分情况考虑 it可以指代天气以及上文已提到过的单数可数名词等 而that通常指代同类不同物的两个东西
例如：
The weather here is warmer than that in Beijing
这里的that就指代北京的weather,和前一个weather是同类不同物
在肯定句中用实意动词,疑问句和否定句中随便用.例如：
You need to hand in your homework tomorrow.
Do you need to hand in your homework tomorrow?
Need you hand in your homework tomorrow?
You don't need to hand in your homework tomorrow.
You needn't hand in your homework tomorrow.
但是以物作主语时,一般不用于情态动词.
如：
The flower needs watering.=The flower needs to be watered.
当然有区别 at接小地点 in接大地点,当然如果不考虑公园的大小 两者相同
谢谢支持 望采纳