Correct or natural use of articles (the, a, an) is probably one of the most difficult things for learners of English to master. For example, why did I write "temperatures are highest" instead of "temperatures are the highest" in the essay I wrote last week? Click here to read an explanation.
There are many other complex 'rules' about article use in English, which grammar books and websites do their best to explain. I also recommend using Google in the way described below.
Task: try Googling the phrases "temperatures were highest" and "temperatures were the highest" (make sure you put the quotation marks so that Google searches for the whole phrase). Compare the number of search results to see which is more common, then look through the results to find good examples of full sentences for each phrase. You might find that good examples help you more than grammar rules do.