public class Test {

    public static void main(String[] args) {
        String str = "晴间少云,20℃～29℃,东北风2-3级,相对湿度最大70%,最小38%,火灾危险信号为红色";
        System.out.println(separate(str, 20));
    }

    public static String separate(String input, int len) {
        StringBuffer sb = new StringBuffer(len);
        int count = 0;
        char[] chars = input.toCharArray();
        for (int i = 0; i < chars.length; i++) {
            char c = chars[i];
            sb.append(c);
            count = (c <= 0xff) ? count + 1 : count + 2;
            if (count >= len) {
                sb.append("<br/>");
                count = 0;
            }
        }
        return sb.toString();
    }
}

------解决方案--------------------
学习。