04

Jul

2016

**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. (For more information on this paper, please contact the IDMP HelpDesk).

**Currently used by:** Bosnia and Herzegovina, Democratic Republic of the Congo, Iran, Pakistan, Tanzania, USA.

15 Comments

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hallo, I am Dr jalab from syria , tishreen uni , I need this information ( percent of normal precipitation)

and thankyou for yor help

Dr. jalab

Dr Jalab, Please be more specific with your request. What exactly do you need?

Dear Dr. Robert Stefanski,

I would like to know about the calculation procedure of Percentage of Normal Precipitation. Since, I want to use this index to analyze the drought severity in Cambodia.

Sincerely your,

Kimhuy

Percent of Normal Precipitation is a 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.

Thanks.

I want pni drought index data

Dear Dr. Robert Stefanski,

I would like to know about criteria to classify the PNP (Percentage of Normal Precipitation) value to identify the drought severity.

Sincerely yours,

Kimhuy

Dear Mr. Kimhuy Sok,

Thank you for your question. The PNP is one of the basic weather parameters that is calculated for weather and climate monitoring. To classify PNP for drought, you will need to understand the previous drought impacts and at what level the PNP was at. What is the application that you are working on (agriculture, hydrology, energy)? I would suggest looking at the SPI which gives a better historical view of the previous drought. The PNP does not do this.

IDMP

The percentage of normal is usually computed as follows ;- ((Value – Average)/StandardDeviation)*100

And the classes are ; 125% Abover Normal.

I hope this help.

Anthony from Nairobi, Kenya

The percentage of normal is usually computed as follows ;- ((Value–Average)/StandardDeviation)*100

And the classes are ; <75% Below Normal; 75-125% Normal; 125% Above Normal.

I hope this helps. (Sorry the previous comment missed some classes)

Anthony from Nairobi, Kenya

Dear Dr. Robert Stefanski,

When using the PNP for a period of study say 1978- 2015 do you calculate the mean and Std Deviation for the whole period (1978- 2015) and them normalize the rainfall values?

Dear Joshua,

Yes, this is correct. You should the longest period of record that you have available.

Best regards,

IDMP Team

Dear Friends

Are there any references (book, journal, etc) which discuss about these indices?

Dear Morteza

The Hanbook of Drought Indicators and Indices includes a short description of selected drought indices: https://www.droughtmanagement.info/find/guidelines-tools/handbook-drought-indicators-and-indices/

You may also find the following publication helpful to learn more about the Percent of Normal Precipitation: https://wwa.colorado.edu/climate/iwcs/archive/IWCS_2007_July_feature.pdf

Best regards, IDMP team

How do i interpret the percentage of normals , for instance if i have a normal of 200 and actual rainfall of 10, this would be equivalent to 2% , the interpretation is that the station is 2% below normal ?or the chances of normal – above normal is 2%. thank you for your reply

The percentage of normal precipitation gives you an idea of how the actual precipitation for a given time period relates to the “normal” precipitation, which is usually considered to be a 30-year mean. So if your normal (30-year mean for your selected time period) precipitation is 200 mm and your actual value (for your selected actual time period) is 10, this gives (10 / 200)*100 = 5 % of normal precipitation. In this case, your actual precipitation is 5% of the normal.

Best regards, IDMP Team