Much ado has been made about the prospect of changing levels of precipitation in Virginia. On one hand, recent moisture shortages (the most recent having been last summer) have elicited speculation that our precipitation might be decreasing, spurring Virginia Drought Task Force activity as well as claims that global climate change will be taking its toll on the state in the form of a drier climate.
On the other hand, speculation that our extremely wet winter and spring is a sign of a trend in precipitation has also been run more than once through the rumor mill, with the (supposedly "global warming"-aggravated) El Niño-Southern Oscillation (ENSO) phenomenon mentioned as a likely culprit. Rumor also has it that hurricanes are becoming more frequent and more intense (as the world supposedly warms) leading to a wetter Virginia.
As is our propensity as climatologists, then, we had to ask ourselves whether or not these rumors had any credibility. After all, there is some basis for serious scientific speculation here. As our previous article on changes in cloud cover around the state mentioned, there have been some locations which demonstrated significant increases in cloudiness under some conditions. In general, such increases in cloudiness can be correlated with increases in precipitation.
In order to answer this question, we had to analyze the long-term precipitation records for any significant trends. We selected a number of stations from around the state, representing the varying topography and general locations, for examination over the period of computerized records, 1949-1997. Then we calculated the annual total precipitation and plotted it for each year of record.
So, are those rumors credible? Not that we've been able to find in Virginia with this analysis. No stations in our study showed evidence of any significant change over the course of the record. For illustrative purposes, we show here the time series plots of annual precipitation totals for Washington National Airport (Ronald Reagan Airport?—We haven't heard of an official name change for the climatological observing station, yet.—Eds.) in Northern Virginia, and Charlottesville in the Western Piedmont.
Although, if a trend line were placed in the data, it would show a slight trend over the course of all the records, such a trend does not turn out to be statistically significant for any of the stations we examined. Since it has no statistical significance, we haven't even included the trend line in these illustrations.
The aggregate data record, showing the overall average of annual precipitation for all 12 stations is shown in our figure below. Note that it reflects the same conclusions reached in the individual station analysis—no statistically significant trend over the period of record despite the appearance of a slight upward trend.
Despite the fact that we've dispelled that one bit of speculation, there are, nevertheless, other interesting bits if information that can be gleaned from examination of the data records. For instance, we see that the wettest year on average over the period 1949-1997 for these stations overall was 1996, and the driest year was 1965. Among other things, this might help explain why the basement of that home you bought 33 years ago has never been so dry as the day you closed on the purchase.
When we look at the averages of total precipitation, summed up by decade, another piece of climate trivia emerges. (Lest we get deluged with notes from Advisory zealots—Never mind that, technically speaking, decades are normally taken as beginning on the year ending in "1" and ending with the year ending in "0," such as 1971-80. For purposes of this analysis, it suited us to use the easier year-0 through year-9 method.) It seems that the 1970's are the wettest decade of the five in the study, and the 1960's showed up as the driest of the five.
It turns out the what we so glibly refer to as "climatological normals," are based on 30-year averages of data, which are re-calculated every ten years. Thus, the current set of "normals" is based on the period 1961-1990. In a few years, the set of "normals" in general use will be those based on the averaging period 1971-2000.
Since we'll be dropping a particularly dry decade at that time (the 1960's) and adding on a wetter one (the 1990's), look for an increase in what's considered "normal" at that time. Will this, then, mean a significant trend has emerged? Alas, as we've already shown, no.
For the individual stations, Wytheville, in the Southwestern Mountains Climatic Division, has been the driest station of the twelve with 37.43 inches of precipitation annually over the course of this record. On the other hand, Wise, also in the southwest, has been the wettest with 46.78 inches of annual precipitation.
Given that these two stations are located within 75 miles of one another, why the big disparity? The difference can be traced to their relative elevations and locations within the mountains. Wise is located at higher elevation and on the west facing side of the mountains; Wytheville is near the valley floor. Locations alee of mountain slopes, such as those in the Valley of Virginia, generally receive less precipitation, and in this case the generalization holds true.
What has all the ado been about, then? Despite the rumor mill, we can see that there is little credibility to claims that there has been any significant change in annual precipitation totals in Virginia in the last 50 years, based on our representative sampling of stations. We generally think that there is a great deal of real information in the climatic record to make ado about, even without rumor mongering!