in winter泛指在冬天，in the winter特指在某个冬天上下文应该有所指

in the winter是特指的某个冬天比如：In the winter,I came to Haerbin for the first time. 这个冬天， 我第一次来到了哈尔滨

而 in winter是泛指冬天比如：It always snows in Haerbin in winter. 哈尔滨的冬天总会下雪