The standard deviation of the rain column, if that's what you are looking for, will be virtually (within floating point error for a vector this size) identical, because the rain_diff_1 vector will be identical to the rain vector with the sole difference that rain_diff_1[1] will be NA.
Update: what was I thinking? They are only closely similar. For a statistic to evaluate the degree to which a differenced series is still autocorrleated, consider the Ljung-Box test described in Hyndman