Temperature - Minimum, 1981-2010 Monthly Average

Based on daily temperature readings from 1981-2010, colors on the map show the long-term average minimum temperature in each climate division for the month displayed. The map reveals the average of “overnight low” temperatures during the month over the previous three decades.

Temperatures for each month come from stations in the Global Historical Climatology Network. A combination of volunteer observers and automated instruments collected the lowest temperature at each station every day from 1981 to 2010, and sent them to scientists at the National Center for Environmental Information (NCEI). After scientists checked the data quality to omit any systematic errors, they calculated each station’s average monthly minimum temperature: they took the sum of all the minimum temperatures and then divided by the number of days in the month times 30 years. NCEI scientists plotted the values for each station on a gridded map; to calculate values for other points on the grid, a computer program applied a scientifically reviewed mathematical filter to account for the distance between stations and variations in elevation. The average of all grid point values within each climate division is the long-term average minimum temperature for that division.

The color in each climate division shows the average of the lowest temperature recorded every day of the month for the 30 years from 1981 to 2010. Shades of blue show where the lowest daily temperatures measured from 1981 to 2010 averaged below 50°F for the month. The darker the shade of blue, the lower the temperature. Climate divisions shown in shades of orange and red have long-term average minimum temperatures above 50°F. The darker the shade of orange or red, the higher the temperature. White or very light colors show climate divisions where the average minimum temperature is near 50°F.

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) - Weather and Climate. To produce our images, we run a set of scripts that access the source data and display them on our base maps.