Whereas the related term "Southwest" as shorthand for Southwestern United States is generally used in a cultural or historical sense (for example, to refer to parts of the United States that were once part of the First Mexican Empire), the term "Pacific Southwest" is more commonly defined strictly by geographic or ecological factors.
For example, the American Society for Photogrammetry and Remote Sensing has a geographical application of the term, defining it as California, Arizona, Nevada and Hawaii.
[1] The Fish and Wildlife Service, on the other hand, uses an ecological approach to define it as California, Nevada, and the Klamath Basin (which includes parts of Oregon).
In addition to the Fish and Wildlife Service mentioned above, the Environmental Protection Agency defines Pacific Southwest as California, Nevada, Arizona and Hawaii,[3] and the Forest Service defines it as California, Hawaii, and other U.S. islands in the Pacific Ocean.
Sub-arctic conditions are common in the high mountains of the region, such as in the Sierra Nevada and on Mauna Kea.