DROUGHT
A dry year and a hot dry year are not the same event, and the Palmer record is where the difference shows up.
What the Palmer index actually is
AND WHAT IT IS NOTThe Palmer Drought Severity Index is a water balance, not a rain gauge. It carries a running account of moisture supply against moisture demand — how much fell, against how much the atmosphere could evaporate away given the temperature — and it has memory, so a dry month after two wet years reads differently from a dry month after two dry ones. Zero is normal; roughly −3 is severe drought and −4 extreme.
Because it responds to temperature as well as precipitation. Two years with identical rainfall, one of them 3°F warmer, do not produce the same drought — the warmer one evaporates more out of the soil and the snowpack, and the Palmer index registers that as the deeper event. This is the single clearest place in the whole record where the warming on the temperature page and the flat trend on the precipitation page stop being two separate facts.
Palmer was designed in 1965 for agricultural soil moisture in the Great Plains, and it transfers imperfectly to a Mediterranean climate that stores a third of its water as mountain snow — it has no snowpack term at all. Its evaporative demand is estimated from temperature alone, which is a cruder method than the modern alternatives. And it is a divisional average, so it says nothing about a specific reservoir or basin. It is used here because it is the longest consistent drought record that exists for California, not because it is the best one that could be built today.
131 years of the water balance
MONTHLY PALMER INDEX, STATEWIDEEvery month since January 1895. Brown below the line is drought, blue above it is surplus. The record's texture is unmistakable — California swings hard and often — but the depth and the duration at the right-hand end are not matched anywhere earlier in it.
—
The severe episodes, ranked
TWELVE MONTHS OR MORE AT PALMER −3 OR BELOWAn episode here is defined and then counted rather than remembered: at least twelve consecutive months at or below −3, so that a single bad month is not a drought and the list matches the events people actually lived through.
Drought is not evenly distributed
THE DEEPEST MONTH ON RECORD, BY DIVISIONEach division's worst month in the whole record. The statewide index averages these into a single number, which is useful for ranking eras and useless for knowing whether a given drought was worst on the north coast or in the southern deserts.
For the current, official picture — updated weekly, built from many indicators including snowpack and reservoir storage that this record does not contain — use the U.S. Drought Monitor. For California reservoir and snowpack conditions, use CDEC. This page is the long historical context behind those, not a substitute for them.