Map from Google - Mauritania

In Mauritania, the climate, hot and desert in the center-north, becomes semi-desert in the south, due to the summer monsoon that brings some rains between June and early October. On the coast the heat is tempered by sea breezes, but the climate is mild only in the northernmost part, which is also affected by a cool sea current. Across the country, the wind can bring sandstorms.

Climatic zones in Mauritania

The north (zone 1 on the map) is occupied by the western part of the Sahara desert; along the coast, however, a cold current flows, so that the sea temperature reaches a maximum of 75 °F in August and September in the north; the cold current affects the climate in the northern part of the coast to the point that in Nouadhibou, the air is often mild or pleasantly warm: the daily average is 68 °F in winter, when there are frequent morning mists, it is still about 73 °F in July, and reaches 77 °F in September, which is the warmest month. However, when the wind blows from the desert, the temperature can rise and reach 99/100 °F for most of the year (except in winter), but it's still better than in the rest of the country.
Here are the average temperatures of Nouadhibou.
Average temperatures - Nouadhibou
NouadhibouJanFebMarAprMayJunJulAugSepOctNovDec
NouadhibouJFMAMJJASOND
Min (°F)595961616364666870666359
Max (°F)737577777779798182827973

The sea temperature in this northern part of the coast is very cool for much of the year, while it becomes warm enough for swimming from August to October.
Sea temperature - Nouadhibou
NouadhibouJanFebMarAprMayJunJulAugSepOctNovDec
NouadhibouJFMAMJJASOND
Sea (°F)666666666668727575737268

In the inland northern areas we are instead in full desert: winter is cooler, so that the daytime temperatures are around 68 °F in January, the warmer months are the "canonical" ones, July and August, with highs around 104/108 °F; the hottest area is the north-east, where in summer the temperature can sometimes exceed 120 °F. The rains are definitely scarce.
Average temperatures - Bir Moghrein
Bir MoghreinJanFebMarAprMayJunJulAugSepOctNovDec
Bir MoghreinJFMAMJJASOND
Min (°F)545561636468777775686355
Max (°F)70737981869110010295867972

Mauritania, desert

The central-southern region is that of the Sahel (zone 2), which in the summer months is affected by the northern offshoots of the African monsoon, with come chance of clouds and rains moving inland from the Atlantic Ocean.
The Mauritanian capital Nouakchott is warm all year round; in winter, from December to February, the maximum temperatures remains around 84/86 °F, even though in this season the nights are cool. In addition, in winter the Harmattan, the dry north-east wind, often blows, carrying dust and sand. In March, the temperature increases, though not as in the rest of the country, thanks to the influence of the ocean, however, the daytime temperature rises up to 91 °F in June, the hottest month. In fact, the temperature can reach 113 °F when the wind blows from inland, and it can even cause sandstorms. From July to September it's the period of the monsoon, which lowers the maximum temperature to 86/90 °F, but it increases moisture, and brings some rains, with a maximum in August. Here is the average precipitation.
Average precipitation - Nouakchott
NouakchottJanFebMarAprMayJunJulAugSepOctNovDecYear
NouakchottJFMAMJJASONDY
Prec. (in)0000000.83.11.60.40.206.3
Days00000124411013

In fact, even in summer the wet ocean current have some breaks, during which the winds from the interior return to blow, bringing the temperature above 105 °F. After the monsoon, in September and October the temperature increases to the point that these are the hottest months, with highs generally around 95 °F. Finally, by mid-November the temperature starts to decline, in view of winter. Here are the average temperatures in Nouakchott.
Average temperatures - Nouakchott
NouakchottJanFebMarAprMayJunJulAugSepOctNovDec
NouakchottJFMAMJJASOND
Min (°F)596164646872737779736861
Max (°F)818688889091868891959082

The cold sea current has evident effects even at Nouakchott for most of the year, in fact the sea temperature is about 66 °F from February to May, it's still 70 °F in June, and becomes warm, reaching 79/81 °F only between August and October: given that the city is located south of the Tropic, it's still not much.
Sea temperature - Nouakchott
NouakchottJanFebMarAprMayJunJulAugSepOctNovDec
NouakchottJFMAMJJASOND
Sea (°F)686666666670757981797572

Nouakchott

East of the capital, the heat is more intense for most of the year, excluding the winter months, due to the distance from the sea. In Tijikja, at 1,300 feet of altitude, in June the daytime temperature is normally about 102 °F. The annual precipitation amounts to 5 in.
In the south, winter is even warmer than in Nouakchott: night-time temperatures remain around 68 °F on the coast and drop to 63/64 °F in inland areas, while highs are around 83/86 °F, the heat begins to increase in February, and in May highs of 104/106 °F are normal, but then the summer monsoon is more intense, so that in total, from 8 to 16 inches per year fall. In Néma, in August 4.3 in of rain fall. The combination of moisture and temperature, which remains around 97 °F during the rainy months, makes the heat annoying.
Average temperatures - Kiffa
KiffaJanFebMarAprMayJunJulAugSepOctNovDec
KiffaJFMAMJJASOND
Min (°F)636472798486828181797063
Max (°F)829095102106106999797999386

In the far south, along the border with Senegal (zone 3 on the map), the summer rains become more abundant, so that they total between 16 and 24 inches per year: this is still a part of the Sahel, but the climatic subdivision called "Sudanese", where even more than 8 inches in August fall, and in the rainy season the landscape changes, becoming definitely green. In Mauritania this happens especially in the southernmost region, called Guidimakha.

Guidimakha

When to go

The best time to visit Mauritania is winter, from December to mid-February: the temperatures are hot in the south and mild in the north, the sun shines and it almost never rains. At night it can get a little cold everywhere, quite often in the inland areas of the north, rarely in the south, while in the south it can sometimes be hot even at this time, but more rarely than in the rest of the year.
In winter it's also possible to swim in the sea, even though the water is a bit cool, and sometimes the air can be cold, at least at night and in the early hours of the morning.

What to pack

In winter: for inland areas, light clothing, long loose-fitting shirt and pants, a scarf for the sand and dust, sunglasses (even graduated instead of contact lenses), a jacket and a sweater for the evening, especially in the north, desert boots or sandals; for Nouakchott and the coast, light clothes for the day, a sweatshirt and a jacket for the evening, scarf for the sand.
In summer: lightweight clothes, long and natural fabric (cotton or linen), desert turban, a sweatshirt for the night, sleeping bag for night outdoors. For women, it is best to avoid shorts and miniskirts. On the coast north of Nouakchott, a light sweatshirt; for the south, a light raincoat for the rain showers.