Gao has a harsh Saharan climate. The coolest and most bearable months are November through February, when temperatures drop to around 20–25°C during the day. From March onwards the heat intensifies rapidly, and by May–June temperatures regularly exceed 45°C. The rainy season (July–September) brings some relief from the heat but can make roads outside the city impassable.
The best time to visit is November to January, though even then the harmattan wind carries fine Saharan dust for days at a time. Check the current security situation carefully before travelling — Gao and the wider Gao Region have experienced serious instability related to jihadist insurgency since 2012, with French and UN forces present. Travel advice from your government's foreign ministry is essential.