It's a major natural disaster that slowly grows in one place and then moves across a region, gaining intensity and size. As it spreads, it destroys land, ruins agriculture and tears apart communities, and it can kill people.
It's a drought.
Researchers are just beginning to view droughts as this type of dynamic force, and some hope that soon they will be monitored similarly to hurricanes — with scientists able to predict their development, helping to protect those living in their path.
Ten percent of droughts travel between 1,400 to 3,100 kilometers from where they begin, according to a recent study. The study, which analyzed 1,420 droughts between 1979 and 2009, identified "hot spots" around the world and common directions in which droughts move.
Some droughts in the southwest United States, for example, tend to move from south to north. In Argentina, they usually migrate the opposite direction. In Central Africa, droughts tend to go southeastern toward the coast.
"It can start somewhere, move throughout the continent, and obviously cause harm throughout its way," Julio Herrera-Estrada, a doctoral candidate at Princeton University and leader of the study, said Thursday.
Droughts that travel are usually the largest and most disastrous, the scientists found. They can cause a loss of agriculture, wildlife, wetlands and human life, according to the National Drought Mitigation Center in Lincoln, Nebraska.
They are also one of the most expensive natural disasters that people face today, according to Herrera-Estrada, who collaborated on the study with the International Institute for Applied Systems Analysis in Vienna.
The most recent moving drought that Herrera-Estrada studied began in 2008 in Ukraine and Russia, and moved 1,700 kilometers northeast, ending in northwest Russia and affecting parts of Kazakhstan on the way. It lasted almost a year.
"People haven't really thought of droughts in this way," he told the Thomson Reuters Foundation.
Future research, Herrera-Estrada said, can shed light on what mechanisms cause some droughts to move and what affects their paths. This can be done accurately, however, only through collaboration among national governments, he said.
"It's important to have a global or a continental understanding about how droughts are behaving," he said. Collaboration "benefits people on the ground, farmers, cities that need water, power plants that need water."
The study was published last week in the journal Geophysical Research Letters.