1. Introduction
Wide Impact range of dust storms is one of the major characteristics that influence thousands of kilometers. The evaluation of relationship between wind speed and direction, visibility, air pressure and precipitation with frequency of dust storm occurrences in long-term, provides a complete knowledge of their content and provides useful information for solving this phenomena. The major sources of dust aerosols entering to west of Iran is Syrian-Sahara and Nofoud-Sahara in north of Arabian Peninsula (Zolfaghari, 2005).
2. Study area
In this study we investigate dust storms, in Sar-Pol-Zohab (lat 45̊ 57́ 45̋, lon 34˚ 3́ 45̋), using meteorological data. So, we intend to analyze speed and direction of winds during dust storms. The study area is a semi arid region, with mean precipitation of 421.3 mm annually.
3. Material and methods
Meteorological data was obtanied from Iranian Meteorological Organization during 1986 to 2009 which includes: Frequency of occurrence, storm types, speed and direction of wind, relative humidity, air pressure, and visibility. WRPLOT View software version 5.3 was used to draw annual, seasonal and monthly windrose plot. Then the data were statistically analyzed using SPSS 17 software. Finally SURFER 8.0 full version software was used to analyze the relationship between speed and direction of the wind and visibility.
4. Results and discussion
In this study we focus on widespread dust in suspension in the air that is not raised by wind or near station at the time period which includes time range between 14th of July 1986 to 2009, totally 152 storms. The frequency of dust storms has increased during recent years. Monthly investigation showed that the maximum days of dust storms occure in July by 4.5 day per month and 108 days in the whole period. The minimal occurrences are in December. The most frequent dust storm days are seen in warm months (June, July and August) and the least frequent dust storm days are seen during cold months (November, December and January). The investigation of visibility, precipitation, relative humidity, pressure and speed and direction of the wind reveals an average decrease of 4313 m that in some cases it is decreased to 100 m. Relative humidity was 35.4% and pressure of 1006.55 hPa is reported. The average speed of wind was about 5.1 knot and the direction was mostly west to south. Correlation investigation between climatological factors and visibility shows a weak relationship between the two, but there is a direct relation between speed of the wind and visibility. Investigation period is divided into two periods: (1) windrose at highest rate and (2) windrose at lowest rate which the following characteristic are seen in each period respectively. In first period western, northern and south western winds are seen which western wind (11 knot) plays a major role. In the second period, western winds are decreased and southern winds are increased, and the speed of wind in this period reach to more than 22 knots.
5. Conclusion
General analysis of whole period shows that western, north western and south western winds play major role during storm events. Monthly windrose analysis reveals that winds directions were western during the months that dust storms had been increased. Also result showed an increase in the number of the storms that are not raised by wind or near station which tends toward west. Monthly trends verify the increase of storms in warm seasons due to a decrease in humidity. With respect to the results, we can calculate that the dust storms are originated outside of this region and Aeolian-sediments are not from this region. Also in this region dust storms occur at low speed of wind. |
