Dust is the first Natural hazard in desert and semi-desert world and Iran. In order to identify the days of the dust of South KHorasan, the daily amount of horizontal visibility, wind speed and direction in the spring and summer of 1991 -2008 were received from Meteorol
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Dust is the first Natural hazard in desert and semi-desert world and Iran. In order to identify the days of the dust of South KHorasan, the daily amount of horizontal visibility, wind speed and direction in the spring and summer of 1991 -2008 were received from Meteorological Organization. Then, based on Shao and Dong index, the days were extracted with dust. In order to determine atmospheric pattern led to the dust, the daily Geopotential height 500 HP from the National Center for Environmental Prediction America/ National Center for Atmospheric Research (NCEP / NCAR), was prepared. In this study, to determine the pressure patterns were used the principle component analysis approach in the state S and hierarchical clustering (ward). Then the composite maps of vorticity, geopotential height, and sea pressure and flow pattern for each pattern produced and were analyzed. The results showed that the tow dominant synoptic patterns respectively are the summer pattern with a subtropical high on Iran and high low on Pakistan in the high levels of the atmosphere and the pressure difference between the South-East of Iran (low pressure) and the Caspian Sea (high pressure) on the land surface. In this pattern, the wind from the East of the Caspian Sea and Turkmenistan desert in the dry lands and deserts to eastern Iran flows and causing dust. This atmospheric conditions is this Connection with the 120-day winds of Sistan and Baluchestan in Iran's East. In the spring pattern, the trough of westerly winds in the center and east of Iran in the high level of atmosphere and low pressure on the ground in this area has led to unstable weather conditions in the central deserts of Iran and the West the province that these flows transfer dust particles and reduce visibility. As well as intense pressure difference between the centers lead to dust storms in the South Khorasan province
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