Pollen transport in the atmosphere is calculated using the HY-SPLIT trajectory model, which comes from NOAA's Air Resources Laboratory (ARL) in Silver Spring, Maryland. For the necessary meteorological input, HY-SPLIT commonly uses outputs of either the ETA model or the AVN (Aviation) model, members of the family of Numerical Weather Prediction (NWP) models used to forecasts short-term weather conditions in and around the United States. This meteorological data is generated initially by National Center for Environmental Prediction (NCEP), a branch of NOAA, then modified by ARL to conform to HY-SPLIT's input requirements. The data of primary interest are the forecast wind fields in the atmospheric boundary layer. In nearly all cases, HY-SPLIT trajectories are provided by ARL via an automated Electronic Mail system. The trajectory is a plot of the future atmospheric pathway of a "parcel" of air likely to contain spores; in other words, the prediction of the spatial and temporal positions of a spore cloud CENTER for the future two days following release from a source site.