# Climatological normal and anomalies

**URL:** <https://forum.ecmwf.int/t/climatological-normal-and-anomalies/1751>\
**Category:** Other - Software and tools\
**Tags:** c3s, toolbox\
**Created:** [2 May 2021 23:53 UTC](https://forum.ecmwf.int/t/climatological-normal-and-anomalies/1751 "2021-05-02T23:53:19Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Brian\_Yalle](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/b/8dc957/32.png) [@Brian\_Yalle](https://forum.ecmwf.int/u/Brian_Yalle)\
**Post date:** [2 May 2021 23:53 UTC](https://forum.ecmwf.int/t/climatological-normal-and-anomalies/1751/1 "2021-05-02T23:53:19Z")

</div>

Hi!

I'm not expert in climate science and I need your help to solve some doubts in the most simple possible way to understand.

1) The&nbsp;[CDS Toolbox documentation](https://cds.climate.copernicus.eu/toolbox/doc/how-to/13_how_to_calculate_climatologies_and_anomalies/13_how_to_calculate_climatologies_and_anomalies.html#calculating-anomalies) states the current climate normal period is January 1981 to December 2010. Is it a worldwide convention? What is the reason behind choosing this period of time?&nbsp;

2) Regarding climatological normal convention (1981-2010), does it not affect or matter that "the data object" period may be outside 1981-2010 range? Can the data object be just a year?

3) In this&nbsp;[section](https://cds.climate.copernicus.eu/toolbox/doc/modules/cdstoolbox.climate.html), what is the difference between "ct.climate.anomaly" and "ct.climate.relative\_anomaly"? How do the outputs look like? and how they can be interpreted?

Any help will be appreciated

Thanks in advance,

Brian