It has been hypothesized that urban regions influence precipitation patterns by increased surface temperature, increased surface roughness and surface convergence. While a number of urban influences exist, it has been shown urban size is among the most important. Therefore it can be expected that as an urban area expands, changes in the local precipitation will follow. A study from Huff and Changnon (1972*) examined the Washington, DC climatology for precipitation, thunderstorm and hail activity for the period 1931-1968 as part of a larger study on urban precipitation effects. In particular, the study placed emphasis on precipitation patterns during the 20-year period from 1949-1968 and the 5-year period from 1964-1968.

Washington, DC has expanded a great deal over the past forty years. In particular the regions of Columbia, MD (opened in 1967) and Sterling, VA (which began to expand heavily in the 1970's) have been transformed from mostly farm lands to suburban and small urban areas. In this study, we will compare similar climatology for more recent years. Surface data will be taken from NWS weather stations using as many of the same stations used in the Huff and Changnon study as possible. In addition, variations in synoptic variables for all time periods will be examined using the NCEP/NCAR reanalysis data.

The examination in precipitation changes is valuable for two reasons. First, such examination will provide evidence for the scale of effect of urbanization on precipitation patterns. Second, it will allow the identification of regions of stable high or low precipitation due to geographic influences of the region. This information in turn will be valuable for forecasting and model-building as well as further studies on mesoscale effects for the region.

* Huff, F. and S. Changnon, 1972. Climatological assessment of urban effects on precipitation: part 2. Final Report to National Science Foundation on Grant GA-18781, Illinois State Water Survey. 237 p.