Influence of a Park on the Surrounding Urban Thermal Environment

Observations and analysis were carried out to study the influence of a park on the surrounding thermal environment. The park has the size of about 0.6 km^2. The observations were carried out in summer season with hourly values of air temperature, relative humidity, wind velocity, radiation condition, temperature of the ground and wall surfaces measured at many points in the park and surrounding urban areas.

Based on the measured environmental factors, the thermal environment inside the park and surrounding areas is evaluated by employing the well-known two-node model for the response of a human body to the thermal environment. The radiation attacking the human body is evaluated by assuming that the human body resembles a rectangular block; and the radiation to each surface of the block from the sky or surrounding surface facets is evaluated by the view factor from the surface of the block for the radiated surface. The mean radiant temperature on the human body is computed from the weighting average of radiation on six surfaces. Since evaporation from the ground surface increases humidity, the influence of humidity on human thermal feeling is evaluated by using the PMV*, proposed by Gagge et al (1986).

Results of the observations and analysis reveal that even with an increase in humidity due to the park, the existence of the park significantly improves the thermal environment in itself and surrounding downwind areas. At noon, the air temperature difference between the park and the nearby commercial area of more than 3K readily offsets the increase in humidity inside the park. Additionally, the reduction of solar radiation due to tree shelter improves much thermal feeling. With a strong local wind, the cool air from the park can improve the thermal environment in the downwind direction to the distance of more than 400m. It is estimated that the are of influence by the park is almost 0.5km^2. Numerical simulation with a turbulent model reveals that even at night, the divergence of air causing by the park can still improve the thermal environment of the surrounding areas.

References: Gagge et al (1986) ASHRAE transaction, Vol. 92, Part 2B,709-731.