How do we measure what qualifies as a drought, or how severe that drought might be?

There are several different ways, but most are incomplete. One measure is how many days have passed without precipitation — but that omits important information, like how many days normally pass in a specific area without moisture falling from the sky.

The Standardized Precipitation Index compares measured precipitation to variable historical rates of precipitation, from one month to 24 months. The Index changes as rates of precipitation rise and fall, a variable not adjusted with other measures.

This naturally raises the question: how do we measure how severe a drought is? One method is the Palmer Drought Severity Index, which “attempts to measure the duration and intensity of long-term, spatially extensive drought, based on precipitation, temperature, and available water content data.”

The PDSI is regularly updated and can be found online at

By: Ellie Cabell

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