No usage restrictions. No usage restrictions. These are calculated by averaging the 4 times daily data into monthly means. In some cases, the units were changed from the original NCEP data. That time period was used to avoid some of the problems before 1968.
Some variables use a different climatology time period.
The long-term mean is subtracted from the mean for each year and that squared number is averaged over all years and then the square root is taken. It represents the average deviation from the mean from the long-term mean. The average of this number is the intraannual standard deviation. Feb 29 data is ignored.