The sd or “standard deviation” measures the variance or spread of a group of numbers. With weather, higher sd connotes higher uncertainty and greater spread and variation in the models. For normal distributions, 2/3 of a group fall within 1 standard deviation. For the NBM model, if the (mean or average) temperature sd is 1.3º, it means that 67% of the models that comprise the NBM are within + 1.3º or -1.3º of the mean temperature.