Two towers located near Hannover and Dickinson in western North Dakota were instrumented to determine the climatology of low level temperature inversions. Platinum bulb thermisters were mounted at 1,2,5, 14, 35, and 91 m heights at Hannover and at 2, 8, 20, 55,104, and 157 m heights at Dickinson. Temperatures were recorded continuously for about 37 months on multipoint strip chart recorders and were averaged over O.5 hr intervals. An inversion condition was defined as a lapse rate ~1C/100 m. Definite seasonal trends were observed. Inversion conditions occur about 36 percent of the time on an annual basis ranging from 28 percent in spring to 50 percent in winter. The average duration of a wintertime inversion was two to three times greater than in summer, but their average intensitives were similar. A nearly linear relationship was found between average intensity and duration (R2= 0.58; significant at 0.01 level). During inversions fumigation conditions occurred about 2.5 percent of the time, varying from less than 1 percent in the summer to over 7 percent in winter. Daytime inversion conditions are rare during the summer but occur with about a 20 percent frequency during the winter. Changes in the inversion commencement and ending times correspond with the seasonal changes in sunset and sunrise times. Inversions occur almost daily during summer nocturnal hours.