عنوان مقاله [English]
The aim of this study is survey of clouds and effective physical indices, in mashhad’s rainfall. at first, we have studied cloudy sky in Mashhhad, According to meteorological yearbook (1986-2010). The importance of medium and low clouds in precipitation, the prevalence and average of them in each month, were calculated. this study shows that highest frequent of low and middle clouds occur on cold and rainy months in Mashhad. Then we have reviewed condition of low clouds. The importance of low clouds with vertical development, type 2 cumulus and cumulonimbus, we have investigated the formation of them. The results show the most of clouds with vertical development occur in Spring. Then we have studied freezing level of clouds in both rainfall and no rainfall cases, using skewt diagrams and data of sonding during 1992 to 2011. condition considered here was overcast sky. We have obtained a regression equation, in order to specify role of physical indices (CAPE، SI، K، PWT) in precipitation. At first, rainfall days of Mashhad were extracted from Khorasan razavi meteorology department archive (1992 to 2011). On these days, six hour rainfall (00 to 06 Z) were determined. Then, instability indices were obtained from data of Mashhad sonding. A regression equation were estimated by analyze relationship between natural logarithm of rainfall as dependent variable and instability indices as independent variable. In this equation, due to significant level, CAPE and PWT were removed. The most relationship is obtained between SI and natural logarithm of six-hour rainfall.