Published: 2008-11-19 10:00:42
Updated: 2008-11-19 10:00:42
Posted November 19, 2008 10:00 a.m. EST
MIKE MOSS SAYS: Sravan, The "normal" value we present on air is computed by the National Climatic Data Center, and is based on 30 years of observations ending with the most recent "zero" year, with the normal values updated once every ten years. While the numbers are quite close, they are not necessarily the same as a straight 30 year average over the same set of years. This is because, in order to produce a smoothly varying normal that doesn't, for example, decrease for a couple of days during the spring or increase an occasional day in the fall, due to random variations and the fairly small sample size of 30 values, NCDC computes these normals in a way that will cause values to increase a little each day through the spring and vice versa for the fall, and to smoothly curve through a minimum in mid-January and a maximum in mid-July. They do this by using the 30 years' data to directly calculate MONTHLY averages, and then fit a smooth (cubic spline) curve to those averages in order to retrieve the daily numbers that vary in the desired manner.
If you're interested in more details about how climate "normals" are arrived at, NCDC provides a fairly complete description at the address below: