Map from Google - Tajikistan

In Tajikistan the climate is arid continental, with cold winters, and hot and sunny summers. Being a mountainous country, both temperature and rainfall vary with altitude and slope exposure. At high altitudes, there are extensive glaciers and snowfields; the easternmost and the south-western areas are the driest, and are virtually desert. The mountainous areas (Alaj-Turkestan Range in the north, Pamir in the east) in winter receive abundant snow, which later, when it melts, feeds the rivers, and in particular the two great rivers of Central Asia, Amu Darya in the south and Syr Darya in the north. Clashes of air masses can cause wind storms, especially in the period from November to April. The Afghanets can bring dust storms from Afghanistan, while in the valleys, a down-slope, warm and dry wind, called Harmsil, can blow. In the eastern part of the country, which is affected by the Asian monsoon, the wind can bring dust storms also in summer.

Khujand (or Khodzhent) is located in the north, at 350 metres (1,150 feet) above sea level, in the Fergana Valley, where the Syr Darya flows. Here winter is cold, with an average temperature in January of 0 °C (32 °F), while summer is hot, since the average in July is 28 °C (82 °F), with a night minimum of 21 °C (70 °F) and maximum of 36 °C (97 °F). Precipitation is very low, only 170 millimetres (6.7 inches) per year, with a relative maximum in spring, of 25/30 mm (1/1.2 in) per month in March-April, and a minimum in summer (it almost never rains from July to September). In winter there are intense cold waves, in which the temperature can drop to -20 °C (-4 °F), while in summer, from June to August, the temperature can reach 45 °C (113 °F).
Here are the average temperatures of Khujand.
Average temperatures - Khujand
KhujandJanFebMarAprMayJunJulAugSepOctNovDec
KhujandJFMAMJJASOND
Min (°C)-3-2411162021191483-1
Max (°C)461422293436322921126
Min (°F)272839526168706657463730
Max (°F)394357728493979084705443

The capital of Tajikistan, Dushanbe, is located at 800 metres (2,600 feet) above sea level, in the west, and is moderately rainy, because it's located at the foot of the mountains: it receives about 600 mm (23.5 in) of rain or snow per year, most of which between November and May, with a maximum in March and April, of about 110 mm (4.3 in) per month, distributed in about 10 days per month, and a summer minimum (even here, from July to September it virtually never rains).
Here is the average precipitation.
Average precipitation - Dushanbe
DushanbeJanFebMarAprMayJunJulAugSepOctNovDecYear
DushanbeJFMAMJJASONDY
Prec. (mm)80751101107520131204570610
Prec.(in)3.134.34.330.800.100.81.82.824
Days111110107301026970

In Dushanbe the climate is continental as well; winters are quite cold, though not freezing, with an average in January of 3 °C (37.5 °F), while summers are hot and sunny: highs in July and August are around 35 °C (95 °F), but with peaks of 40 °C (104 °F). In winter the temperature often drops below freezing (0 °C or 32 °F) at night, but it remains quite mild during the day; however, even here cold waves may occur, with lows around -10 °C (14 °F), though less intense than in the rest of the country, because the mountains in the north partly protect the city. However, some winters can be very cold, like that of 2008, when the temperature dropped to -19 °C (-2 °F). Here are the average temperatures.
Average temperatures - Dushanbe
DushanbeJanFebMarAprMayJunJulAugSepOctNovDec
DushanbeJFMAMJJASOND
Min (°C)-2-14101317181611730
Max (°C)8915222733363430231711
Min (°F)283039505563646152453732
Max (°F)464859728191979386736352

Dushanbe

In the south-west, in Qurghonteppa (or Kurgan-Tyube), at 400 metres (1,300 feet) above sea level, the climate is similar to that of Dushanbe, but more arid, so that only 280 mm (11 in) of rain or snow per year fall, and more exposed to temperature extremes. The wind that descends from the mountains can cause sudden increases in temperature, with peaks of 45 °C (113 °F) in summer, while in winter the temperature can drop more easily to -20 °C (-4 °F).
Average temperatures - Qurghonteppa
QurghonteppaJanFebMarAprMayJunJulAugSepOctNovDec
QurghonteppaJFMAMJJASOND
Min (°C)-216121519201712841
Max (°C)111117243036373531221811
Min (°F)283443545966686354463934
Max (°F)525263758697999588726452

