
Because this is the same happening when you are opening the refrigenerator door in a hot summer day: the ice is melting and the water is going down and into atmosphere, and in real life a warmer atmosphere with more water in it will trigger more often and more stronger storms.
Watching the last night live broadcast you can join NASA’s Chief Scientist Dr. Kate Calvin and climate expert Dr. Michelle Hawkinson on NASA Science Live as they discuss the science behind these changes. Have questions? Some people have asked them in NASA live chat during the broadcast about hot stuff (hot considering the wheather). It's too late to join them and ask more stuff, at least for now, but you can expect the next live event and you can start asking.