30-yr averages by month: Precip

How much rain and snow usually fall this month?

Based on daily observations from 1981-2010, colors on the map show long-term average precipitation totals in each climate division for the month displayed. The darker the color, the higher the total precipitation. 

Where do these measurements come from?

Daily totals of rain and snow come from weather stations in the Global Historical Climatology Network. Volunteer observers or automated instruments gathered the data from 1981 to 2010 and submitted it to the National Center for Environmental Information (NCEI). After scientists checked the quality of the data to omit any systematic errors, they calculated each station’s monthly total and plotted it on a gridded map. To fill the grid, a computer program applied 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.

What do the colors mean? 

White areas on the map received an average of zero measurable precipitation during the month from 1981-2010. Areas shown in the lightest green received a monthly average of less than one inch of water from rain or snow over the 30 years. The darker the color on the map, the higher the average precipitation total for the month. Areas shown in dark blue received an average of 8 or more inches of water that fell as either rain or snow.

Why do these data matter? 

Understanding these values provides insight into the “normal” conditions for a month. This type of information is widely used across an array of planning activities, from designing energy distribution networks, to the timing of crop and plant emergence, to choosing the right place and time for recreational activities. 

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 run a set of scripts that access the source data and display them on our base maps.