I am a 23 yo male. I have my own place, but recently have been spending more time with my parents at their house. My Dad and Mom have been married 28 years. My dad is bipolar. He is very successful at his job, they have a beautiful house. My mom is so patient with him, caters to him and does so much for him. He is incredibly polite to most every person, including me. But he is absolutely awful to my mom. He does not ever hit her, but any little thing that sets him off is my mom's fault. He will yell at her, throw things, not speak to her. 

As a kid, I never realized that this was abnormal. I grew up to be afraid of my dad. Now that I am older, I really want to stand up to him and tell him what an absolute asshole he is to my mom. If I was her, I would have divorced him long ago.  It's like he bottles all of it up and is nice to everyone throughout the day, and then unloads on my mom. It is heart breaking and I feel like less of a man for not standing up for her. If a stranger talked to her like that I wouldnt think twice to give them the what for. Don't get me wrong. I love my dad. But he needs another man to tell him to get his shit in line with my mom.

How do I do this? I want to do it in a way that minimizes his anger and dosent result in any bad fallout for my mom. I'm not a kid anymore, he needs his adult son to tell him that he's wrong.