I am trying to do trajectory analysis for an event in which we have observed meteorological data. Is it possible to download HYSPLIT to the PC and feed it observed data from meteorological stations?

There is no existing program to take observational data for input to HYSPLIT. HYSPLIT requires gridded 4 dimensional fields (x,y,z,t) of the meteorological variables. Data interpolation can be a tricky process and we felt that most simple interpolation techniques due not properly address the problem. Others in your situation use a meteorological model, such as RAMS or MM5, to assimilate observational data into a gridded framework that is consistent with atmospheric dynamics.

Roland Draxler