Weather anomalies

An anomaly is a deviation of a measurable unit over a specific period in a given region. These measurable units can be temperature or precipitation. Now in the study of climate changes, we study temperature anomaly quite closely, temperature anomaly is more important than an absolute temperature when studying weather. It can be defined as deviation from the long term average or reference value or baseline temperature. The baseline temperature is calculated by averaging temperature data of 30 or more years.

Now, we can have two types of temperature anomalies:

  • Positive anomaly: It includes that the observed temperature is warmer than the baseline or reference temperature.
  • Negative anomaly: It includes that the observed temperature is cooler than the reference or baseline temperature.

Global temperature anomaly is used as a global scale climate diagnostic tool that provides the overview of average global temperature compared to the base value. There are so many severe weather phenomenons that are hazardous to human property and life, this tool provides us with an edge in predicting the weather more accurately thus preventing these phenomenons. 

We know that absolute temperature also plays an important role in determining the weather, so what’s the difference between the two? Absolute temperature calculations take into consideration things like elevation or station location whereas when calculating anomalies, these factors are less critical. The absolute estimate of the global temperature is sometimes difficult to compline such as some regions may have few temperature measurement stations ( Sahara Desert) and then the interpolation has to be made over data-sparse large regions. Most observations come from inhibited valleys in mountain regions thus elevation also affects the average temperature.

Firstly, reference values are computed on a small region over the same period of time, this establishes a baseline for the calculation of anomalies. Now, this effectively normalises the whole temperature data, thus they can be now combined and compared for an accurate representation of temperature patterns with respect to what’s considered normal ranges in that region. Anomalies are more effective than absolute temperature especially in large areas as they give a frame of reference for better understanding and meaningful comparison of temperature trends.1981-2010 is the baseline period which is used in order to comply with the WMO ( World Meteorological Organization) policy recommendations. The reference period is adjusted according to the 20th century average for conceptual simplicity in the calculation of the global-scale averages. This adjustment though does do not change the trends or their shapes.

Using temperature anomalies in weather forecasting and climate study helps in minimizing problems when stations are closed, removed, added or are missing from the monitoring network. Different datasets are used in calculating the average global temperature anomaly. GHCNm (Global Historical Climate Network-Monthly)  is used for Land surface temperature. ERSST (Extended Reconstructed Sea Surface Temperature) is used for determining sea surface temperature. The dataset used in ERSST is ICOADS (International Comprehensive Ocean-Atmosphere Data Set).

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