what本应是 what (a)good weather it is!但weather是不可数名词,不能要a、而it is 就省略了
What....如果用how...How good the weather is!
此举为感叹句,WHAT的句式是WHAT+形容词+名词HOW的句式是HOW+a+形容词+名词
此句为感叹句,WHAT的句式what是+形容词+名词后面主和谓可省HOW的句式是how+形容词+a/an/the+名词