Synoptic Causes of Heavy Rain falls on Larestan Region

ABSTRACT: One of the problems in the most arid areas is the low amount of rainfalls and this little amount also falling as heavy showers. Dearth of vegetation makes the effect of these showers more acute and may as well cause different problems in those regions. Lar is one of the cities of Fars province, at the southern part of Iran. In this research, the reasons of heavy rains that are related to 4th February 1993 with 103 mm of rainfall and 24th July 1995 with 40 mm of rainfall have been studied. The advantage of this study is that the understudy times belong to summer (July 95) and winter (February 93). The rainfall on July 24th is because of the settlement of a thermal low pressure on the surface of the earth and a high pressure center on the upper levels of the atmosphere, which causes infusion of humidity into the system from above the Indian Ocean and Oman Sea. The main factor affecting this process (intense warmness of the surface of the earth) is break of inversion layer and therefore lay the grounds for convection rainfalls. The rainfall on February 4th, 1993 is a result of the combination of the Mediterranean low pressure and Sudan low pressure. The center of this pressure is one of the constructive elements of the general atmosphere flow at north of Africa and is formed on the most days of the year on the west of Ethiopia and Sudan. The slight vacillation of this low pressure center is in harmony with the sun’s outward movement during the year. Since this (Sudan) low pressure is a thermal type, it has a minimal depth and is strengthened in specific conditions and then is changed to a dynamic center.