By averaging the data over long time spans for typically 30 years or more, datasets are developed that show average climate conditions. These datasets are references for comparing points in time to what would be expected. For example, is the temperature warmer or cooler for this month than would be expected from the historical average? Temperature climatologies compiled from in situ (on the ground) readings are very important for measurement, model, and reconstruction analyses of World temperature changes.
The Global Historical Climatology Network monthly analysis from the National Climatic Data Center (NCDC) uses data from thousands of land & ocean temperature stations the worldwide to compute global temperature averages and anomalies.
The majority of these locations date from the 1950's. The image above plots the temperature anomaly for the past month's time. The blue areas are cooler than the historical average, the red areas are warmer. Data in the high latitudes, around the Arctic & Antarctic circles, are not plotted because there are few weather monitoring stations in these regions, data is just too sparse for inclusion into the analysis. The dataset is processed every month on the 15th with data that describes the full previous month, and coincides with the global climate analysis conducted by the NCDC.
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