Why is the weather in Shenzhen hotter than that in Dongguan?

The weather in Shenzhen and Dongguan should be similar. It is normal to say that Shenzhen is hotter than Dongguan, because Shenzhen has more people, cars and roads, and emits more carbon dioxide than Dongguan. Moreover, there are more high-rise buildings in Shenzhen than in Dongguan. The sun shines on the glass of high-rise buildings, reflects on people and feels hot.