The radioactive pollutant is assumed to be passively transported by the wind. A trajectory is the time integration of the position of a parcel of air that contains the pollutant. The trajectory is represented as a collection of points, which are computed using a 4-dimensional wind field, that varies with horizontal position, height, and time. The trajectory represents the integration of the position vector from the measurement location going upwind following the air flow to determine the origin of the air parcel. The trajectory computation uses the HYSPLIT model. It is also possible to run HYSPLIT interactively.

The backward trajectories shown are computed for each 6-h period, arriving at the monitoring location at hourly intervals. The sampled air could have come from any point along the trajectory that arrived over the detector. Trajectories (red) were started at the time indicated by the plot label and each previous hour (blue green cyan magenta olive). The computations are made with the most current meteorological data using forecast data fields from NOAA's RAP model. The forecast data are available at 1-hour intervals at 20 km resolution over most of the U.S. All trajectories are started at 100 meters above ground.