The Importance of Perspective
by Brian Sutherland
By now, the word climate is familiar to almost everyone. But what everyone knows about climate depends a lot on what one is told about climate, and how it is told.
There are many different ways of describing the climate in terms of what are officially known as “Means, Normals, and Extremes”. A certain few parameters are included in these statistics, but many others are not.
These statistics are derived from the day to day readings- most obviously of temperature and precipitation- taken at stations that meet certain minimum requirements as determined by the National Climatic Data Center (NCDC) in Asheville, North Carolina.
Fundamental to this record keeping is what is known as the “Period of Record”. This is simply the number of years that continuous records are kept at any one location. Climate normals are determined from these records over a span of 30 years, and are updated every 10 years.
Currently, the climate normal period we are using goes from 1971 to 2000. Before it was updated in the year 2000, the climate normal period used ran from 1961 to 1990. The climate normal period will change again in 2010, and will describe the period from 1981 to 2010.
This is done because climate is always changing. Climate is changing now, climate has always changed, and climate will always change. That is the nature of climate.
The term “normal” is, of course, a bit misleading, and really should be thought of as the "average" conditions. Use of the word normal, although correct from the standpoint of statistics, implies that anything that’s not normal is abnormal. To avoid the confusion, some organizations no longer even use the word normal, but speak of averages instead.
In addition to the “Normals”, and the records that get the most attention, are the “Extremes”. Most common are the extremes of temperature and precipitation.
It seems that recently many high temperature records are being set, in particular in Bluefield, West Virginia. This is where the length of the period of record becomes significant.
The statement could be made that all of the temperature records at Bluefield have been set in the last 50 years. This would be true, but would be highly misleading, since official records only began there in 1959.
In the Blacksburg Weather Forecast Office area of responsibility there are five climatological stations for which we issue daily and monthly climate summaries and, of course, records. At four of these stations- Bluefield, Blacksburg, Roanoke, and Danville- the period of record is only about a half century.
The fifth station for which we maintain climate records has a period of record over twice as long as the others. At Lynchburg, Virginia, official records- as recognized and accepted by NCDC- have been kept since 1893. This creates quite a different perspective.
As I mentioned earlier, there are many different ways to look at climate statistics.
One such parameter is the daily high temperature record.
In each of our daily climate bulletins the high temperature for that day is mentioned, and right along side is shown the record for that day; that is, the highest temperature ever recorded on that day, and in what year that daily record was set.
For the purpose of this article, I took the period of record at Lynchburg and divided it roughly in half. I used the cut-off of 1950, and determined how many daily records were set before 1950 and how many were set during or after 1950. I looked at records through 2006 since that is the last complete year in the period of record.
236 daily high temperature records were set at Lynchburg before 1950, 129 were set in or after 1950. Put another way, 65% of the 365 daily high maximum temperature records at Lynchburg were set before 1950.
Another parameter one could look at is the number of days in a year that the temperature reached 100 degrees or more. 100 degrees is a nice, round number and certainly is an indication of extreme heat.
First off, Lynchburg has not recorded a 100 degree day since 1988. In the period of record, Lynchburg reached or exceeded 100 degrees on a total of 56 days. 43 of those days were before 1950, 13 of those days were after 1950. That is, 77% of the 100 degree days occurred before 1950.
Was there a period of time before 1950 when the majority of those excessively hot days occurred? There certainly was. In 1930 there were 8 days at 100 degrees or higher. 7 days in 1934 reached 100, 6 in 1936, and 5 in 1932.
Over half of the hottest days in Lynchburg occurred in roughly the first half of the 1930s. This is, of course, the period in which much of the country became known as the “Dust Bowl”.
It would not be unreasonable to assume that these statistics would be mirrored at the other four stations if their period of record was as long. It could also be said that far fewer records might have been set recently in Bluefield if the period of record there was over a century long.
So, a period of record of over a century gives quite a different perspective than one of less than half a century. The difference is in how the climate story is told.