Map from Google - Tibet

The vast Tibetan Plateau, the "roof of the world", spans a large area which goes beyond the political boundaries of the Chinese autonomous region of Tibet (Xizang), since it also includes much of the neighbouring Qinghai Province, which lies to the east. This area has a fairly uniform climate: freezing and windy but dry in winter, and mild or cool in summer, with considerable variations between night and day, because of the strong sun's rays of high altitudes. The north is a bit colder than the south, but the temperature varies mainly with altitude; the only significant difference between the areas is that the eastern part is affected by the summer monsoon, so it receives a bit of rainfall (sometimes even abundant) in summer. Lhasa, the capital, is located in the eastern part of the province as well, while the central and western parts of Tibet are virtually uninhabited, since they are desert. While in winter the coldest areas of China are the northermost ones, in the other seasons the coldest region is Tibet, due to the high altitude.
Lhasa is located at 3,500 metres (11,500 feet) above sea level: at this altitude the air contains 68% of the oxygen that is found at sea level. In addition, at high altitudes water boils at lower temperatures: it boils at 90 °C at 3,000 metres (195 °F at 9,800 feet), at 87 °C at 4,000 metres (189 °F at 13,100 feet), and at 84 °C at 5,000 metres (183 °F at 16,400 feet). In Lhasa, winter nights are freezing cold, and on colder nights the temperature can reach -15 °C (5 °F), but during the day it almost always exceeds freezing (0 °C or 32 °F). It should be remembered that we are at a low latitude (29 ° north), so the sun is quite strong even in winter, and also that winter here is the sunniest season. The cold is usually bearable, even because the air is dry, except when the wind blows: as already mentioned, Tibet can experience strong winds especially in winter. Despite the fact that the winter is dry, sometimes light snowfalls may occur. Summer in Lhasa is mild, quite cold at night, about 9/10 °C (48/50 °F), and pleasantly warm during the day: the average is 23 °C (73 °F) in June, but the almost tropical sun, especially at an altitude so high, in summer is very strong, so in the sun it feels warmer. In the hottest days, the temperature can reach 27/30 °C (81/86 °F), from May to August.
Here are the average temperatures of Lhasa.
Average temperatures - Lhasa
LhasaJanFebMarAprMayJunJulAugSepOctNovDec
LhasaJFMAMJJASOND
Min (°C)-10-7-315910981-5-9
Max (°C)791216192322212016118
Min (°F)141927344148504846342316
Max (°F)454854616673727068615246

Summer, however, is not always sunny: the summer monsoon brings rainfall from June to September; July and August are the rainiest month, with 120/125 mm (4.7/4.9 in) per month on average. Even Mount Everest is often covered with clouds, so in summer it is more difficult to spot.
Here is the average precipitation.
Average precipitation - Lhasa
LhasaJanFebMarAprMayJunJulAugSepOctNovDecYear
LhasaJFMAMJJASONDY
Prec. (mm)11252570120125601021420
Prec.(in)000.10.212.84.74.92.40.40.1016.5
Days001151015151020060

Lhasa

The vast areas of Tibet located at higher altitudes, and especially the ones located in the north, have a worse climate than the capital. Between 4,000 and 4,500 metres (13,000 and 14,800 ft), the temperature often remains below freezing all day long in winter, while at night it normally drops to -20 °C /-25 °C (-13 °F /-4 °F), but sometimes it can plunge below -40 °C (-40 °F), after outbreaks of cold air from the north. Summer at this altitude is cold at night, with lows around 3/5 °C (37/41 °F), and mild during the day, with highs around 15/17 °C (59/63 °F). Here are the average temperatures of Nagqu, located north of Lhasa, at 4,500 metres (14,800 ft) above sea level.
Average temperatures - Nagqu
NagquJanFebMarAprMayJunJulAugSepOctNovDec
NagquJFMAMJJASOND
Min (°C)-21-18-14-9-32430-6-14-20
Max (°C)-3-137111516151382-2
Min (°F)-607162736393732217-4
Max (°F)273037455259615955463628

Xining, located at 2,260 metres (7,400 feet) above sea level, is the most populous city of the Tibetan Plateau and the capital of the Qinghai Province. Here, the lower altitude is balanced by the higher latitude (the city is located at the north-eastern edge of the plateau), so that winter is colder than in Lhasa, while summer is a bit warmer: the average in January is -6.5 °C (20.5 °F), and in July it's 18 °C (64 °F). Here, 360 mm (14 in) of rain per year fall, with a maximum of 80 mm (3.2 in) in August, and very little rain (or rather snow) from November to March.
Average temperatures - Xining
XiningJanFebMarAprMayJunJulAugSepOctNovDec
XiningJFMAMJJASOND
Min (°C)-14-11-4269121182-6-12
Max (°C)14101620232424191472
Min (°F)71225364348545246362110
Max (°F)343950616873757566574536

As already mentioned, the western part of Tibet, even more arid and located at very high altitudes, which often exceed 5,000 metres (16,400 ft), is almost totally uninhabited.

When to go

The best time to visit Tibet is summer: in Lhasa and in the east you may prefer May and September to avoid the bulk of the summer rains, but at higher altitudes, and maybe in the western area which is dry, you may prefer the June-August period, to reduce the risk of cold nights, though above 4,000 metres (13,000 ft) it is inevitable. In spring and autumn, in Tibet the climate is variable, it can be very cold with possible snow and frost especially at night, while the days can be sunny and mild. Those who don't suffer from the cold can choose spring and autumn, to enjoy the clear skies, which among other things allow to see the high mountains in the distance.

What to pack

In winter: in Lhasa, warm clothes, fleece, hat, gloves, parka, being ready to take off the outer layer during the day. In the north and at the highest altitudes, clothes for the big cold, synthetic thermal long underwear, parka, Gore-Tex jacket, gloves, hat, scarf.
In summer: in Lhasa, spring/autumn clothes, t-shirts for hot days, sweater and jacket for the evening, sunscreen, sunglasses, raincoat or umbrella, scarf for the wind, hiking shoes. For the north and the mountains, long jacket for the wind, down jacket, hat, gloves, scarf. For overnight stays outdoors, sleeping bag, warm jacket.