha - Average temperatures
Min (°C)581114172020202016106
Max (°C)171922252729282827262320
Min (°F)414652576368686868615043
Max (°F)636672778184828281797368

Above 2,000 meters (6,500 feet), winter are quite cold, with nights usually below freezing (0 °C or 32 °F). In the capital, Thimphu, located in a valley, at an altitude varying between 2,200 and 2,600 meters (7,200 and 8,500 ft), the average temperature ranges from 5 °C (41 °F) in January to 20 °C (68 °F) in the period from June to August. Winter is dry and sunny, but snow can sometimes fall. In summer, the monsoon rains are plentiful, so much so that 1,400 mm (55 in) of rain fall in a year, of which more than 250 mm (10 in) fall per month from June to August, and up to 370 mm (14.5 in) in July. In the nearby city of Paro, a similar climate is found. Here are the average temperatures in Thimphu.
Thimphu - Average temperatures
Min (°C)-31471315151615105-1
Max (°C)121416202324252523221815
Min (°F)273439455559596159504130
Max (°F)545761687375777773726459

In some inland valleys, like that of Jakar, at 2,500 meters (8,200 ft), the monsoon penetrates with more difficulty, so much so that the annual precipitation is lower, and doesn't go above 750 mm (29.5 in) per year, though it still follows the same pattern, with a winter minimum and a summer maximum.
Above 2,800 meters (9,200 ft), the daily average temperature in January drops around freezing, and even summer nights become cold.
Glaciers and perennial snows begin at high altitudes, above 5,000 meters (16,400 ft). In the northernmost areas, the summer rains are not very heavy, but the sky in summer is often cloudy, and you can find fogs, or rather clouds lying on the mountain slopes.
The highest peak of Bhutan is Gangkhar Puensum, 7,570 meters (28,836 feet) high.

Temple in Bhutan

The sun in Bhutan shines regularly in winter, while it does not shine very often in the monsoon season. Here are the average daily sunshine hours in Thimphu.
Thimphu - Sunshine

Tropical cyclones

Bhutan is too far from the sea to be directly hit by tropical cyclones, but the remains of cyclones affecting Bangladesh can cause heavy rains in the plains as well as snowfalls in the mountains, as happened in May 2009 with Cyclone Aila.
Cyclones typically occur from mid-April to early December, and most likely in May-June and in October-November.

Best Time

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

What to pack

In winter: for the plain, bring spring/autumn clothes (light for the day), a jacket and a sweater for the evening. For Punakha and around 1,500 meters (5,000 feet), bring spring/autumn clothes for the day, warm for the evening, a jacket, a hat, a scarf, sunglasses, sunscreen. For Thimphu and above 2,000 meters (6,500 feet), warm clothes for the evening, but during the day, a fleece and a light jacket may be sufficient; hiking shoes. At higher altitudes, cold weather clothing, synthetic, thermal long underwear, a fleece, a parka, a wind jacket, warm boots.

In summer: for the plain, bring lightweight clothing of natural fibers, a light raincoat or umbrella. For Punakha and around 1,500 meters (5,000 feet), light clothing, a sweatshirt for the evening, a raincoat or umbrella. For Thimphu and above 2,000 meters (6,500 feet), spring/autumn clothes (light for the day), a sweater and a jacket for the evening. At higher altitudes, a warm jacket, a down jacket, a hat, gloves, a scarf. A sleeping bag and a warm jacket for outdoor overnight stays.