Rain in Yokohama

Rain is really something different from Germany. Everyone who witnessed a rainy season knows what I'm talking about ;) But today was even for me something new. As you can see, there were no dark clouds. I mean okay, for rain that isn't necessary but we also had thunder and storm. And the thunder sounded like it cracked right above us :D Doesn't that mean there have to be dark clouds? Is it physically possible? Questions over questions ... ;)

View outside of my workspace