  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.
