Из Английской Википедии узнаем
Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm drizzle impacts at about 2 m/s, while large 5 mm drops impact at around 9 m/s. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater. See droplet's sound...
То есть от 2 до 9 метров в секунду при размерах капли от 0.5 до 5 милиметров