A hybrid model is proposed for calculating the observed trajectories of 
constant level particles (e.g. balloons) in the atmosphere using standard 
meteorlogical fields. Particles in the model are advected by a linear 
combination of the Eulerian ECMUF winds exptrapolated to the desired level and a 
correctional velocity calculated by integrating the acceleration due to 
pressure- gradient, the Coriolis force and a drag force modeled by Rayleigh 
friction term. The model trajectories are at least twice as close to 
trajectories observed during the EQUATURE experiment conducted in the summer of 
1998 than those that result from advection by the airflow. This significant 
improvement is achieved with a 15% weight of the correctional velocity i.e. when 
the balloon's velocity is about 85% that of the surrounding airflow. The 
relaxation time of the Rayleigh friction coefficient is on the order of 1 day in 
all cases. In addition to the individual best fit of each of the observed 
trajectories, the model yields a global, single best fit, set of parameter 
values in which the model trajectories are 33% closer to the observed ones 
compared to the advection by the airflow. The parameter values of this global 
best fit set are only slightly different from the three individual sets. A 
simple, Stokes drift, model where the balloon velocity is allowed to vary 
slightly from that of the airflow for short times (i.e. where the drag relaxes 
the balloon to the air speed) yields trajectories that are worse than those of 
the pure advection for realistic values of the relaxation coeeficient.