A viewer asks, How do you figure  yearly average rainfall? 

The rainfall averages are based on the daily rainfall measurements. Daily rainfall is defined as the 24-hour accumulated amount on a given  day. Yearly rainfall is obtained by summing over the individual monthly rainfalls.

The average rainfall for a specific year is the average of 30 yearly values. The calculation requires the existence of all the monthly rainfall data. So every 30 years we change the averages.  The same is done for temperatures.