Map from Google - Niger

In Niger the climate is desert in the north, semi-desert in the centre, and semi-arid of the savanna in the south. In fact the rains increase from north to south, because of the African monsoon, which brings humidity and clouds from the Atlantic Ocean, in the period June-September.

Rainfall in Niger, in inches (left) and millimeters (right)

The north, part of the Sahara desert, is mild or pleasantly warm in winter, and very hot in the rest of the year, and it is almost always sunny.
Here are the average temperatures of Bilma, located in the north-east, at 350 metres (1,150 feet) above sea level.
Average temperatures - Bilma
BilmaJanFebMarAprMayJunJulAugSepOctNovDec
BilmaJFMAMJJASOND
Min (°C)8101519232425252218129
Max (°C)242832374141403939363025
Min (°F)465059667375777772645448
Max (°F)75829099106106104102102978677

The rains in the north are rare and sporadic, however, there may be some rare showers in summer. Here is the average precipitation in Bilma.
Average precipitation - Bilma
BilmaJanFebMarAprMayJunJulAugSepOctNovDecYear
BilmaJFMAMJJASONDY
Prec. (mm)100111310520024
Prec.(in)0000000.10.40.20.1000.9
Days0000011110004

In winter, in the Djado Plateau and in the Aïr Massif, but sometimes also in the Ténéré Desert, nights can be cold, and the temperature can sometimes drop to around freezing (0 °C or 32 °F), while daytime temperatures are around 20/25 °C (68/77 °F). In winter, a dry wind called Harmattan blows, raising dust and sand, which can veil the sun, especially from January onwards. On the top of Aïr, in Mont Idoukal-n-Taghès 2,022 metres (6,634 feet) high, night frosts are frequent. In the rest of the year, in the north, the scorching heat reigns, at least in the lowlands, with highs around 40 °C (104 °F) from April to September, and peaks of 50 °C (122 °F).

Ténéré Desert

The central-southern region, known as Sahel, receives rainfall during the summer months. The northernmost part, at the limit with the previous region, is in fact still desert, and receives little rainfall: in the Aïr Massif and in Agadez the annual amount is slightly above 100 millimetres (4 inches), with a maximum in August, when in Agadez 55 mm (2.2 in) of rain fall. In the central part, which also includes the area of Lake Chad in the far east, precipitation is about 250/300 mm (10/12 in) per year.
The map above depicts the lines of equal rainfall (called isohyets) of 100 mm and 600 mm (4 and 23.5 in) per year in the country.
As we can see, only in the southernmost part, rainfall is moderate. In the capital Niamey, 585 mm (23 in) of rain per year fall, with the first sporadic showers in the afternoon or evening as early as in May, which increase progressively until August, when rainfall reaches 205 mm (8 in), and then decrease in September, and cease on early October. Here is the average precipitation.
Average precipitation - Niamey
NiameyJanFebMarAprMayJunJulAugSepOctNovDecYear
NiameyJFMAMJJASONDY
Prec. (mm)01173585140205902010585
Prec.(in)0000.31.43.35.58.13.50.80023
Days001146913720043

In Niamey and in the rest of the south, even the temperature follows a different pattern compared with the north. Winter is hotter, so that highs exceed 30 °C (86 °F) even in January; sometimes it can get quite cold at night, but with peaks of 7/8 °C (45/46 °F), and not around the freezing point as happens in the north. In spring the temperature rises quickly, so that in March the daytime average reaches 40 °C (104 °F), and in May it can exceed 45 °C (113 °F), then the monsoon lowers the temperature in summer, but at the cost of increased humidity: in August, highs return to 33 °C (91 °F), as in January, although night temperatures are higher and the humidity makes the heat muggy. After the monsoon, there is room for the return of the hot weather, with highs around 35/37 °C (95/99 °F) in October and November, before winter, which brings a little cool at least at night.
Here are the average temperatures in Niamey.
Average temperatures - Niamey
NiameyJanFebMarAprMayJunJulAugSepOctNovDec
NiameyJFMAMJJASOND
Min (°C)161923272826242324242017
Max (°C)333639414037343334383633
Min (°F)616673818279757375756863
Max (°F)9197102106104999391931009791

In winter, the sky is clear, though the sun may be veiled, due to the Harmattan, which raises a fine dust that penetrates everywhere and makes the sky whitish. South-central Niger is one of the hardest-hit areas by the Harmattan across the Sahel.
To the east of the capital, cities such as Maradi and Zinder have a climate similar to that of Niamey.

Mosque in Niamey

In the far south, rainfall becomes more abundant, so that it exceeds 600 mm (23.5 in) per year, therefore we are out of the Sahel proper. In Gaya, 825 mm (32.5 in) of rain per year fall, of which 70 mm (2.8 in) already in May, and 250 mm (10 in) in August. Here the landscape becomes definitely green in the summer months. The hottest month in the far south is April, because as early as in May the temperature begins to decrease, while in summer the heat is tropical, that is, uncomfortable because of high humidity.

When to go

The best time to travel to Niger is winter, from December to mid-February, when the sun shines in the whole country, the air is dry and the temperature is usually acceptable. In the south it can get definitely hot during the day even at this time, but nights remain cool. In the north it can get cold at night, while during the day the temperature is mild or pleasantly warm. In January, sand storms become more frequent, and in February the heat begins to increase in the south, heading towards 40 °C (104 °F), so all in all is the best month is December.

What to pack

In winter: light clothes for the day, long and made of natural fabric (cotton or linen), desert turban for the wind-borne sand and dust, sunglasses (even graduated instead of contact lenses), a jacket and a sweater for the evening, desert boots or sandals, sweatshirt or sweater for boat excursions in the Niger River.

In summer: in the desert, loose fitting, light-coloured clothing, long and made of natural fabric (cotton or linen), desert turban, sunglasses, comfortable and breathable shoes. A sweatshirt for the night, sleeping bag to sleep outdoors. In Niamey and the south, light clothes, umbrella or light raincoat for the rain showers.
For women, it is best to avoid shorts and miniskirts.