Look at these black clouds .-----.

1.will do 是表意愿eg.Now if you will take off your clothes,we will fit the new clothes on you in front of the mirrow.
2.be going to 表将来 a) 主语的意图,即将要做某事eg.What are you going to do tomorrow?b)计划,安排要发生的事eg.The play is going to be produced next month.c)有迹象要发生的事eg.Look at the dark clouds,there is going to be a storm.
3.will be doing 是指将来某一时间正在进行的动作,或表示要在将来某一时间开始,并继续下去的动作,一般有时间状语.
4.be to do 是指按计划或正式安排将发生的事eg.We are to discuss the report next Saturday.
本句意思是看着眼前的乌云认为马上要下雨,表示有迹象表明要发生某事,则应使用be going to do