The only sensible time to be in Néma is the cool season, roughly November to February, when daytime temperatures stay in the high twenties and nights drop sharply. From March through October the heat is punishing — fifty degrees on summer afternoons is normal — and travel along the Route de l'Espoir becomes genuinely dangerous.
The harmattan blows red Saharan dust through the town from December to February, which can ground the weekly flight and reduces visibility on the road. Time a visit around it if you can.