It was ______ weather that they decided to go out for a picnic.

考查形容词such和so的辨析。so…that…和such …that…都有“如此……以致……”的意思，其区别是：so后面接形容词或副词，而such后面接名词。故由关键词weather可选出正确答案。

用such...that时，要注意中间的名词是可数还是不可数，weather是不可数名词，所以such后面不加冠词。