By snobuilder on Thursday, April 10, 2008 - 06:03 pm: Edit Post
Hey John., as a meteorologist, do you think the daily mean in temps and such should be referred to as the days "normal" or the days "average"?
I noticed that one of the local forecasters has switched from using normal to using average, which in fact IS exactly what it is, ain't nothin' "normal" about it!
i believe that using terms like normal leads people to believe that this is the way things SHOULD be for that particular day, which IS NOT even close when you look back at past years daily stats.
By admin on Friday, April 11, 2008 - 08:49 am: Edit Post
I have always used the term "average" as I believe it to be more representative of what we are talking about. One thing is certain, there is not such thing as "normal" when it comes to the weather!
By frnash on Friday, April 11, 2008 - 03:08 pm: Edit Post
"i believe that using terms like normal leads people to believe that this is the way things SHOULD be for that particular day
Actually snobuilder nailed it exactly.
Not to start an argument with John, but this exact problem here has been recognized by NOAA/NWS and other climate wonks for quite a long time, as noted below.
The problem is that what we are referring to here is a "Climate (or climatological) Normal", which has a very precise definition. Unfortunately, that definition is obviously not well understood by the layman.
From the Frequently Asked Questions at the National Climatic Data Center:
I can well understand why John has "I have always used the term 'average'", particularly when addressing the layman rather than a member of the scientific community.
What is a 'Climate Normal'?
The term climatic "normal" has faced a dilemma since its introduction a century and a half ago. A climate normal is defined, by convention, as the arithmetic mean of a climatological element computed over three consecutive decades (WMO, 1989).... a normal value is usually not the most frequent value nor the value above which half the cases fall." The casual user, however, tends to (erroneously) perceive the normal as what they should expect. Dr. Helmut E. Landsberg, who became Director of Climatology of the U.S. Weather Bureau in 1954 and, later, Director of the Environmental Data Service, summarized the dilemma quite well over four decades ago (Landsberg, 1955): "The layman is often misled by the word. In his every-day language the word normal means something ordinary or frequent. ...When (the meteorologist) talks about 'normal', it has nothing to do with a common event..... For the meteorologist the 'normal' is simply a point of departure or index which is convenient for keeping track of weather statistics..... We never expect to experience 'normal' weather."
It might be "normal" for the weather to swing radically between extremes from day to day and year to year, but the "climatic normal" is simply an arithmetic average of what has happened at such a "swinging" place. This is why it's important to use a measure of the variability of climate (such as the standard deviation and extremes) in conjunction with the climatic normal when studying the climate of a location (Guttman, 1989).
In accordance with national and international convention, the official climate normals computed for U.S. stations by NCDC consist of the arithmetic average of a meteorological element over 30 years. The 'official' normals are provided solely by NCDC, which should be noted in light of other non-official means computations from a myriad of sources.
Is a "Normal" the Climate You Would "Expect"?
Climate normals are a useful way to describe the average weather of a location. Several statistical measures are computed as part of the normals, including measures of central tendency (such as the mean or median), of dispersion or how spread out the values are (such as the standard deviation or inter-quartile range), and of frequency or probability of occurrence.
Over the decades the term "normal", to the lay person, has come to be most closely associated with the mean or average. In this context, a "climatic normal" is simply the arithmetic average of the values over a 30-year period (generally, three consecutive decades). A person unfamiliar with climate and climate normals may perceive the normal to be the climate that one should expect to happen.
It's important to note that the normal may, or may not, be what one would "expect" to happen. This is especially true with precipitation in dry climates, such as the desert southwestern region of the United States, and with temperature at continental locations which frequently experience large swings from cold air masses to warm air masses.
As a "mathemagician" myself, I can tell you that it is possible to engender similar confusion with the terms average and/or mean. (Average over what period of time? Over a fixed period of time, or a rolling average? etc.)
Sometimes you just have to understand the formal definition of the terms being used.
By snobuilder on Friday, April 11, 2008 - 08:48 pm: Edit Post
Thanks for the lengthy info, I had a hunch that the word normal when applied in the field of climatology has a very precise meaning, but in everyday use of the word, to us laymen, it can be misleading when referring to weather stats.
I believe it tends to help out the GW enthusiasts make there point to those who think weather is constant or "normal" and any wide swings up or down from the norm could be construed as a man made calamity.