ain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).
