rain不可数.请看剑桥英语英英词典对名词rain的解释：
drops of water from clouds
examples:
Rain is forecast for tomorrow.
Come inside out of the rain.
We had heavy/light rain all day.(注意这个,并没有加a）
We got caught in pouring/torrential (= a lot of) rain without either raincoats or umbrellas.
There will be showers of rain/rain showers (= short periods of rain) in the east.
It looks like rain (= as if rain is going to fall).