ArealRain.avx is available on ESRI ArcScripts site
http://gis.esri.com/arcscripts/details.cfm?CFGRIDKEY=0B8A90A2-CD29-11D5-944B
00508B0CB419 for a free download. It calculates precipitation for a
particular area (e.g. catchment) from points' precipitation measurement
using the Thiessen Polygon method.
Tired of pushing planimeter? Have a fun using ArealRain.avx.

Ivan Petras