Associated Press

The ongoing drought in the United States has ties to La Nina, National Oceanic and Atmospheric Administration meteorologists say. La Nina is a weather pattern that consists of an abnormal cooling of the Pacific Ocean that brings dry conditions to the southwestern United States and Mexico. La Nina has been linked to the Dust Bowl of the 1930s, the dry spells in the Southwest of the 1950s and the drought from 1998-2002. See 5 of the worst droughts in the United States.

The Dust Bowl (1930s)
Associated Press

According to a National Climatic Data Center and the National Oceanic and Atmospheric Administration website, the Dust Bowl drought came in three waves: 1934, 1936 and 1939-1940. The "dust bowl" effect came when drought conditions and land management practices left topsoil susceptible to wind forces. The dry soil was lifted by the wind into clouds of dust and sand. The Dust Bowl drought hit farmers already trying to recover from economic losses suffered during the Great Depression. LiveScience reports that at least 50,000,000 acres of land were affected.

1950s drought
Associated Press

Residents of the Great Plains and the southwestern United States were hit with a drought that ran from 1950 to 1956. In Texas, LiveScience reports, rainfall decreased by 40 percent between 1949 and 1951. The drought caused Texas to kick off the modern era of water planning in the state. The Statesman reports that after the 1950s drought, Texas policymakers developed drought contingency plans, expanded water storage and sought new sources of groundwater, while state and federal departments of agriculture put in safeguard programs to help farmers deal with severe droughts.

Northeastern drought, 1960s
Associated Press

LiveScience reports that the drought between 1962 and 1966 that hit much of the Northeastern United States occurred when the temperatures were lower than average, but precipitation also hit an abnormal low.

1980s drought
Associated Press

The 1980s drought, which occurred from 1987 through 1989, covered 36 percent of the United States at its peak, aquaamerica.com reports. The drought began along the west coast before extending into the northwestern U.S., the northern Great Plains and into the eastern half of the United States. The drought became the costliest in U.S. history, with estimates for the cost at $39 billion.

Drought of 2012
Associated Press

A July report on the ongoing drought in the U.S. suggests that the current drought is the worst since the 1950s. "Indicators suggest that the 2012 drought is similar to the 1950s drought in extent, pattern and intensity, although not in duration," the report said. A July LiveScience article suggested that the combination of a heat wave in June and July and other factors can be blamed for the drought. Mark Svoboda, a climatologist with the U.S. National Drought Mitigation Center at the University of Nebraska-Lincoln, said the current drought is parching similar amounts of land as the droughts of the '50s and '30s, but it is not as severe or as long-lasting. It ranks among the five largest U.S. droughts in modern record keeping.