H10Scratch

Considering a Sonic Rainboom is similar to a Sonic Boom, when an object (in this case, rainbow dash) goes faster than the speed of sound. I can't remember the episode but there is one when Rarity goes up to the sky with her butterfly wings, and the sun burns them. Depends on a lot of things but assuming it was a really hot day, the speed of sound is faster in warmer air, so that makes the speed of sound faster on that day. It can't really be told what number the speed of sound was at, but it was probably over 761 mph, the speed of sound on the average day. To save time, let's call the speed of sound on that day 'S'. Rainbow Dash would have to go faster than S, again, probably over 761 mph! When she was going down, she was starting to do the sonic rainboom at that point, but Rarity was falling FAST, and she noticed that, and I don't know if it was luck, but I'm guessing 'adrenaline' kicked in and that made her REALLY fast, able to save Rarity. I can't remember the end but I do believe she pulled off a sonic rainboom, meaning she went faster than 761 mph, again bringing us back to the very beginning, considering a Sonic Rainboom is similar to a Sonic Boom. In conclusion, on the average warm day, she has to go over 760 mph to pull off a sonic rainboom! Doesn't surprise me that her cheeks flap- Hope this helped!