Calculation of normals 

Calculation of rainfall averages
The rainfall averages are based on the daily rainfall measurements. Daily rainfall is defined as the 24-hour accumulated amount of rain  from 8:00 AM on a given  day  until 8:00 AM the following day. Monthly rainfall is obtained by summing over the individual daily rainfalls.
The average rainfall for a specific month is the average of 30 monthly values for the period  1961-1990. The calculation requires the existence of all the daily rainfall data. Since rainfall is highly variable from day to day (range 0 to  100mm or more), some of the monthly rainfall totals were obtained by interpolating for a few missing daily rain amounts in order to calculate the 1961-1990 averages.

Time period to obtain rainfall normals
The average rainfalls over a 30-year period are sufficient to adequately characterize the rainfall regime at a station and its surroundings.  This is in accordance with the World Meteorological Organization (WMO) which recommends  continuous 30-year records to calculate rainfall normals.  Furthermore, reliable climatic differences between station rainfalls requires the normals to be calculated over the identical 30-year periods. This also allows for the comparison of averages for different time periods. In compliance with WMO, the average rainfall in Israel is calculated for the years 1961-1990, beginning the 1960/1961 season.
This is refered to the data presented in the rainfall normals page and the rainfall observations page.

However, please pay attention to the fact that in the climate information page the normals are updated for the period 1971-2000.