Is this correct?

[pmda]

Here is a question and my answer. I'm sure it's not elegant but does it look OK.

Cur multi homines urbem relinquunt aestate?

Multi homines urbes relinquunt quia urbes nimis callidus sunt at villas suas qui petunt et quae procul ab domibus frigidiores sunt.

[Imber Ranae]

Multi homines urbes relinquunt quia urbes nimis callidus sunt, at villas suas qui petunt et quae procul ab domibus [missing verb] frigidiores sunt.

You mean calidus (or caldus) "hot" rather than callidus "shrewd". Also check the gender and number of urbes, as calidus must be in agreement with it.

After at it's a bit of a mess: Pay close attention to the grammatical role each noun and pronoun is meant to play within its clause, and change the cases accordingly. You also inexplicably changed the gender of the relative pronouns, though I'm guessing you meant them both to refer to the same antecedent.

[pmda]

Thanks. I wasn't paying nearly enough attention as I wrote it.