Total irrigated land area has been expanding in Nebraska over the last 40 years, propelling the state into a leading position within the US in terms of irrigated acreages. Typically, those counties which display the largest degree of irrigation development had a significant portion of their land area already irrigated in 1978. Large-scale irrigation in Nebraska affects its atmospheric environment. During the typical irrigation season of May to August, counties with the largest rate of irrigation expansion have cooled by about 0.2 – 0.3 °F per decade in the summer months of June, July and August, while counties with the smallest rate of development warmed by about 0.15 °F per decade during the same period. In the summer months, relative humidity increased three times faster, by about 1.5 % per decade, in well irrigated counties than in least irrigated ones. Finally, ET rates increased by 0.05 inch (1.3 mm) per decade in well-irrigated counties while they stayed about constant in the least irrigated ones.