MAY HIGHLIGHT: When In Drought, When In Doubt…Leave It Out

By Halverson, Jeffrey B

The title of this month’s column refers to a time-tested maxim employed by National Weather Service forecasters when faced with forecasting the likelihood of rainfall during a drought. In May, a particularly pernicious drought dug in its dusty heels throughout the southeastern United States, particularly in Alabama, northern Florida, Georgia, southern Kentucky, and Mississippi. A great many locations in Georgia, for example, received zero rainfall during May, and drought conditions were categorized as “extreme,” a classification that is expected once every 50 years. Satellite remote sensors help scientists quantify the severity of the drought by examining the stress on vegetation and the rapid spread of wildfires. Figure 1 shows the extent of stressed vegetation, all of which was dying or experiencing stunted growth. The image, acquired by NASA’s MODIS sensor on the Terra satellite, is a composite view constructed over three weeks in May. The brown colors indicate sparse, severely stunted vegetation; green color (what little there is) reveals healthy, abundant vegetation. The oval gray patch over the Georgia/Florida border indicates a fire-scorched zone.

The demise of vegetation is one of the key factors that perpetuates drought. Healthy vegetation wicks up enormous quantities of soil moisture. The water is crucial for biological processes, then it’s liberated through the leaves in the form of water vapor. The flux of water through plants and into the air is known as “transpiration.” The combination of transpiration and normal evaporation of soil water helps moisten the overlying air mass. But in the case of Alabama and Georgia, the soil was desiccated and stressed plants clamped shut their stoma in an attempt to preserve precious water. Without evapotranspiration, the dewpoint of the air mass lowered. Normally, evaporation helps cool the surface because some of the solar energy is used to change the phase of water from liquid to gas. But without surface water, all of the sun’s energy was funneled into heating the overlying air. The result was a drier, warmer air mass. And without the local recirculation of atmospheric water, the hydrological cycle ground slower, meaning less rainfall. Surface temperatures soared into the upper 90s, reaching more than 100[degrees]F throughout the Southeast.

Of course, there are additional meteorological factors at play in sustaining major droughts, such as a shift in the position of the jet stream’s flow and the attendant zone of high pressure. But on a local or regional level, the loss of the southeast’s normally lush vegetation enhances the aridity.

Georgia also contains vast swampland. When swamps dry, the rich peat provides abundant fuel for fires. The second figure shows the extent of fires ignited across the Southeast on May 30, again captured in false color by Terra’s MODIS instrument. This shows the Bugaboo Fire raging in the Okefenokee Swamp, which ignited during the first week of May. The brown regions show scorched earth. Turquoise reveals smoke mixing with high clouds, and red squares pinpoint the actual location of fires.

By the end of June, according to the U.S. Drought Monitor, the locus of drought shifted into central Alabama and increased in severity from Extreme to Exceptional. During the first few days of June, widespread rains associated with Tropical Storm Barry added moisture to parched ground over parts of Georgia. Barry’s broad circulation imported water vapor over the region from the ocean and recharged the regional air mass that was depleted in moisture. But the drought continued unabated in some parts of Georgia, and the only real hope for breaking it is a succession of similar tropical disturbances during late summer and early fall.

Contributing Editor JEFFREY B. HALVERSON is an associate professor of geography at the University of Maryland, Baltimore County.

Copyright Heldref Publications Sep/Oct 2007

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