عنوان مقاله [English]
Strong winds have been named storms, which, in various forms and at high speed, blow over for a short time and are usually accompanied by unstable air. If unstable air is humid, a thunderstorm, and if it is dry, a dust storm will form (Alijani, 2000). In all stations of Hamadan province the majority of thunderstorms occur in the spring. In this season, the greatest number of thunderstorms have occurred due to increased solar radiation energy, which is characterized by significant atmospheric moisture content due to evaporation and vegetation cover, wet lands and transitional thermodynamic systems from the west and southwest of the country. The fall season is second in terms of the number of Thunder storms. In this season, due to the high radiation energy and the arrival of unstable systems with an appropriate moisture source, the number of thunderstorms is significant. The smallest number of thunderstorms belongs to winter.
The maximum average of dust storms occurs mainly in dry areas without vegetation, and the number of these storms is less in the mountainous areas of Hamadan.
The seasons with most frequent storms in the districts of Ekbatan, Hamadan, Noghe and Malayer are spring and summer, and at Nahavand station the summer season. The study of the trend of hourly changes in storms frequency shows that the maximum occurrence of these storms is from 9 am on, and at 6 p.m. their intensity and frequency decreases, and in all stations the frequency of these storms is higher in days than in nights. According to the conducted surveys, the most activity of dust storms is in the spring and summer. Although during this period of time the ground for local instability is provided due to the high radiation energy and suitable topographic conditions, but the energy of the systems is evacuated as dust storms because of the lack of adequate moisture.