PHILADELPHIA - Commuters along this city's Schuylkill River have a grim midsummer view of an alarming number of trees that appear to be dead or dying. Leaves of many smaller trees are brown and lifeless weeks before they should even be turning color. Larger and healthier trees are frequently tinged with yellow. Suburban backyards for miles around are littered with prematurely fallen leaves.
The cause, monitored by orbiting satellites, is one of the most severe countrywide droughts in recorded history.
Like Philadelphia, half the nation is overly parched, according to calculations made last week by the NOAAs National Climatic Data Center (NCDC) in Asheville, N.C. The situation is worse in much of the Southwest. For most places, the end is not in sight, and some trees simply will not survive, scientists say.
In several states, drought has been the norm for three years or more. And conditions have worsened. The 12 months ending in July were the driest on record in Georgia, South Carolina, North Carolina, Virginia, Colorado and Wyoming. It was the second driest period during 107 years of record keeping for Arizona, Nevada and Delaware.
In the East, where summer thunderstorms are the source for the bulk of annual rainfall, billowing afternoon clouds and dark squall lines have been mere memories from years gone by.
At the end of July, 49 percent of the contiguous United States was in moderate to extreme drought, according to the Palmer Drought Index. The worst drought on record, peaking in 1934, involved 80 percent of the country.
Seeing green
The Palmer index is produced by NOAA, parent organization of the National Weather Service, and is derived in part from temperature and moisture data collected from space. These measurements help fill gaps between ground-monitoring stations.
But the drought watch also involves looking at long-term effects. Satellites also keep their orbiting eyes on vegetation; detecting healthy green areas versus stressed regions. Alan Basist, a research meteorologist at the NCDC, develops computer products that produce imagery based on such satellite data.
In a telephone interview, Basist explained that visible-light monitoring from space is used to determine active photosynthesis. Healthy plants emit radiation differently than those that are not converting sunlight to energy.
In a separate effort, microwave energy detected by military satellites is used to see through clouds and
at, and just under the surface.No single set of data accurately reflects long-term drought conditions. But these two products, combined with others, allow for a more complete picture, Basist said.
Basist's efforts are beginning to have global implications. He is working with the United Nations to identify regions where crops are under drought or flood stress. The World Health Organization is interested in using the processed satellite imagery to predict outbreaks of waterborne diseases and to figure out when regions are dry enough to allow relief caravans to pass without getting stuck in the mud.
Hot and hotter
In Philadelphia and elsewhere, it's not hard to figure out why leaves are falling. Along with the lack of rain has come an incredible stretch of miserable heat. As of Monday, the region had experienced nine straight days with temperatures in the 90s, according to Jim Gross of the National Weather Service in nearby Mount Holly, New Jersey. It only takes three days above 90 for Gross and his colleagues to declare a heat wave.
"The trees adjust to that and shed their leaves early," Gross said. "It's a normal occurrence in drought situations."
Many trees won't survive, though.
With the prolonged drought comes a lower water table, explained Dale Mohler, a senior meteorologist at AccuWeather. In some places, the underground water is below the reach of shallow-rooted trees and some young trees. Mohler said brown leaves are probably a sign of dying trees rather than early leaf drop.
"Some trees that are struggling might make it," Mohler said. "Certain varieties are starting to die off."
Satellites also examine the potential for a vicious cycle of heat begetting heat, further stressing parched plants.
"If you've got less vegetation, you've got more bare ground, which heats up more and shows on satellite sensors," Mohler said.
The forecast
Relief may come, but the majority of the country probably won't experience much of it for weeks.
Satellites are used to track large-scale weather patterns and ocean temperatures, both of which in turn help forecasters make long-range predictions. So what's going on, and what's to come? Weather in the East, Mohler said, has been dominated by a region of high pressure most of the summer. Cold fronts that would normally move through and produce storms have been thwarted or weakened.
"Come late September or early October we may gradually start to back out of the pattern," Mohler said.
That doesn't mean the drought will end. The National Weather Service predicts September rainfall will be near normal across the United States. Experts say prolonged rainfall over weeks or months would be needed to restore water tables and reservoir levels to normal.
For localized drought information,
advertisement
