Climate Normals and Normalised Departures

Climate “Normals” are reference points used by climatologists to compare current climatological trends to that of the past. A Normal is defined as the arithmetic average of a climate element over a 30-year period. A 30 year period is used, as it is long enough to filter out any inter-annual variation or anomalies, but also short enough to be able to show longer climatic trends. The current climate normal period is calculated from 1 January 1961 to 31 December 1990. Meteorologists and climatologists regularly use Climate Normals for placing recent climate conditions into a historical context. As discussed by WMO (2007), Climate Normals are not only used as predictors of future climate conditions, but are also used to provide a reference value for the computation of climate anomalies. In addition to weather and climate comparisons, Climate Normals are utilized in seemingly countless applications across a variety of sectors. These include regulation of Power Companies, Energy Load Forecasting, Crop Selection and planting times, Planning Construction and Seasonality .

Climate Normals were not designed to be metrics of Climate Change. In fact, when the widespread practice of computing Climate Normals commenced in the 1930s, the generally-accepted notion of the climate was that the underlying long-term averages of climate time series were constant in a stationary climate. With a Non-stationery climate, changes from one installment of Climate Normals to the next would not be as constant as before. However, care must be taken when interpreting changes between one Climate Normals period and the other. It is true that differences between the reported 1971-2000 Climate Normals and the 1981-2010 but also may be due to station moves, changes in methodology, changes in instrumentation .Further local research is required to confirm these climate normal changes from one period to another to determine the true attribution emanating from of climate change

ICPAC has been using a reference climate normal of 1961-1990 for its operations as it was considered the time when Climate change had less impacts than the current and also the period with the more solid data as compared to 1951-1980.

However, with the advent of Satellite gridded datasets ICPAC is currently using both 1961-1990 and 1981-2010 for its operations. 

Click to see the inter-annual normalized graphs.
Click to see the climatological maps.

© 2016 IGAD Climate Prediction & Applications Centre.