An extensive dust devil census taken near Tucson, Ariz., has provided, from two independent areas, quantitative information on the relation of environmental conditions to the spatial distribution, frequency and duration of a particular type of dry atmospheric thermal. The observations indicate that for these two areas, each approximately 100 mi2 in area, dust devils are initially sighted most frequently in the vicinity of dry river beds which lie in the lee of small hills. Once initiated, the visible lifetime of the observed dust devils ranged from a few seconds to approximately 20 rain. These data show that dust devil duration increases with dust devil size. The results indicate that the energy for the dust devil is the warm boundary layer air which is continually being fed into the visible vortex as it moves along with essentially the same speed and direction as the environmental wind.The highest dust devil frequency occurs with the lowest atmospheric stability and not necessarily with the highest shelter air temperature. The diurnal variation of dust devil occurrence follows a Gaussian type distribution with the maximum dust devil activity between 1300-1400 LST, which is approximately at the time of maximum soil surface temperature and convective heat flux. The appearance of dust devils suggests that the smaller scale eddies may be unable to accomplish the required vertical heat transfer; hence, larger elements such as thermal plumes and dust devils are necessary to accomplish the transfer through a deep layer of the atmosphere.The mean number of dust devil occurrences per day varied from 50-80 between the two areas investigated. While the dust devil frequency increased from June to July, the observations indicate that on days with cumulus cloud cover dust devil activity is suppressed or terminated.