In this research paid attention to investigate cloudiness in Mashhad plain, based on the meteorological yearbook (1986-2008 that the most frequent overcast occurrence has been shown. Then we studied the prevalence of low and medium height clouds and average of them in each month، from Mashhad synoptic station data during 2001-2010، considering to the importance of medium and low clouds in the cloudseeding process، and was paid to review condition of sky with low clouds. We investigate to the formation of low height clouds with vertical development، including cumulus (type 2) and cumulonimbus، in this region, given the importance of this clouds. The investigation of these cloud formations showed the most cloudiness has been occurred during cold and rainy seasons i.e. winter and early spring. The evaluation of the cloud base temperature and the level of freezing in this region has been done according charts and data of Mashhad station and using skewt with rainfall and without rainfall during 1992-2011. The basis of information has been obtained on the basis of overcast condition and the average of the cloud base temperature Some instability indexes (K, Showalter, Precipitable water, CAPE) was obtained in rainy days at 00Z, using data of Wyoming university website, during 1992-2011. Then was paid to investigate the correlation between instability index (K Index، Showalter Index, Precipitable water and CAPE Index) with 6 hours rainfall (from 00 to 06 Z). The studies showed a correlation is positive and significant at 1% between precipitable water, K index with cumulative 6-hour precipitation. According to the correlation and significance level, the Sowalter index and CAPE index can^’t be used in determining the appropriate threshold of cloudseeding. the data showed a very high scattering.