as sonn as possible中的possible是什么词性

possible 是形容词.
as loud as thunder 可以补充完整如“the sound is as loud as thunder.”“那声音跟打雷一样响亮.”也就是两者都是名词,对其共有的特点进行比较.
而as soon as possible 是一个常用词组,意思是“尽快”,而没有和某物做比较的意思,所以一般常见的用法是”Please do it as soon as possible.”,“请尽快完成.”