The rain and internet speed

Can someone explain to me why I have to reboot my modem at home every time it rains. What is going on there ? Whenever it rains my internet slows down dramatically at home. I have to turn the modem down and redo all of it's settings and all of a sudden the net is going as fast as Patrick Johnson. What's that about ? Why should the rain effect my internet speed....and why is it so easy to fix ? Does anybody know the answer to that ?