Percent of Normal Precipitation
Index name: Percent of Normal Precipitation.
Ease of use: Green.
Origins: The percentage of any quantity is a simple statistical formulation. The exact origin or first use is not known in describing precipitation anomalies.
Characteristics: Simple calculation that can be used to compare any time period for any location. Can be computed on daily, weekly, monthly, seasonal and annual timescales, which will suit many users needs. Calculated by dividing actual precipitation by normal precipitation for the time being considered and multiplying by 100.
Input parameters: Precipitation values suitable for the timescale being calculated. It is ideal to have at least 30 years’ worth of data for calculation of the normal period.
Applications: Can be used for identifying and various impacts of droughts.
Strengths: A popular method that is quick and easy to calculate with basic mathematics.
Weaknesses: Establishing the normal for an area is a calculation that some users could confuse with mean or average precipitation. It is hard to compare different climate regimes with each other, especially those with defined wet and dry seasons.
Reference: Hayes, M.J., 2006: Drought Indices. Van Nostrand’s Scientific Encyclopedia John Wiley & Sons, Inc., doi: 10.1002/0471743984.vse8593.