This post on average temperatures was taken from the NOAA
Was the month cooler or warmer than usual?
Colors on the map above, courtesy of the NOAA, show where and by how much the average monthly temperatures differed from the month’s average temperature from 1981-2010. Red areas were warmer than the 30-year average for the month and blue areas were cooler. White and very light areas had temperatures close to the long-term average.
Where do these measurements come from?
Daily temperature readings come from ground-based weather stations in the Global Historical Climatology Network. Volunteer observers or automated instruments collect the highest and lowest temperature of the day at each station over the entire month, and submit them to the National Center for Environmental Information (NCEI). After scientists check the quality of the data to omit any systematic errors, they calculate each station’s monthly average temperature and plot it on a gridded map. To fill the grid, a computer program applies a mathematical filter that accounts for the distribution of the stations and variations in elevation. The average of all grid point values within each climate division is the average monthly temperature for that division.
To calculate the difference-from-average temperatures shown on these maps—also called temperature anomalies—NCEI scientists take the average temperature in each climate division for a single month and year, and subtract its 1981-2010 average for the same month. If the result is a positive number, the region was warmer than average. A negative result from the subtraction means the region was cooler than usual.
Shades of blue show climate divisions where average monthly temperatures were below their long-term average for the month. Climate divisions shown in shades of pink to red had average temperatures that were warmer than usual. The darker the shade of red or blue, the larger the difference from the long-term average temperature. White and very light areas show where the average monthly temperature was the same as or very close to the long-term average.
Data Snapshots are derivatives of existing data products: to meet the needs of a broad audience, we present the source data in a simplified visual style. This set of snapshots is based on climate division data (nClimDiv) produced by and available from the National Centers for Environmental Information (NCEI). To produce our images, we use an automated process to access the source data and represent them according to our selected color ramps on our base maps.