雨水下降的時速

It could be detected/measured by radar using Doppler effect.

In fact, the velocity of water droplets, both horizontal and vertical velocities, in the atmosphere is an important parameter in determining the prevailing wind speed. It is used in [wind shear radar](風切變雷達) in detecting the existence of wind shear (風切變) near airports.

It makes use of the principle that the speed of wind would affect the velocity of raindrops in the air. Thus a correlation could be established by measuring the speed of rain using microwave (which uses Doppler effect to measure the speed of rain, thus such kind of radar is commonly called Dopler effect radar 多普勒雷達) and from which to infer the speed of wind.

當雨水下降的一瞬間
雨水係作自由落體運動
但一瞬間過後
雨水須克服air resistance (空氣阻力)下落
而air resistance = kv ( k= constant )
在下落一段時間後
雨水達到 terminal speed( 終端速度 )
最後勻速下落