Khartoum has a hot desert climate, and you do not visit casually in summer. From May through September daytime temperatures regularly top 40°C, sometimes pushing 45°C, and dust storms — haboobs — sweep over the city in afternoon walls of red air. The Nile rises in August and September, the only time the confluence colours of the two rivers are really visible.
The good months are November through February, when daytime highs are typically 30°C to 35°C and nights drop pleasantly. December and January are coolest, occasionally below 20°C at night, and this is when most travel to Sudan happens. March and April are bearable but warming up fast.
Ramadan in Khartoum shifts the rhythm of the city dramatically — most restaurants are closed during the day, and the streets come alive only after sunset, when families gather for iftar and the markets stay open until dawn.