Topic: When do they adjust 'Normals'?

[WeatherHost]

We often see discussions of temperatures and precipitation 'above normal' or 'below normal' and there are monthly and annual charts of 'normal' temperatures and precipitation.

We see so many months in a row with average temperatures' above normal'.  So after so many of those, doesn't 'normal' need to be adjusted over time, even if only by tenths of degrees?

[Jáchym]

Hi,
this is actually not accurate.

"Normal" in meteorology has very specific definition, it is always a 30-year period and if you were absolutely strict, it is only a particular 30year period.

The strict normals are:
1961-1990
1931-1960
1901-1930

so the next normal will be 1991-2020 and we should now still be using 1961-1990.

In some cases you will see they are not using it so strictly and also use "normals" like 1981-2010 and 1971-2000. But strictily speaking these are not normals.


What people often refer to as "normal" should be called "long-term average"

[PaulMy]

This is what I refer to if I need to use Canada Climate Normals:

http://climate.weather.gc.ca/climate_normals/index_e.html#1981

http://climate.weather.gc.ca/climate_normals/results_1981_2010_e.html?stnID=4789&prov=&lang=e&dCode=4&dispBack=1&StationName=london&SearchType=Contains&province=ALL&provBut=&month1=0&month2=12

Enjoy,
Paul

[SteveFitz1]

In the US, the "normals" are calculated every 10 years for the previous 30-year period. The "normals" currently being used are 1981-2010. In 2021, we'll switch to 1991-2020.
https://www.ncdc.noaa.gov/data-access/land-based-station-data/land-based-datasets/climate-normals/1981-2010-normals-data

Steve

[Jáchym]

Yes thats correct, thats what NOAA does, but technically speaking if you take strict normals, this is not correct and they were the first who started doing this and it is now used in other cases as well. But if you wanted to be perfectly correct, normal was defined differently based on the WMO standard:

http://www.wmo.int/pages/prog/wcp/wcdmp/GCDS_1.php

[Jáchym]

WMO:

Period averages: Averages of climatological data computed for any period of at least ten years starting on 1 January of a year ending with the digit 1 (Technical Regulations).

Normals: Period averages computed for a uniform and relatively long period comprising at least three consecutive ten-year periods (Technical Regulations).

Climatological standard normals: Averages of climatological data computed for the following consecutive periods of 30 years: 1 January 1901 to 31 December 1930, 1 January 1931 to 31 December 1960, etc. (Technical Regulations). WMO publishes the climatological standard normals which are computed by the WMO Members for their observing stations. (CLINO), WMO No.847. The latest version of this publication includes the standard normals for the period 1961-1990.

[alanb]

Thanks for the definitions ... I learned something new

[WeatherHost]

OK, that makes even less sense than I anticipated.  If we happen to be in a warm cycle now and they set this period as 'normal' in the future when we may be in  cooler cycle, then every year could be 'cooler than normal'.  I was expecting it to be the overall average since records were recorded for a given area, updated at some period ... 10 years for example.

[weatherc]

1971-2000 and 1981-2010 are also what meteorologic institutes at least in Norway, Sweden and Finland use.

[Jáchym]

OK, that makes even less sense than I anticipated.  If we happen to be in a warm cycle now and they set this period as 'normal' in the future when we may be in  cooler cycle, then every year could be 'cooler than normal'.  I was expecting it to be the overall average since records were recorded for a given area, updated at some period ... 10 years for example.

I agree it is stupid that only some 30-year periods are used, it should be any 30year period, however comparing it with overall series would create a bias because each station has different period of measurement.

[Dko]

It seems like everyone does it differently. You could even do a rolling 30 year average, updated every year.

[Jáchym]

Not really.

It is defined very specifically, not like everyone does it differently

Normal - 3 decades (eg. 1981-2010, 1971-2000)
Climate normal - defined 3-decade periods - 1901-1930, 1931-1960, 1961-1990

Yes you will often see people referring to long-term averages as "normals", but technically speaking this is incorrect

[WeatherHost]

My problem with that is 61-90 was a cool period for some reason.  I see a lot of low temperature records and 'low highs' during that period.  In contrast, I see a lot of high temperature records and 'high lows' during the 30s, 40s and early 50s that we still have not broken.  Strings of high records in '47 for some reason.

To me, that says any month now noted as 'above normal' or 'below normal' is improperly labeled.  And it's skewing the arguments of warming.  If our 'normal' period was a rolling 50 or 75 year period, we may not be seeing 'above normal' notations.  And I'd like to see a 'lifetime normal'  of all records since recordkeeping began for of each WFO/region.

[Dko]

It would be interesting to do some analysis on the 30 year CET periods and see what comes out. I'll try to do it and see what we get. I suspect if will still reveal a substantial warming.

[Jáchym]

My problem with that is 61-90 was a cool period for some reason.  I see a lot of low temperature records and 'low highs' during that period.  In contrast, I see a lot of high temperature records and 'high lows' during the 30s, 40s and early 50s that we still have not broken.  Strings of high records in '47 for some reason.

To me, that says any month now noted as 'above normal' or 'below normal' is improperly labeled.  And it's skewing the arguments of warming.  If our 'normal' period was a rolling 50 or 75 year period, we may not be seeing 'above normal' notations.  And I'd like to see a 'lifetime normal'  of all records since recordkeeping began for of each WFO/region.

Not really, because NOAA doesn´t use the "strict normals" and currently uses 1981-2010 instead.

[WeatherHost]

81-10 is not sufficient to determine anything.  Any average should be based on a minimum of 50 years and should be 100 years if records are complete for that period.

[ValentineWeather]

Its asinine to use a 10-20 year period as normal. 50-70 should be considered the minimum. 

[Jáchym]

OK, we were not talking about how representative the data is but rather what a normal is.

Of course in an ideal world you would use a time series that is hundreds of years long... but how many stations like that, with direct measurements, do you have... you then have to choose what is better - longer series, but very few stations, or shorter series, with more stations available. There is no right or wrong answer and it always depends on the purpose of your research.

[SLOweather]

It seems like everyone does it differently. You could even do a rolling 30 year average, updated every year.

I did that for local rain here: http://www.wxforum.net/index.php?topic=31942.0

30 year rolling graphs of rainfall, average, and median...