English expressions generally like to use pronouns to refer to the above things, especially in the expression of "comparison of similar things", which is usually only referred to as that (singular) or that (plural), without other words such as it, them and one. This is an idiom, why not. In this sentence, that refers to the weather above, which has the advantage that the sentence is concise and not verbose, which is equivalent to the weather in Wuhan being hotter than that in Beijing. The more in the sentence should be changed to much, right? Hotter is already a comparative.

In addition, I would like to add that in English articles, we also like to use different words to express the same thing or action mentioned above or repeatedly. If we know this clearly, it will help to improve the quality of reading and writing (flexible and diverse writing is more attractive to readers and not boring).