Drought is lack or inadequacy of rainfall over a long period of time, so that it leads to a lack of moisture in soil and reduces the flowing waters, thereby interrupting human activities and plant and animal life. The time between last rainfall and observation of tangible decrease in rivers’ water and soil moisture is not the same in different climates. Therefore, a precise and comprehensive definition of drought cannot be provided. For this reason, experts have each proposed a different definition of drought from different points of view.
In this paper, eleven meteorological factors that play a role in severity of dryness and drought have been selected and their effects on determination of dry years have been classified. Among these factors, six factors of precipitation, maximum daily rainfall, number of rainy days, dew point, relative humidity and cloudiness have been selected as factors reducing drought severity, and factors of maximum temperature, average temperature, sunny hours and evaporation as those intensifying drought; these factors have been classified at the synoptic meteorological station of Hamedan during the statistical period from 1989 to 1997 using the scalogram model. This classification can be applied to more factors and longer periods at each station, so that in determination of drought years, efforts will not be limited to the amount of precipitation, and other factors will be involved in calculations according to their priority.
According to the classification and use of the tables at Hamadan station, eleven meteorological factors were selected which can be named in terms of priority as follows:
1. Maximum temperature 2. Minimum temperature 3. Average temperature 4. Rainfall 5. Maximum daily precipitation 6. Number of rainy days 7. Dew point 8. Relative humidity 9. Cloudiness 10. Sunny hours 11. Evaporation