Map from Google - Tunisia

In Tunisia the climate is Mediterranean on the northern coast, with mild, rainy winters and hot, sunny summers, while it is semi-desert or desert in inland areas. The east coast, from the Gulf of Hammamet towards the south, receives little rainfall, and going towards the south becomes even less rainy, until it becomes desert in the southernmost part (from the Gulf of Gabès to the border with Libya), but the temperatures along the coast remain Mediterranean-like, with highs around 16/18 °C (61/64 °F) in winter, and 31/32 °C (88/90 °F) in summer.
Here are the average temperatures of Tunis, the capital, located in the northern part of the coast.
Average temperatures - Tunis
TunisJanFebMarAprMayJunJulAugSepOctNovDec
TunisJFMAMJJASOND
Min (°C)77810141720211916118
Max (°C)161718212529333330252117
Min (°F)454546505763687066615246
Max (°F)616364707784919186777063

The rains follow the Mediterranean pattern on the coast, so they are more abundant in late autumn and winter, and become very rare in late spring and summer. They are more abundant in the northern coast, where the weather fronts of Mediterranean or Atlantic origin pass more frequently in the winter months: in Tunis, precipitation amounts to 520 millimetres (20 inches) per year, most of which occur from October to April, while in summer it hardly ever rains; in Bizerte, in the far north, the rains are a bit more abundant, around 570 mm (22.5 in) per year.
Here is the average precipitation in Tunis.
Average precipitation - Tunis
TunisJanFebMarAprMayJunJulAugSepOctNovDecYear
TunisJFMAMJJASONDY
Prec. (mm)65655545251051035706075520
Prec.(in)2.62.62.21.810.40.20.41.42.82.4320.5
Days88875312577869

Precipitation decreases to 300/400 mm (12/16 in) per year in the Gulf of Hammamet, and to about 200 mm (8 in) in the Gulf of Gabès.
Here is the average rainfall in Sousse, in the southern part of the Gulf of Hammamet.
Average precipitation - Sousse
SousseJanFebMarAprMayJunJulAugSepOctNovDecYear
SousseJFMAMJJASONDY
Prec. (mm)35303030151051035553545330
Prec.(in)1.41.21.21.20.60.40.20.41.42.21.41.813
Days66654212465653

In the northwest, there are hilly and mountainous areas (see the Tunisian Dorsale or High Tell, and Tebessa Mountains), which in winter can be cold and may even experience snowfalls, while in summer they are hot during the day, but remain quite cool at night.
Here are the average temperatures of Kasserine, located at 675 metres (2,200 feet) above sea level.
Average temperatures - Kasserine
KasserineJanFebMarAprMayJunJulAugSepOctNovDec
KasserineJFMAMJJASOND
Min (°C)123611151818141053
Max (°C)111215192531353428221613
Min (°F)343637435259646457504137
Max (°F)525459667788959382726155

Kasserine is located at the foot of Jebel ech Chambi, the highest mountain in Tunisia with its 1,544 metres (5,066 feet).

In the desert, which is found in central and southern inland areas, the summer heat is remarkable: in July and August, the maximum temperature reaches on average 37 °C (99 °F) in Gafsa, and 39 °C (102 °F) in Tozeur. In the desert, the temperature range is wider than on the coast, so that in winter, night-time temperatures can drop a few degrees below freezing (0 °C or 32 °F), while in summer the highest recorded temperatures are remarkable: about 52 °C (125.5 °F) in the southern part of the desert.
Here are the average temperatures of Tozeur, located in the desert, west of Gabès.
Average temperatures - Tozeur
TozeurJanFebMarAprMayJunJulAugSepOctNovDec
TozeurJFMAMJJASOND
Min (°C)781114182225252217117
Max (°C)171922263136393934282217
Min (°F)454652576472777772635245
Max (°F)63667279889710210293827263

The rains in the desert region are scarce throughout the year, but they are even scarcer in summer, when only very few showers occur. Here is the average rainfall in Tozeur.
Average precipitation - Tozeur
TozeurJanFebMarAprMayJunJulAugSepOctNovDecYear
TozeurJFMAMJJASONDY
Prec. (mm)151520151051515152015155
Prec.(in)0.60.60.80.60.40.200.20.60.60.80.66.1
Days43232201323429

In the desert area, there is also a large salt lake, Chott el-Jerid, almost entirely dry, witness of a past when the climate was more humid. The few remaining waterholes have different colours, depending on the salt that prevails in each of them, offering an unusual view.

Coastal and mountainous regions of Tunisia may be affected by the desert wind, which causes an increase in temperature by several degrees, but also a drop in relative humidity, and may also bring sandstorms. During these periods it can be hot, especially from June to September, when the temperature can exceed 40 °C (104 °F) in Tunis and along the east coast.
In the south, the islands of Djerba and Kerkennah have a mild climate, but they too can receive the torrid wind from the desert.
Average temperatures - Djerba
DjerbaJanFebMarAprMayJunJulAugSepOctNovDec
DjerbaJFMAMJJASOND
Min (°C)9911131620222321181310
Max (°C)161820222629323330262218
Min (°F)484852556168727370645550
Max (°F)616468727984909186797264

The amount of sunshine in Tunisia is very good throughout the year on the east and south coast and in inland areas, while on the northern coast, from October to April there's no shortage of sunny days, which, however, alternate with cloudy and rainy periods, which last for a few days.

Monastir

In the northern part of the coast, the sea is warm enough for swimming from July to September, while it is definitely cool, if not cold, from December to May.
Sea temperature - Tunis
TunisJanFebMarAprMayJunJulAugSepOctNovDec
TunisJFMAMJJASOND
Sea (°C)151515151721242625221917
Sea (°F)595959596370757977726663

In the southern part of the coast, the trend is similar, although the sea is slightly warmer, and it's still warm in October.
Sea temperature - Djerba
DjerbaJanFebMarAprMayJunJulAugSepOctNovDec
DjerbaJFMAMJJASOND
Sea (°C)161516171922262827252218
Sea (°F)615961636672798281777264

When to go

To visit Tunis and the northern coast, and also the northern hills and mountains, you can choose spring and autumn (April, May and October).

For a beach holiday, you can go from June to September. The favorable season for a beach holiday is quite long, especially in the east coast, which is usually sheltered from Atlantic disturbances, so that it can easily last until mid-October.

The desert can be visited in winter, or when the maximum temperatures are normally below 30 °C (86 °F), ie from November to March. In winter, the daytime temperature is mild and the sun usually shines, while at night it can get cold.
Summer is not the best season to visit the Tunisian desert, because of the torrid heat. In the desert, there are hotels equipped with swimming pools, but in summer the heat is so intense that even sunbathing can be difficult. A compromise between excursions in the desert and moments of relaxation at the pool could be found in spring and autumn, ie in April and October. On the contrary, as mentioned, the coast is good for a beach holiday in summer.

What to pack

In winter: for Tunis and the coast, spring/autumn clothes, a jacket and a sweater, raincoat or umbrella; for the mountain areas, warm clothes and down jacket; for the desert, light clothing, but also sweater and jacket for the evening, scarf for the wind-borne sand and dust.

In summer: for the coast, lightweight clothes, light scarf for the sea breeze, light sweatshirt for the evening. For the desert, loose fitting clothes, made of natural fabric (cotton or linen), desert turban for the wind-borne sand and dust; sunglasses (even graduated instead of contact lenses); a sweatshirt for the evening or the mountains, sweater or sweatshirt and sleeping bag for overnight stays in tents; hiking boots or sandals.
For women, it is best to avoid shorts and miniskirts, especially in rural and non-tourist areas.