The UAE has a desert climate with extreme summer heat and mild winters. June through September is brutal: temperatures regularly exceed 45°C with high humidity near the coast — outdoor activity is essentially impossible in the middle of the day. Air conditioning is omnipresent.
The best time to visit is October–April: daytime temperatures of 20–30°C, minimal rain, and comfortable evenings. December–February can occasionally be cool (15–20°C) and even rainy. Dubai's famous outdoor events and markets (Global Village, Expo) run October–April. The beach season is October–May.