What determines the temperature of rain water?
Most raindrops form at temperatures below freezing, in the cold high layers of the atmosphere. Raindrops can exist in a cloud in the form of supercooled droplets of liquid; this tends to happen when there is a lack of condensation nuclei available for the drops to form around. If condensation nuclei are present, then snow flakes or ice pellets form. Either way, when the precipitation leaves the cloud it is below the freezing point.
As the precipitation falls toward the earth, it passes through layers of air at various temperatures. If it passes through warm air layers, the ice melts and the precipitation falls as rain. The exact temperature of the rain is determined by the temperature of the air the droplets travel through on their way down.