Difference from Average - Monthly

Difference from Average - Monthly current snapshot

Was the month drier or wetter than usual?

Colors show where and by how much monthly precipitation totals differed from average precipitation for the same month from 1981-2010. Green areas were wetter than the 30-year average for the month and brown areas were drier. White and very light areas had monthly precipitation totals close to the long-term average. 

Where do these measurements come from?

Daily measurements of rain and snow come from weather stations in the Global Historical Climatology Network. Volunteer observers or automated instruments gather the data and submit it 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 total and plot it on a gridded map. To fill the grid, a computer program applies a mathematical filter that accounts for the distribution of stations and the terrain. The total precipitation for each climate division is the average of all grid point values within its boundaries.

To calculate the difference-from-average precipitation values shown on these maps—also called precipitation anomalies—NCEI scientists take the total precipitation 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 wetter than average. A negative result from the subtraction means the region was drier than usual.

What do the colors mean? 

Shades of brown show climate divisions where total precipitation was below the long-term average for the month. Climate divisions shown in shades of green had more rain and/or snow than they averaged from 1981 to 2010. The darker the shade of brown or green, the larger the difference from the average precipitation. White and very light areas show where precipitation totals were the same as or very close to the long-term average.

Why do these data matter? 

Comparing an area’s recent precipitation to its long-term average can reveal insights about the magnitude of how wet or how dry conditions are compared to usual conditions. These insights can encourage people to pay close attention to on-the-ground conditions that affect daily life and decisions. Some of the reasons people check these data include consideration of crop progress; monitoring reservoir levels; checking the health of native vegetation, lawns, and landscaping; potential demands for irrigation; and possibilities of flooding.

How did you produce these snapshots? 

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.