As they descended the mountain, the temperature began to rise.

descended 是动词 descend 的过去式。descend 由前缀 de-（表示"向下"）和词根 scend（来自拉丁语 scandere，意为"爬"）组成，字面意思是"向下爬"，引申为"下降、下山"的意思。