Digital Climatic Atlas of Texas - Texas Water Development Board

Appendix B
Thirty year monthly means (1971-2000) of precipitation, maximum and minimum
temperature, dew point temperature and mean monthly wind speed (60 data layers) were
used to identify unique climatic zones of varying size across Texas. In order to identify
the unique climatic zones, each climatic parameter was normalized using the maximum
and minimum values of the climatic parameters to make sure that the resultant data layer
was unit less. Using these 60 layers of information the Iterative Self-Organizing Data
Analysis Technique (ISODATA) (Tou and Gonzalez, 1974) was used to find clusters
with unique climatic properties. This clustering algorithm identifies unique patterns of
temporal variations in the five climatic parameters and then assigns each pixel to a unique
class iteratively until about 95% of the pixel does not change classes within subsequent
iterations. The procedure was used to classify Texas into 5, 10, 25 and 50 different
climatic classes. Appendix B contain illustrations of these climatic divisions along with
an overlay of the NCDC’s 10 climatic division (image on the left) along with the new
climatic division boundaries corrected to align with county political boundaries (image
on the right) based on the dominant climatic zone of the county. It is interesting to note
that the 10 climatic zones produced by ISODATA are different from the current NCDC
climatic divisions, except for the High-Plains climatic division.