عنوان مقاله [English]
Introduction: Considering population growth and urbanization development, one of the main requirements of the urban society is to create appropriate life condition for dwellers. The microclimate is considered as one of the factors that affect the activity of people in the environment and in terms of tourist and recreational industry, it is considered as an important source of economic. Many climatic parameters such as temperature, relative humidity, radiation and wind speed can affect the quality of people's recreational activities. So far, many studies have been conducted in the world about the positive effects of vegetation specially trees in the urban landscape on the optimizing of microclimate and human thermal comfort. The cooling mechanism of trees and clouds in an area mainly by directly shading the ground surface and indirectly by supplying humidity are similar. Thus, perhaps it be possible to calculate the changes in temperature and relative humidity based on cloud categories and impute it to tree canopy and vegetation size. The purpose of the present study was to answer followed questions, 1) does the use of single tree produce a sensible difference in temperature and humidity relative to open space? 2) Does the acacia tree as an indicator for a broad-leaved species compare to the pine tree as an indicator for the needle species in terms of the microclimatic variation of around themselves act differently? 3) Do the microclimatic elements (Temperature and relative humidity) under the canopies are comparable to various cloudiness conditions?
Materials and Methods: The experiments were conducted at Ferdowsi University of Mashhad. The climate data under two species of false acacia (Robinia pseudoacacia) and pine trees (Pinus eldarica) and open space were received from Mashhad weather station over a period of 19 years, recorded and analyzed. In the comparison of the effects of vegetation and cloudy conditions on the temperature and relative humidity an assumption was proposed that has been mentioned in the following. The differences between the means of temperature and relative humidity obtained under SKC (Sky Clear) cloudiness conditions and in open space; under SCT (Scattered Sky) conditions and pine trees; and under BKN (Broken Sky) conditions and false acacia tree must be equal or these differences must be statistically not significant. In addition to the effect of tree canopy, the effects of days and hours on the temperature and humidity were investigated. To analysis and also mean comparison, SPSS 16 software was used.
Results and Discussion: The results showed that the temperature (decreasing) and relative humidity (increasing) were significantly different among various cloudiness conditions during 19 years. The results also indicated that between the temperatures recorded under the canopy of pine and false acacia trees, and also among of recorded relative humidity in two trees specious with corresponding outdoor spaces there were significant differences. Also, there were no significant differences among the temperature of the clear sky and the partly cloudy with that of the mostly cloudy sky. Further, clear sky and mostly cloudy sky showed significant differences in terms of relative humidity. Mostly cloudy sky and the partly cloudy sky compared with pine and false acacia trees were about 4.6 and 4.5oC cooler, respectively. The cloud covers, also could enhance the more level of relative humidity in the environment in comparison with single tree canopy, so that were caused a wetter environment equal to16.6 and 8.4 percent, respectively. The results also showed that temperature and relative humidity created by the shade from false acacia and pine trees are not comparable with the same climatic factors created by partly cloudy and mostly cloudy skies. In the present study, false acacia as a broad-leaved tree compared with pine tree and also open space could cool the environment about 0.65 degree C more that is in line with previous studies that showed the microclimatic impact of vegetation depend on crown and leaf size. Temperature and relative humidity in vegetation (trees) and cloudy condition are not comparable with microclimate and their impact on the environment is not the same. The cloud covers, also could enhance the more level of relative humidity in the environment in comparison with single tree canopy, so that were caused a wetter environment equal to16.6 and 8.4 percent, respectively.
Conclusion: This finding showed that climatic effects of cloud covers had no similarity compared with two studied trees species. It may also indicate that green space and greenery should not also be ignored in areas where cloudy skies are mostly present throughout the year.