it's rainy指的是下雨天，it's raining是说正在下雨，理论上来说两个是不相等的。比如外面正在下雨你就可以和别人说it's raining now。如果是描述今天的天气状况可以说it's rainy today。
it's rainy和it's raining的区别
raining
1.It was raining when we set out. 我们出发时正下着雨。
2.Yesterday the radio forecast drizzle,and today it is indeed raining. 昨天预报有小雨，今天果然下起来了。
3.It's raining hard. 雨下得正急。
4.It's raining rustlingly. 雨刷刷地下起来了。
5.Hurry in,children.It's raining! 孩子们，快进来，下雨啦！
rainy
1.They walked along the promenade on a rainy night. 他们在一个雨夜沿着海滨道散步。
2.The rainy season in the Andes normally starts in December. 安第斯山区的雨季通常在12月份来临。
3.The rainy season has ended at last. 雨季总算过去了。