The changing natural world at our doorsteps | Illustration and text by Patterson Clark
August 9, 2011
A drier August appears to be the new normal
With Dan Keating / The Washington Post
Washington's August has dried up.
What was once the rainiest month of the year is now one of the driest. It has become a month marked by withering foliage and aborted blooms.
Average rainfall at Reagan National Airport in August has fallen from almost five inches in the middle of the last century to less than three inches today. The tropical storms that used to roll through the area bringing 12 and 14 inches of rain at a time have tapered off. The most rainfall August has seen since 1980 was seven inches in 1990. And in the past decade, August has exceeded five inches of rain for the month just once.
"That is quite an eye-opener," says Mike Hapert, deputy director of NOAA's Climate Prediction Center. "My recollection has been that August has been one of the wettest months."
So what happened?
Experts at the center are reluctant to point to a reason. "It could be chance that there weren't any" tropical storms recently, says Dan Collins, a meteorologist with the center. "I am sure that there is no good evidence that [human-caused global warming] caused the change."
Rick Schwartz, a weather historian and author of "Hurricanes and the Middle Atlantic States," calls it remarkable that so few hurricanes have made it to the East Coast during the past decade, even though the North Atlantic is in the midst of an active hurricane period. "Hurricanes are either weakened or turned out to sea," he said, speculating that the region's increasingly hot weather seems to be acting as a shield, which keeps hurricanes away.
"But if there is a cause and effect," he said, "it definitely won't be established for a long time."
Whatever the reason, a drier August has had a significant impact on plant and animal life in myriad ways.
Because most of the growth for many summer crops is in June and July, less rain in August has little impact on yield if rainfall is near normal the rest of the summer, says Robert James Kratochvil, a University of Maryland agronomist.
Less rainfall might even be good for some wetlands. Lower water levels can open up large areas usually inundated in late spring and summer, says Changwoo Ahn, a wetlands ecologist at George Mason University. This would trigger germination and growth of a variety of moist-soil plants that would otherwise have been still dormant, Ahn said. Such a vegetative explosion would enrich the wetland's diversity and productivity.
But a boost for wetland plants might come at the expense of some wetland animals: Bullfrog tadpoles need water in August to complete their development, says Kevin T. Munroe, manager of Huntley Meadows Park in Fairfax County. "Although we still have a strong bullfrog population, we do appear to have less now than we did 15 years ago," he said.
"There are also several species of dragonflies that lay their eggs in shallow, late-summer wetlands," he adds. He has noticed fewer bar-winged skimmers whenever August is dry.
Perhaps one of the most visible effects of a drier August can be seen in the oaks, says Rod Simmons, a plant ecologist with the city of Alexandria. "Oaks are generally drought-tolerant," he says. "But they do require sufficient and consistent moisture during growing season for good health and long-term survival."
He called the extensive crown dieback of numerous old-age oaks throughout the region a sign of doom for the trees.
There could be hope, however. Although Northern Virginia still has an August rainfall deficit, Maryland's regional totals have been above average for the past two years. Could this be the start of an upward swing?
*Comparing difference in normal rainfall between the periods of 1900-1980 and 1981-2010.
Labeled changes are statistically significant within a 90% confidence interval.
Bold labels indicate significance within a 95% confidence interval.