Redistribution of air masses due to atmospheric circulation causes loading 
deformation of the Earth's crust which can be as large as 20 mm for the 
vertical component and 3 mm for horizontal components. Rigorous computation 
of site displacements caused by pressure loading requires knowledge of the 
surface pressure field over the entire Earth surface. A procedure for 
computing 3-D displacements of geodetic sites of interest using a 6-hourly 
pressure field from the NCEP numerical weather models and the Ponte and 
Ray [2002] model of atmospheric tides is presented. We investigated possible 
error sources and found that the errors of our pressure loading time series 
are below the 15% level. We validated our model by estimating the admittance 
factors of the pressure loading time series using a dataset of 3.5 million 
VLBI observations from 1980 to 2002. The admittance factors averaged over 
all sites are 0.95 -+ 0.02 for the vertical displacement and 1.00 -+ 0.07 
for the horizontal displacements. For the first time horizontal displacements 
caused by atmospheric pressure loading have been detected. The closeness 
of these admittance factors to unity allows us to conclude that on average 
our model quantitatively agrees with the observations within the error budget 
of the model. At the same time we found that the model is not accurate for 
several stations which are near a coast or in mountain regions. We conclude 
that our model is suitable for routine data reduction of space geodesy 
observations.