The data used by Hysplit is gridded four-dimensionsal (x,y,z,t) meteorological fields output as analysis or forecast fields from the NOAA NCEP weather prediction models. Model output field resolution varies according to the model, anywhere from standard pressure levels (1000, 925, 850 hPA) to every 25 hPa intervals for the regional models. See the data set description information on the READY web site (forecast data and archive data). See the Technical Memo for further details.
Roland Draxler
NOAA ARL