The Study of Effective Air Instability Conditions and Synoptic Patterns on Occurrence of Dust Storms

Dust storms are natural phenomena which occur in arid and semi-arid areas of the world. In this study، to evaluate the dust storms which occurred in Mashhad over a period of 30 years (1981-2009)، first، the number of days on which dust storms happened in Mashhad (days with a wind speed greater than 15 m. s-1 and horizontal visibility less than 1000 m) were identified and counted for each month; then، the average of storm days over the 30 years were calculated. The severest storm day was chosen and the amount of pressure، temperature and dew point in different layers of the atmosphere were obtained by radio sound during that day and the day before it، being plotted on thephigram by digital atmosphere software. After analyzing the thephigram، the most important instability indexes were calculated. Finally، to confirm the results، the synoptic maps of Geopotential and Isotherm were also considered. The results showed that most storms in Mashhad happened in early summer (June)، when warm air mass enters the area. The results also show that during December and January، there happened the fewest stormy days in Mashhad. The results also show that there was moderate instability before the stormy days and severe instability (SWEAT=431. 7) during the stormy days، indicating the strength of the storm in the region. Synoptic analyses also confirm the analysis of atmospheric vertical profile، both showing that the interaction of south warm and north cold air masses over the region caused severe instability of the air and created the dust storm.