# Percent of Normal Precipitation

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.

## 25 Responses

1. Use the following formula to calculate PON for this categories of below normal, normal and above normal.

The percentage of normal is usually computed as follows PON = (current value of pre/long mean value of Pre)*100
And the classes are ; <75% Below Normal; 75-125% Normal; 125% Above Normal.

Thanks.

2. Eidith says:

why is the result of my calculation pi / p * 100 different from the result from the MDM application? Thank you for your help

1. Katrin Ehlert says:

Dear Eidith

Thank you for your question. In order for us to respond to your question, could you please specify your request further: 1) MDM application – what are you referring to?, 2) Please name the parameters used in your calculation.

Best regards
IDMP TSU

3. Robert Stefanski says:

Dear Logeshan,

Thank you for your question. Please go to the online link on the percent of normal index. 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.

Best regards,

IDMP TSU

1. Jignesh says:

I have perticular standardized precipitation index value and I want to convert it in percentage how can I do it

1. Valentin Aich says:

Dear Jignesh, Thank you for your question. Since the SPI is already standardized, there is usually no need to convert it. If you could explain in a bit more detail what you have in mind, we would be happy to clarify that question with you.

Kind regards,
Valentin

4. Exavery says:

Dear Anthony

May you please give me the reference(s) for this categories of below normal, normal and above normal.

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.

Thanks.

5. Logeshan says:

how to calculate percent of normal drought index
plz tell me with example

6. Katrin Ehlert says:

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

7. Dwayne Lanferman says:

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

8. Morteza says:

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

9. Joshua Kuria says:

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?

1. Robert Stefanski says:

Dear Joshua,

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

Best regards,
IDMP Team

10. Anthony says:

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

1. Exavery says:

Dear Anthony

May you please give me the reference(s) for this categories of below normal, normal and above normal.

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.

Thanks.

11. Anthony says:

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

12. Mr. Kimhuy Sok says:

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

1. Robert Stefanski says:

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

13. akash says:

I want pni drought index data

14. Mr. Sok Kimhuy says:

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

1. Robert Stefanski says:

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.