"In the summer, it is hot."
Translation:L'été, il fait chaud.
En été il fait chaud is also correct.
L' or le + a season/a month/or a day of the week means each (summer/month/day), and we often translate that to "in or on."
Example: Le samedi je vais au supermarché. / On Saturday I go to the supermarket. (Each Saturday I go to the supermarket.)
It is an idiomatic part of French grammar that differs from English. "C'est chaud" is used to refer to objects, but never weather. The verb "faire" is used for weather.
Il fait chaud. = It is hot.
il fait beau. = It is nice/beautiful.
il fait froid. = It is cold.
There are a number of instances where the verbs are different in French than in English. For example to say that "I am hot" it is j'ai chaud. "Avoir" is the verb used with people, not "être". Using être could be a tad embarrassing! One must appreciate that French is an entirely different language with its own idiomatic ways of expressing things.
Unfortunately for English and French speakers alike, the prepositions don't often correspond between the two languages. Eventually, with time and practice, French prepositions become more natural for English speakers and the reverse is also true for French speakers.
You are correct that "dans" doesn't work here.