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.