Calculating temperature anomalies

Temperature anomalies indicate how much warmer or colder a value is compared to the long-term average for a given climatological period (e.g. 1961-1990).

Long-term averages can be calculated using the daily HadCET temperature series. These monthly averages then form the baseline for anomaly calculations.

Steps to calculate anomalies

1. Calculate long-term monthly averages
   For each calendar month (e.g. January, February, etc.), compute the average from the daily CET values over the chosen long-term period (e.g. 1961-1990).


2. Calculate the anomaly
   Subtract the appropriate long-term average (LTA) from the actual observed value. This can be done at different temporal resolutions:
   
   
   • Daily anomaly - Subtract the monthly average from the actual daily value.
     Example: 1st January 2024 anomaly = 1st January 2024 actual - January average (1961-1990)
   
   
   • Monthly anomaly - Subtract the monthly average from the actual monthly value.
     Example: January 2024 anomaly = January 2024 actual - January average (1961-1990)
   
   
   • Yearly anomaly - Subtract the weighted average of monthly long-term means from the annual value.
     The weights correspond to the number of days in each month (i.e. Jan=31, Feb=28.25, Mar=31, etc.).
     Example: 2024 anomaly = 2024 actual - weighted average of all monthly averages (1961-1990)

Example 1: Calculate January 2024 Anomaly

To calculate the anomaly, we compare the actual temperature for January 2024 to the long-term average for January (1961-1990).
Note, values derived from the CET V2.1.0.0 series.

Example 2: Calculate 2024 Anomaly

To calculate the anomaly, we compare the average temperature for 2024 to the long-term average (1961-1990).
Note, values derived from the CET V2.1.0.0 series.