DOI: 10.3886/e233226v1
title: Climate.gov Data Snapshots: Precipitation - Monthly Percent of Average

description: <b>Q: Was the month drier or wetter than usual?
</b><b>
 </b><br><b>A:</b> Colors
 show where and by how much monthly precipitation totals differed from 
average precipitation for the same month from 1991-2020. 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.

 
 <br><b>Q: Where do these measurements come from?
</b><b>
 </b><br><b>A:</b> Daily
 measurements of rain and snow come from weather stations in the Global 
Historical Climatology Network (GHCN-D). Volunteer observers or 
automated instruments gather the data and submit them to the National 
Centers 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 5x5 km gridded map. To fill in 
the grid at locations without stations, a computer program interpolates 
(or estimates) values, accounting for the distribution of stations and 
various physical relationships, such as the way temperature changes with
 elevation. The resulting product is the NOAA Monthly U.S. Climate 
Gridded Dataset (NClimGrid).

To calculate the percent of average precipitation values shown on 
these maps—also called precipitation anomalies—NCEI scientists take the 
total precipitation in each 5x5 km grid box for a single month and year,
 and divide it by its 1991-2020 average for the same month. Multiplying 
that number by 100 yields a percent of average precipitation. If the 
result is greater than 100%, the region was wetter than average. Less 
than 100% means the region was drier than usual.

 
 <br><b>Q: What do the colors mean?</b><b>
 </b><br><b>A:</b> Shades
 of brown show places where total precipitation was below the long-term 
average for the month. Areas shown in shades of green had more liquid 
water from rain and/or snow than they averaged from 1991 to 2020. 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. Note that snowfall totals are reported as the amount of liquid 
water they produce upon melting. Thus, a 10-inch snowfall that melts to 
produce one inch of liquid water would be counted as one inch of 
precipitation.

 
 <br><b>Q: Why do these data matter?</b><b>
 </b><br><b>A:</b> Comparing
 an area’s recent precipitation to its long-term average can tell how 
wet or how dry the area is compared to usual. Knowing if an area is much
 drier or much wetter than usual can encourage people to pay close 
attention to on-the-ground conditions that affect daily life and 
decisions. People check maps like this to judge crop progress; monitor 
reservoir levels; consider if lawns and landscaping need water; and to 
understand the possibilities of flooding.

 
 <br><b>Q: How did you produce these snapshots?</b><b>
 </b><br><b>A:</b> 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 data (NClimGrid) 
produced by and available from the National Centers for Environmental 
Information (NCEI). To produce our images, we invoke a set of scripts 
that access the source data and represent them according to our selected
 color ramps on our base maps.

 
 
 <br><b>Additional information</b>
 The data used
 in these snapshots can be downloaded from different places and in 
different formats. We used these specific data sources:

NClimGrid Total Precipitation<br>
NClimGrid Precipitation Normals

 
 References
 
 NOAA Monthly U.S. Climate Gridded Dataset (NClimGrid)<br>
NOAA Monthly U.S. Climate Divisional Database (NClimDiv)<br>
Improved Historical Temperature and Precipitation Time Series for U.S. Climate Divisions<br>
NCEI Monthly National Analysis<br>
Climate at a Glance - Data Information<br>
NCEI Climate Monitoring - All Products<br>Source: https://www.climate.gov/maps-data/data-snapshots/data-source/precipitation-monthly-difference-averag...<br><br><br>This upload includes two additional files:<br>* Precipitation - Monthly Percent of Average _NOAA Climate.gov.pdf is a screenshot of the main Climate.gov site for these snapshots (https://www.climate.gov/maps-data/data-snapshots/data-source/precipitation-monthly-difference-averag...)<br>* Cimate_gov_ Data Snapshots.pdf is a screenshot of the data download page for the full-resolution files.<br><br><br>
description: Data is divided in to folders by image resolution -- full size (zip), small (png) and large (png). Occasionally there may be one image missing in a particular size but available in another size.<br><br>Data downloaded from climate.gov 6/17/2025.