「[寒冷]{かんれい}」 is a fairly technical/academic word and very few people, if any at all, would use it to say everyday things like "It's so cold today." As you said, the word we use for that purpose is 「[寒]{さむ}い」. (Dialectally, people from the north may, for instance, opt to use 「しばれる」, but that has the same level of informality as 「寒い」. Point is 「寒冷」 is too big a word for everyday use everywhere.)
We use 「寒冷」 in formal, technical or academic speech to talk about cold regions and cold weather. You may hear/see us say:
「[寒冷]{かんれい}な[地方]{ちほう}ではオレンジは[育]{そだ}たない。」 "Oranges do not grow in cold regions."
「シベリアは[寒冷]{かんれい}な[気候]{きこう}で[知]{し}られている。」 "Siberia is known for its cold climate."
In compounded technical terms, 「寒冷」 is also often used.
「[寒冷低気圧]{かんれいていきあつ}」 "cold-core cyclone"
「寒冷[前線]{ぜんせん}」 "cold front"