Journal of Applied Sciences1812-56541812-5662Asian Network for Scientific Information10.3923/jas.2008.1299.1304Ameneh SajjadiS. 7200887The study of Precipitable Water (PW) and determining the cloud seeding threshold are of great importance in reducing the considerable cost of unsuccessful seeding or over- seeding which well result in a decrease in rainfall potentiality. The present study was carried out within four-year period covering from 2000-2003 where the amount of rainfall in North-Western Iran (Tabriz) was recorded by precipitation stations. PW was calculated using the data obtained through radiosonde, thermodynamic charts and MODIS satellite. Using the curve and upon choosing the fittest index PW as independent variable and the rainfall average precipitation (P) as dependent variable, from among the PW, the suitable cloud seeding threshold was estimated. The thresholds obtained through MODIS and radiosonde for cloud seeding and PW were compared across 21 cases. The two methods employed showed high correlation. The thresholds obtained through MODIS in Tabriz was 7 mm, respectively, while the thresholds obtained through radiosonde on the same days for PW was 9 mm in Tabriz. In general, the amount of PW obtained through radiosonde proved to be greater than those achieved through MODIS. It is mainly due to the fact that the clouds` build up time and satellite picturing were not synchronous; moreover, the MODIS had higher sensitivity to the back scattering of the infrared waves by heavy clouds and at the same time and the low sensitivity in fogy condition on earth.]]>Costa, S., P. Mezzasalma, V. Levizzani, P.P. Alberoni and S. Nanni,2001567388Czys, R.R. and R.W. Scott,1993329961005Fraser, R.S. and Y.J. Kaufman,198523625633Gao, B.C. and H. Goetz,199095935493564Gao, C.B. and G. Kaufman,200330871884King, R.W. and Y. Bock,200389107112Krauss, T.W. and J.R. Santos,2004713550Manzato, A. and G. Morgan,200367-68455473Manzato, A.,200368-67417454Platnich, S., M.D. King, S.A. Ackerman, W.P. Menzel, B.A. Baum, J.C. Riedi and R.A. Frey,200341459473Rogers, R. and M. Yau,19963rd Edn.pp: 290pp: 290Simoenov, P. and C.G. Georgiev,200367-68629643