Climatologists define a climatic normal as the arithmetic average of a climate element such as temperature over a prescribed 30-year interval. The 30 year interval was selected by international agreement, based on the recommendations of the International Meteorological Conference in Warsaw in 1933. The 30 year interval is sufficiently long to filter out many of the short-term interannual fluctuations and anomalies, but sufficiently short so as to be used to reflect longer term climatic trends. Currently, the 30-year interval for calculating normals extends from 1971 to 2000.
In constructing a climatic normal for a station, certain precautions are maintained by the National Climatic Data Center (NCDC). The data record for the thirty year span should be “homogeneous”, that is, no changes in location, instruments, or observation procedures should have been made during the span that would influence the resulting climatic average. The record should be complete, with no missing values.
A climate normal is computed by averaging the appropriate 30 values of the particular climate element obtained during the current 1971-2000 interval. If changes in exposure or instruments have occurred, a correction is made to remove the non-climatic inhomogeneities. (For additional documentation, consult NCDC.) Corrections are also made to the temperature records for stations in the Cooperative Network to adjust for the time of observations. Typically, daily observations made at these stations are not made at midnight, but at some other time, such as in the morning. Therefore, the maximum and minimum temperature records for the previous 24 hours from these stations would not be consistent with those compiled for the calendar day at all first order stations operated by the National Weather Service. The normals computed by NCDC for all cooperative stations include the time of observation adjustments to make the records consistent with the calendar day. No time of observation adjustments are made to the precipitation data.
National Climatic Data Center, U.S. Climate Normals 1971-2000, Products
National Climatic Data Center, 2002: U.S. Climate Normals 1971-2000: Inhomogeneity Adjustment Methodology) [PDF] is available http://www.ncdc.noaa.gov/oa/climate/normals/normnws0320.pdf
National Climatic Data Center, 2002: Climatography of the US No. 81: Monthly Station Normals of Temperature, Precipitation, and Heating and Cooling Degree Days, 1971-2000, Wisconsin. National Oceanic and Atmospheric Administration, Asheville, NC.
Documentation available at http://www.ncdc.noaa.gov/oa/climate/normals/usnormalsprods.html#CLIM81.
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