Gordon M. Heisler

Quantifying urban forest structure, function, and value: the Chicago Urban Forest Climate Project

This paper is a review of research in Chicago that linked analyses of vegetation structure with forest functions and values. During 1991, the regions trees removed an estimated 5575 metric tons of air pollutants, providing air cleansing worth 9.2 million. Each year they sequester an estimated 315 800 metric tons of carbon. Increasing tree cover 10% or planting about three trees per building lot saves annual heating and cooling costs by an estimated 50 to 90 per dwelling unit because of increased shade, lower summertime air temperatures, and reduced neighborhood wind speeds once the trees mature. The net present value of the services trees provide is estimated as 402 per planted tree. The present value of long-term benefits is more than twice the present value of costs.

2 – Effects of Windbreak Structure on Wind Flow

Abstract Functional effects of windbreaks are directly related to the effects of windbreaks on air flow. Additionally, the indirect effects of windbreaks on air temperature and humidity are interrelated with the effects of air movement. The horizontal extent of windbreak effects upwind and downwind is usually assumed to be proportional to windbreak height, h . Measureable reductions in windspeed have been recorded as far as 50 h to the lee of windbreaks, and rarely, even farther. Reductions of 20% or more may extend to about 25 h from the windbreak. For windbreaks that are long relative to their windbreak height, the most important structural feature is porosity. Maximum wind reductions are closely related to porosity, with low porosity producing high maximum reductions. Barriers with very low porosity create more turbulence downwind than medium-dense barriers. The higher turbulence may result in recovery of mean horizontal windspeeds to upwind speeds closer to low-porosity barriers, thus resulting in a shorter protected distance. However, the reduction in protected distance with very dense windbreaks compared to medium dense windbreaks is much less than much of the older literature suggests. Turbulence in the approach flow reduces windbreak effectiveness, particularly at far downwind positions. The turbulence may be caused by thermal instability, a rough ground surface, or other upwind barriers to flow. Differences in approach-flow turbulence, differences in height of measurement relative to windbreak height and differences in vertical porosity gradients are responsible for much of the scatter in experimental data. There is a triangular ‘quiet’ zone below a line beginning near the top of windbreaks and extending to near ground level at a distance of about 8 h to the leeward. In this zone, the turbulent velocity fluctuations are reduced below values in the approach flow. Above and downwind of the quiet zone is a ‘wake’ zone with turbulent fluctuations greater than those in approach flow. The magnitude of turbulent velocity fluctuations in the lee of windbreaks is inversely proportional to porosity. However, there is a larger difference in turbulence generated between solid barriers and slightly porous barriers than between slightly porous and very porous barriers. Windbreaks generally reduce turbulent eddy length, thus increasing the peak frequency of turbulent velocity fluctuations, regardless of their structure. Peak frequency of velocity fluctuations close to windbreaks tends to increase with porosity.

Impacts of vegetation on residential heating and cooling

Computer simulation has been used to test the effects of irradiance and wind reductions on the energy performance of similar residences of 143 m2 in four U.S. cities — Madison, Salt Lake City, Tucson and Miami — representing four different climates. Irradiance reductions from vegetation were modeled using SPS, which simulates shade cast from plants on buildings, and MICROPAS, a microcomputer-based energy analysis program. Space cooling costs were found to be most sensitive to roof and west wall shading, whereas heating costs were most sensitive to south and east wall shading. Irradiance reductions were shown to substantially increase annual heating costs in cold climates (

Photosynthetically-active radiation: sky radiance distributions under clear and overcast conditions

128 or 28% in Madison), and reduce cooling costs in hot climates (

Estimation of Ultraviolet-B Irradiance under Variable Cloud Conditions

249 or 61% in Miami). Dense shade on all surfaces reduced peak cooling loads by 31% – 49% or 3108 – 4086 W. A 50% wind reduction was shown to lower annual heating costs by

Obscured Overcast Sky Radiance Distributions for Ultraviolet and Photosynthetically Active Radiation

63 (11%) in Madison, and increased annual cooling costs by

Effects of Supplementary Ultraviolet-B Irradiance on Maize Yield and Qualities: A Field Experiment¶

68 (15%) in Miami. Planting designs for cold climates should reduce winter winds and provide solar access to south and east walls. This guideline also applies for temperate climates, however it is also important to avoid blocking summer winds. In hot climates, high-branching shade trees and low ground covers should be used to promote both shade and wind.

