Map from Google - Chad

The climate in Chad is desert in the north, while it becomes semi-desert in the center, and tropical with a wet season in the south. In fact, the summer rains, due to the African monsoon, affect only the center-south of the country, and in a progressively more substantial manner as we proceed towards the south. As a result, the rainfall is lower than 2 inches per year in the north, it reaches 8 inches in the center, and exceeds 40 inches in the south. The rains occur from May to early October, although in the far south, in April some early showers and thunderstorms already occur. Winter is dry everywhere.

In the desert north, winter is sunny and pleasantly warm during the day, with highs around 80 °F in January, and lows around 56 °F in Faya-Largeau; in the Ennedi plateau that lies in the north East, the temperatures are a bit lower, and even lower in the Tibesti Range, which has a lunar landscape and reaches the highest point of the country with Emi Koussi, 11,302 feet high. Every now and then, at night the temperature can approach freezing at lower altitudes, while this happens quite often in the mountains. By contrast, during the day the temperature can touch 85 °F even in winter, at least at low altitudes. The rest of the year in the desert area is hot, with highs around or above 105 °F from April to October, but the highest recorded temperature are about 122 °F.
Here are the average temperatures of Faya-Largeau.
Average temperatures - Faya-Largeau
Min (°F)575964727779797979736457
Max (°F)798493102106108106104104998882

The hottest months, albeit slightly, are May and June, then in July and August the area is reached by the southern currents, the extreme offshoot of the monsoon, which are almost never able to bring some rainfall, but prevent further increase in temperature. So here the rains are very rare: in July 2010, a strong thunderstorm hit Faya-Largeau, as had not been seen in living memory.
Here is the average precipitation in Faya-Largeau.
Average precipitation - Faya-Largeau
Prec. (in)000000.10.10.400000.7

In the mountainous area of the Tibesti, it's hot during the day, but even in summer the nights can be cool. In the Tibesti Massif the rainfall is slightly higher than the surrounding area, going just above 4 inches per year, so it's still a desert. However, there are also springs of fresh water (oases) as well as salt lakes.
Throughout the year, the wind can raise dust storms across the desert area.
Tibesti, Chad
Continuing south, in summer the influence of the southern air masses becomes more remarkable, so that there is a decrease in temperature and an increase in cloudiness and humidity. In the area of Lake Chad, from 8 to 12 in of rain per year fall. Lake Chad is a shallow lake, that has almost halved over the past 50 years, anyway its size vary according to seasonal rainfall and from year to year. It is not necessarily going to disappear, but surely its fate depends on the performance of the monsoon rains and on climate change, as well as on exploitation of its tributaries for agricultural purposes.
Continuing south, the rains become more abundant. In the capital, N'Djamena, south of the lake, the total annual rainfall amounts to 22 in, with the first rains in May, a maximum in July and August, and the last rains in early October.
Here is the average precipitation.
Average precipitation - Ndjamena
Prec. (in)0000.312.

Here we are quite to the south (only 12 degrees north of the Equator), so the temperature is high even in winter, around 86 °F during the day, and it can even go above, although the air is dry and the nights are cool. This is still the least hot period of the year, given that from February the heat starts to go up, reaching a peak between April and May, when the temperature often exceeds 105 °F. In summer, the monsoon gradually lowers the temperature, bringing it down to 86/90 °F in August, although the high relative humidity makes the heat sultry and therefore less tolerable than in winter. After the monsoon, there is time for a new increase in the heat, which returns to be intense in October and November.
Here are the average temperatures.
Average temperatures - Ndjamena
Min (°F)636673818179757373736863
Max (°F)86909710210097908690959588

Further south, we enter the area of the savannah, where the landscape becomes definitely green during the rainy season. Floods are frequent, especially as a result of the most intense downpours and in the years when the rains are more abundant. The Zakouma National Park is located in the south-east, in an area where precipitation goes from 31 to 35 inches per year, and it's open from December the 1st to May 31th.
In the far south, the annual rainfall even exceed 40 in, as happens in Moundou, where it amounts to 43 in, with the first showers occurring already in April, and more than 8 in of rain per month from July to September. Here is the average precipitation.
Average precipitation - Moundou
Prec. (in)

Here it's definitely hot even in December and January, with highs around 95 °F, but fortunately at night the temperatures drop a lot, dropping to around 59 °F. The hottest month is March, with 101 °F as normal maximum. In summer, the monsoon lowers the temperature around 86/90 °F, but the heat becomes stifling.
Here are the average temperatures.
Average temperatures - Moundou
Min (°F)596473757572707070706359
Max (°F)93991021009790868688919593

Moissala, Chad

When to go

The best time to visit Chad is winter, and in particular the months of December and January: the sky is clear, even though sometimes the north-east wind, the Harmattan, can raise the dust, creating a kind of haze. During the day it is very hot in the south, but nights are generally cool. In the north, however, it can get cold at night, while during the day the temperature is pleasant.
Already in February, the temperature rises significantly in the south, approaching 105 °F.

What to pack

In winter: for the day, long, light, loose fitting clothing of natural fabric (cotton or linen), especially in the south where it is warm even in winter; 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. For Tibesti and outdoor nights in the desert, warm clothes, warm jacket, gloves, hat.

In summer: for the desert, loose fitting, light-colored clothing, light and long shirt and pants, a wide-brimmed hat, a light scarf, sunglasses, comfortable and breathable shoes, desert turban. A sweater for the night, sleeping bag to sleep outdoors. For Tibesti, at high altitudes, jacket and sweater. For N'Djamena and the south, tropics-friendly, light clothing, umbrella or light raincoat for the rain showers, light sweatshirt.
For women it's best to avoid shorts and miniskirts.