There are various situations in which this sentence would be correct: 1. If you and your friend had been discussing winter in your country, you could say "winters are getting colder" (in our country). 2 If you were making a general statement about climate change, for example, you might say "winters are getting colder" (on the planet). 3 You could also say "compared to 20 years ago, the winters are getting colder in northern Europe".