The eastern part of Tajikistan (Gorno-Badakhshan Autonomous Region) is occupied by the spectacular mountainous region of Pamir, the "roof of the world", whose highest point is the Ismoil Somoni Peak (formerly Communism Peak), 7,495 metres (24,590 feet) high. The vast plateau is cold, desert and windswept, but in the many areas located above 4,500 metres (14,800 ft), snowfalls are frequent, and there are large snowfields and glaciers (among them the huge Fedchenko Glacier). The region is crossed by the Pamir Highway, one of the highest highways in the world, whose conditions, however, are often not good.
In Rangkul, located in the east at 3,800 metres (12,500 ft) above sea level, winter is long and cold, with a January average of -16 °C (3 °F); still in May, the daily average is 0 °C (32 °F), and only from June to September the daytime temperature reaches or exceeds 10 °C (50 °F), even though night temperatures remain around freezing, and sometimes may go below, however this period from June to September is the best time of year, especially in July and August, when highs are around 13/15 °C (55/59 °F). Precipitation is very low, lower than 100 mm (4 in) per year: it is not surprising that the landscape changes directly from the desert to the glaciers of the highest altitudes. The two lakes in the valley, Rangkul and Shorkul, remain frozen for many months of the year, while during summer, near the lakes you can watch several species of migratory birds.
Average temperatures - Rangkul
RangkulJanFebMarAprMayJunJulAugSepOctNovDec
RangkulJFMAMJJASOND
Min (°C)-22-19-14-8-5021-2-7-13-20
Max (°C)-11-7-225101413102-3-9
Min (°F)-8-27182332363428199-4
Max (°F)121928364150575550362716

More to the north, near the shores of Lake Karakul, at 3,900 metres (12,800 ft) above sea level, and to the south, near Lake Zorkul, at 4,100 metres (13,500 ft), the conditions are similar, with mild days and cold nights in summer, and the lake breeze that during the day can increase the feeling of cold.
Pamir is grooved by deep valleys through which the rivers flow. In the west, we find Khorog (or Chorugh), the capital of the Gorno-Badakhshan Region, which is located at 2,000 metres (6,500 feet) above sea level, in the valley of the Panj River, and has a continental climate, with cold winters and warm summers: the average of January is -6 °C (21 °F), while that of July and August is 23 °C (73 °F). Here are the average temperatures.
Average temperatures - Khorog
KhorogJanFebMarAprMayJunJulAugSepOctNovDec
KhorogJFMAMJJASOND
Min (°C)-11-9-259121515104-1-7
Max (°C)-11716212730302618102
Min (°F)121628414854595950393019
Max (°F)303445617081868679645036

Here you can go from May to mid-October, bearing in mind that it could be hot during the day in July and August, and cold at night in May and September-October, when the temperature at night can hit freezing. Here the precipitation amount is that of the steppe climate: 235 mm (9.3 in) per year, most of which occur from November to May, while even here in summer it almost never rains. Here is the average precipitation.
Average precipitation - Khorog
KhorogJanFebMarAprMayJunJulAugSepOctNovDecYear
KhorogJFMAMJJASONDY
Prec. (mm)30304040259301102025233
Prec.(in)1.21.21.61.610.40.1000.40.819.2
Days66664210133543

Khorog

When to go

The best times to visit Tajikistan are spring and autumn, to avoid the weather extremes, and in particular the months of April and October. Between the two, October is preferable, because it is less rainy, especially in Dushanbe and in non-desert areas, however, in these two months, it can sometimes be cold at night and hot during the day as well. For hiking at high altitudes, the summer is preferable, because it is the only season when the temperature exceeds freezing.

What to pack

In winter: in Dushanbe and the major cities, below a thousand metres (3,300 feet), warm clothes, sweater, down jacket, hat, scarf, gloves; some lighter clothes for mild days, scarf for the wind. In Pamir and high mountains, cold weather clothing, synthetic thermal long underwear, fleece, parka, wind jacket, warm boots.

In summer: in Dushanbe and the major cities, below a thousand metres (3,300 feet), lightweight clothes, made of natural fabric (cotton or linen), sun hat, scarf for the wind, a sweatshirt for the evening. In the mountains, at intermediate altitudes, light clothes for the day, sun hat, sweatshirt and light jacket for the evening, hiking shoes; above 3,000 metres (9,800 ft), sweater and jacket for the evening; above 4,000 metres (13,000 ft), down jacket, hat, gloves, scarf.
For women, it is best to avoid shorts and miniskirts.