The standard deviation (StDev) is a measure of how far the observations in a sample deviate from the mean. It is analogous to an average distance (independent of direction) from the mean. The standard deviation is the most commonly reported measure of dispersion. It also serves as an estimate of the dispersion in the broader population from which a sample is taken. Like the mean, the standard deviation is very sensitive to extreme values.

If the data are normally distributed, then the standard deviation and mean can be used to determine what proportion of the observations fall within any given range of values. For example, 95% of the values in a normal distribution fall within + 1.96 standard deviations of the mean.

The standard deviation for the precipitation data is 2.378. This tells you that on average, the values in the data set tend to differ from the mean by + 2.378.

The large value of 10 days with precipitation for April increases the standard deviation quite a bit. Without this value, the standard deviation would be 1.155 instead of 2.378. Conversely, if April had 30 days of rain, the standard deviation would be 8.210!