This Summer was California’s Driest on Record in More Than 100 years, Here’s What That Means

In another alarming measure of California’s historic drought, the summer months this year were the state’s driest on record since 1895, when data-gathering for the government’s standard drought index began.
The monthly average dryness for July, August and September 2021 was -6.8 on the Palmer Drought Severity Index, which indicates extreme drought. Anything below -4.0 on the Palmer scale is considered “extreme drought.” A year with normal precipitation would fall between -0.49 and 0.49 on the scale, and an “extremely wet” year would land above 4.0.