为什么 if the weather were warm 要用were weather不是不可数的么?那weather又

这里是虚拟语气,虚拟语气中,所有人称的be,都要用were,这是固定用法.
weather是不可数名词,没有复数的.
如何判断虚拟语气：一般由if引导,对过去的事情或不可能的事表遗憾.
要注意的是它与if引导的条件从句不同.
If I were you,I will…… 如果我是你（不可能的事）,
If I had worked hard in high school,I could go to a better uneriversity.（对过去的事表遗憾）
看下面的句子,条件从句和虚拟：
If I have enough money,I will go to shop with you.(条件,我不确定我是否有足够的钱,可能要查一下)
If I had enough money,I would go to shop with you.（虚拟,我没有钱,很确定）