Abstract
Using state monthly values of the Palmer Drought Index from January 1895 through April 1981, thespatial and temporal features of dry and wet episodes over the contiguous United States were analyzed. Thevariance spectrum of the area under both drought and wet spells in the western United States (17 westernmoststates) was also investigated.
The main results are as follows. Consistent with the findings of previous investigators, the interior andwestern portions of the United States are found to be more drought-prone than other parts of the country.By contrast, the likelihood of drought occurrence in states near coastal areas is considerably less. Prolongedmoisture abnormalities also tend to occur over the more drought-prone states indicating a tendency towardbimodality (either too dry or too wet).
The variance spectrum of the area under drought in the western United States exhibits a red-type spectrum,whereas the wet spell area exhibits relatively greater variance at the highest and intermediate frequencies(2 and 3-9 years). In a few regions of the United States, the initiation and termination of drought episodestend to occur more often at certain times of the year. For most regions, however, this preference is onlymarginal or non-existent.
