The data for upload in millimeters for both snow and rain is described in their protocol for weather station data transmission. Precipitation volume, not chance, is stated in their JSON API v.1.0 (prior one).
Overall, it's the volume of precipitation in millimeters.
Also,
Forecast.io provides precipitation likelihood with a forecast (but not history, as far as I'm aware).
{"time":1421080800,"precipIntensity":0.0401,"precipIntensityError":0.0099,"precipProbability":0.47,"precipType":"rain"}
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