Map from Google - Mali

In Mali there are three types of climate: 1- the sub-tropical climate of the Sahara desert in the north, 2- the Sahel semi-desert climate in the centre, with a rainy season from June to September, and precipitation going from 100 to 600 millimetres (4 to 23.5 in) per year, 3- the tropical climate of the savanna in the south, with a more intense and long rainy season, so that precipitation exceeds 600 mm (23.5 in), the majority of which occurs between May and October.

Precipitation in Mali

The vast northern area (zone 1 on the map), virtually lacking settlements except for a few rare oases, is dominated by the desert, and has an average temperature in January between 15 and 20 °C (59 and 68 °F), with some cold nights in the winter months (the cold records are around freezing), while in summer it's scorchingly hot, even though the temperature decreases slightly in July and August, when there can be a bit of cloudiness and some rare showers.
Here are the average temperatures of the northern city of Tessalit, located at 500 metres (1,600 feet) above sea level.
Average temperatures - Tessalit
TessalitJanFebMarAprMayJunJulAugSepOctNovDec
TessalitJFMAMJJASOND
Min (°C)121418222528272626241914
Max (°C)273032374043424040373328
Min (°F)545764727782817979756657
Max (°F)81869099104109108104104999182

The cloudiness and showers in the summer period are are caused by the extreme offshoots of the African summer monsoon, so that damp winds move inland from the Atlantic Ocean, but up here they are not felt much.
Here is the average precipitation in Tessalit.
Average precipitation - Tessalit
TessalitJanFebMarAprMayJunJulAugSepOctNovDecYear
TessalitJFMAMJJASONDY
Prec. (mm)10002515301510068
Prec.(in)00000.10.20.61.20.60002.7
Days00001234310014

In the intermediate central area (zone 2), the climate becomes tropical, since the average temperature in January is equal to or greater than 20 °C (68 °F), while the hottest months of the year are April, May and June, when the maximum temperature normally exceeds 40 °C (104 °F), but with peaks of 47/48 °C, because then the monsoon comes, which is able to lower the temperature by a few degrees, and to bring some downpours, more intense and frequent than in the northern area. This is the area called Sahel, that stretches from west to east, south of the Sahara.
In Timbuktu (or Tombouctou), the legendary city of the past Mali Empire, 180 mm (7 in) of rain per year fall, almost all from June to September, with a maximum of 75 mm (3 in) in August.
Here is the average precipitation.
Average precipitation - Timbuktu
TimbuktuJanFebMarAprMayJunJulAugSepOctNovDecYear
TimbuktuJFMAMJJASONDY
Prec. (mm)1001515557530500183
Prec.(in)00000.20.62.231.20.2007.2
Days00011378510025

After the rainy season, in which the temperature remains high, at around 35/36 °C (95/97 °F), between September and October the heat increases slightly again, with highs around 37/38 °C (99/100 °F), but also with peaks over 45 °C (113 °F), and then it decreases in the winter months, between 27 and 30 °C (80.5 and 86 °F). These are the only non torrid months of the year, during which, in theory, the sky would be clear, even though a wind from the north-east often blows, the Harmattan, which often raises dust and sand, making the sky whitish. The cause of the development of such an flourishing city as Timbuktu over the past centuries, have not been the few summer rains, but rather the waters of the great Niger River, which rises in the mountains of Guinea and arrives in this semi-desert area, then bends and returns to the south.
Here are the average temperatures in Timbuktu.
Average temperatures - Timbuktu
TimbuktuJanFebMarAprMayJunJulAugSepOctNovDec
TimbuktuJFMAMJJASOND
Min (°C)131519232627262525231814
Max (°C)303337404242393738393530
Min (°F)555966737981797777736457
Max (°F)869199104108108102991001029586

Timbuktu

Continuing south, the summer rains become more abundant. Already in Mopti, in the area of the "Inner Niger Delta", the rainfall amounts to 500 mm (20 in) per year.
In the southernmost area (zone 3), it exceeds 600 mm (23.5 in) per year, and in the extreme south even 1,000 mm (40 in).
In the capital Bamako, the annual precipitation reaches 1,000 mm (40 in), and the rainy season becomes a little longer, with the first pre-monsoon showers already in May, a maximum of 300 mm (12 in) in August, and the last rains on early October.
Here is the average precipitation.
Average precipitation - Bamako
BamakoJanFebMarAprMayJunJulAugSepOctNovDecYear
BamakoJFMAMJJASONDY
Prec. (mm)11220551302252901956551991
Prec.(in)000.10.82.25.18.911.47.72.60.2039
Days00136817181560074

In the southern area, the maximum temperature is lower in the summer months, because of the cloud cover and the rains, so that it hovers around 30/32 °C (86/90 °F), while in winter the southern sun pushes daytime temperatures up to around 33 °C (91 °F), even though the air is dry and the nights are quite cool. In Bamako, the hottest month is April, with highs around 40 °C (104 °F), because already in May the temperature drops a bit, under the blows of the first thunderstorms.
Average temperatures - Bamako
BamakoJanFebMarAprMayJunJulAugSepOctNovDec
BamakoJFMAMJJASOND
Min (°C)172023252524222222211817
Max (°C)333639403935323132353533
Min (°F)636873777775727272706463
Max (°F)919710210410295908890959591

When to go

The best time to visit Mali is winter, from December to February. In February, the heat rises and it starts to get hot in the south, while in January the sandstorms raised by the Harmattan begin to be more frequent in the north-central, so all in all the best month is December. However, in this season the nights can be cold in the north, and the temperature may drop near freezing at night, while in Bamako and in the south it can get to around 8/10 °C (46/50 °F).

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 to sail the Niger River.

In summer: for the desert, lightweight clothes, of natural fabric (cotton or linen), desert turban; a sweater for the night, sleeping bag to sleep outdoors. For Bamako and the south, light clothes, umbrella or light raincoat, possibly a sweatshirt for the rain showers. For women, it is best to avoid shorts and miniskirts.