بررسی رابطه بین ضریب دید به آسمان و پوشش گیاهی، نمونه موردی: پارک لاله شهر تهران

ضریب دید به آسمان یکی از متغیرهایی است که در تشکیل جزایر گرمایی شهری نقش دارد و بیانگر نسبت تابش دریافتی یک صفحه مسطح به تابش تابیده از کل محیط تابنده نیم کره ایست و کمیتی بدون بعد بین صفر تا یک است و در زمین کاملاً صاف به واحد میل م یکند. حال آ نکه مکان های دارای مانع، مانند ساختما نها و درختان، موجب کاهش svf  به طور متناسب می شوند. هدف اصلی این پژوهش بیان رابطه میان ضریب دید به آسمان و پوشش گیاهی با استفاده از دمای سطح در پارک های شهری است. در این پژوهش، مطالعه میدانی در 15 ایستگاه پارک لاله شهر تهران در 5 روز بهاری انجام شده است. تصاویر از طریق دوربین با لنز چشم ماهی استخراج و ضرایب با برنامه محاسباتی sky view factor calculator و دمای سطح نیز با دماسنج 2-wt اندازه گیری شده است. در فضای درون پارک لاله همبستگی بین ضریب دید به آسمان و دمای هوا سطح معنی دار و مثبت بوده است( 262 / 0= r2 و 035 / p-value=0 ).همچنین همبستگی بین تفاوت دمای هوا و سطح با ضریب دید به آسمان تنها در ایستگاه fl  معنی دار است ( 929 / 0= r2 و 023 / .=p-value). سپس همبستگی بین دمای سطح و میانگین قطر تاج پوششی بررسی شد و نتایج حاکی از عدم معنی داری همبستگی بود ) 202 / 0= r2 و 47 / p-value=0 ). در نهایت مشخص شد که میزان همبستگی بین دما و اندازه برگ پوشش گیاهی در فضای درون پارک لاله معنی دار بوده است (911 / 0= r2 ). گونه های پهن برگی چون ارغوان، چنار، سنجد، ترون و نارون و پوتوس در ترکیب با گونه های سوزن یبرگ بید مجنون، سرو شیراز، سرو نقره ای بهتر از سایر گونه ها می توانند دمای سطح را خنک نمایند. یعنی اکثر گونه هایی که در ایستگاه fl به کار رفته اند.

Comparing Common Space of Urban Parks of Tehran SVF to Reduce Urban Heat Islands, Case Study: Parks of District 6

The Sky View Factor, in abbreviation form known as SVF, is a parameter which is a key in Urban Heat Island (UHI) formation. This coefficient denotes the ratio between the received radiation on a planner surface and the whole available radiation from the entire radiating environment. Having no unit, the quantity is ranged between 0 from an absolutelyobstructed view toward the sky to 1 which is assigned to an unobstructed planner surface. Measuring SVF is quite prevalent approach in urban studies in many developed countries, though in Iranian context it has to be dealt yet. The lag is the direct cause of fractional consideration of environmental chapter in urban planning/design studies that has beenof partial importance to the experts. However, deteriorating environmental condition of the country’s metropolitans urges the issue to tackle and leads to raising awareness. Thus taking environmental parameters; and SVF in particular; into account is relatively pervasive amid specialists. While studying SVF in Iran is inevitable, it carries two folds. Primarily, the transformation of environment as the consequent of excessive changes in material from natural to humanmade has intensified the urban heat island formation. Numerous unwanted impacts including citizens’ health issues, increasing mortality ratio and higher energy consumption is assumed and experienced accordingly. Secondarily, scientific precedent and field studies lead to globally well approval that application of a proper green coverage is the most accepted method to urban heat island reduction in urban open spaces such as squares, parks and parking lots. Nevertheless, one could not find a solid and robust interpretation of ‘proper’ in the coined term of ‘proper green coverage’ in Iranian academic literature with good agreement within climatic requirement. Therefore, on top of identifying the correlation between the SVF and geometrical proportion of the green coverage or community, it is crucial to develop a better understanding of theimpact of green coverage on the temperature of the urban open spaces which are planted with the same green objects. Ultimately, it would be feasible to propose selection and combination of that community suited and tailored for Iranian cities. The main objective of this research is to identify the correlation between the Sky View Factor and the green coverage and subsequently, studying its effect on the ambience as well as surface temperature in a case like an urban park. To achieve this, Tehran Metropolis; the largest city of the country; is selected as the case so that the Iranian wide variation of climatic region narrows down to the limitation of MSc thesis and an article. For the purpose of  surveying diffident combination of green community in an urban open space, a park named Laleh Park is chosen. For attaining a better calibration, a field study comprising of 15 stations in that park is carried out. The time span for collecting data is five days in spring of 2014. The study is equipped with a fisheye lens assembled on a professional camera in order to shoot and record SVF picture. The primary data were keyed in a computer software known as ‘Sky View Factor Calculator’ to make it ready for analysis. It is worth to mention that a standard Wt2 thermometer is utilized to measure the surface and atmosphere temperature. The result; asserted by precedent studies; show that there is a significant and meaningful correlation between the green community and the Sky View Factor. Additionally, it shows that the correlation between the SVF and temperature is meaningful and positive (r2=0.262 & ρvalue=0.035) within the park space. The correlation between the differential temperature of surface and that of the atmosphere with the SVF is meaningful only in FL station (r2= و 0.929 ρvalue=0.023). This implies that some of the green community combination have no important and significant effect on surface temperature reduction which could be due to the density and number of planted objects in adjacency of the corresponding station. In the next step, the investigation of the correlation between the surface temperature and the canopy of the tree yields in no meaningful relationship (r2= و 0.202 ρvalue=0.47), hence one can sum up that the crown diameter crown of the surrounding plants carries no important meaning to the ambience temperature. Finally, the meaningful correlation between the temperature and the leaf size of the green area (r2=0.911) reveals the prominence of the factor for the interior of the park. In conclusion, this is revealed that some of the plants and their combinations have more influence on the temperate of the surface and surroundings in Tehran climatic region. This has implications for urban planners and designers. The boardleaved plants such as Judastree, Planetree, Boxwood, Seabuckthorn, Elmtree and Pothos must be considered with combination of needleleaved trees like Weepingwillow, MediterraneanCypress and Cypresses in order to have a cooler environment.