Mean Monthly and Annual Number of
Days with Measurable Precipitation
Conterminous United States

Statistic: 30-year average number of days with measurable (at least 0.25 mm) precipitation 
Time period: 1961-1990
Time interval: monthly
Spatial format: grid
Data format: ARC/Info ASCII GRID

 Projection: Geographic
Spatial resolution: 1.25 arc-minutes (~2 km)
Spatial Extent: Conterminous United States
Units: Days
Date created: August 2000
Methods: Statistical derivation from mean monthly and annual precipitation, mapped with the PRISM modeling system and Gaussian filter resolution enhancement. See FGDC-compliant metadata for details.

Examples 
Mean number of days with measurable
precipitation, May, entire region
Mean number of days with measurable
precipitation, January, north-central Colorado
Features
  • Reviewed and approved by the US National Climatic Data Center 
  • Incorporates topographic effects on precipitation 
  • Captures rain shadows in great detail 
  • Includes coastal and lake effects on precipitation 
  • Accounts for the varying ability of terrain features to enhance precipitation 
  • Consistent treatment and spatial quality control of station data 
  • Consistent coverage across all state boundaries 
  • "Universal" geographic (latitude/longitude) projection allows conversion to any other coordinate system 
  • Data format allows for easy import into ESRI's GIS products, as well as MAPINFO and Intergraph's GeoMedia 

Data used in analysis

  • 6,320 National Weather Service stations for mean number of days with precipitation 
  • 9,110 stations for mean precipitation
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