Clear sky radiance distributions in ultraviolet wavelength bands

The photosynthetically active radiation (PAR), defined as the wavelength band of 0.400 μm to 0.700 μm, represents most of the visible solar radiation. Although the proportion of global irradiance that originates from diffuse sky radiation is higher for PAR than for all solar shortwave radiation, it is often assumed that the PAR diffuse sky radiation is distributed identically to that of all shortwave solar radiation. This assumption has not been tested. PAR sky radiance measurements were made in a rural area over a wide range of solar zenith angles. The distribution of PAR sky radiance was modeled using physically-based, non-linear equations. For clear skies, the normalized sky radiance distribution (N) was best modeled using the scattering angle (ψ) and the zenith position in the sky (Θ) as N(Θ,ψ)=0.0361[6.3+(1 + cos2Θ)(1 − cosψ)][1 − e−0.31 secΘ]. The angle Ψ is defined by cos ψ = cosΘcosΘ∗ + sinΘsinΘ∗cosΦ, where solar zenith angle is Θ* and the difference in azimuth between the sun and the position in the sky is Φ. Modeling of the overcast sky depended on the visibility of the solar disk. The translucent middle/high cloud overcast conditions (cloud base greater than 300 m above ground level) were best modeled as: N(Θ∗, ψ) = 0.149 + 0.084Θ∗ + 1.305e−2.5ψ while the translucent low cloud overcast conditions (cloud base less than 300 m above ground level) were best modeled as: N(Θ∗, ψ) = 0.080 + 0.058Θ∗ + 0.652e− 2.1ψ. The obscured overcast sky condition (solar disk obscured) was best modeled as: N(Θ) = 0.441[1 + 4.6cosΘ][1 + 4.6]. The unit of N for all equations is π Sr−1, so that integration of each function over the sky hemisphere yields 1.0. These equations can be applied directly to the sky diffuse irradiance on the horizontal, Idiff, to provide radiance distributions for the sky. Estimates of actual sky radiance distribution can be estimated from Na(Θ,ψ) = IdiffN(Θ,Φ).

Estimation of Pedestrian Level UV Exposure Under Trees

Abstract Methods to estimate the irradiance of ultraviolet-B (UVB; 280–320 nm) radiation are needed to assess biological effects of changes in atmospheric composition. Measurements of the spatial distribution of sky cloud cover, temporal variability of photon flux density of photosynthetically active radiation (PAR; 400–700 nm), and UVB irradiance (I-UVB) were made on 23 days during the summer of 1993 in a rural area (West Lafayette, Indiana). Prediction equations for the measured UVB irradiance under partly cloudy skies were developed based on the photosynthetically active photon flux density (PPFD), cloud cover fraction, probability of cloud obstruction of the sun, and a semiempirical combination of cloud probability and cloud cover. The I-UVB was linearly related to the PPFD, with the variability in PPFD accounting for 77% of the I-UVB variability. Normalized PPFD (PAR F) and I-UVB (UVB F) values, calculated by dividing the observed value by the expected cloud-free sky PPFD and I-UVB, were also linearl...

Solar ultraviolet-B and photosynthetically active irradiance in the urban sub-canopy: A survey of influences

Abstract Sky radiance measurements in the wavelength bands of ultraviolet-B (0.28–0.32 μm), ultraviolet-A (0.32–0.40 μm), and photosynthetically active radiation (0.40–0.70 μm) were made under obscured overcast skies in a rural area. Radiance distributions were modeled for seven measurement scans with solar zenith angles varying from 19° to 49°. For the seven scans, the atmospheric transmittance of photosynthetically active photon flux density varied from 0.16 to 0.25. The corresponding fraction of cloud-free sky photosynthetically active photon flux density ranged from 0.21 to 0.32. The corresponding fraction of ultraviolet-B cloud-free sky irradiance was between 0.20 and 0.34, with typically lower fractions of cloud-free sky irradiance in the ultraviolet-B than in the photosynthetically active photon flux density. The sky radiance was modeled from the ensembled measurements according to the standard overcast sky radiance distribution for each of the wavelength bands. Although the ultraviolet wave bands ...