Milo E. Hoffman

Vegetation as a climatic component in the design of an urban street: An empirical model for predicting the cooling effect of urban green areas with trees

Abstract The cooling effect of small urban green wooded sites of various geometric configurations in summer is the object of this study. It was studied experimentally at 11 different wooded sites in the Tel-Aviv urban complex during the period July–August 1996. An empirical model is developed in this study for predicting the cooling effect inside the wooded sites. The model is based on the statistical analysis carried out on 714 experimental observations gathered each hour from the 11 sites on calm days, when urban climate is expressed. Two factors were found to explain over 70% of the air temperature variance inside the studied green site, namely, the partial shaded area under the tree canopy and the air temperature of the non-wooded surroundings adjoining the site. The specific cooling effect of the site due to its geometry and tree characteristics, besides the shading, was found to be relatively small, about 0.5 K, out of an average cooling of about 3 K at noon. The cooling effect of the green wooded areas on their immediate surroundings at noon was also analyzed. The findings corroborate earlier studies that the range is noticeable. At small green sites, the cooling effect estimated in this study is perceivable up to about 100 m in the streets branching out from the site. The empirical findings in this study permit development of tools for incorporating the climatic effects of green areas in the urban design. Some policy measures are proposed accordingly, for alleviating the “heat island” effect in the urban environment.

Geometry and orientation aspects in passive cooling of canyon streets with trees

As streets usually cover more than a quarter of the urban area, canyon street morphology plays an important role in creating the urban climate. It directly influences the air temperature, moisture and wind flow within the streets as well as the urban surrounding area and has been the topic in several urban climatology studies. Recently, studies based on the street cluster thermal time constant (CTTC) model have been carried out by the authors with a view to assessing the thermal effects of alternative architectural designs of the flanking buildings and inner courtyards. The effect of green spaces, especially that of shade trees which plays a significant role in solar radiation penetration, has not yet been considered. In the CTTC model, passive cooling of the street by solar heating attenuation is governed mainly by the street orientation and its geometry as measured by the aspect ratio of flanking buildings height to street width. The tree shading coverage largely offsets the contribution of these two factors. Moreover, significant thermal effects are provided by the tree canopy, in addition to the direct solar radiation. Accordingly, adjustments are called for in the currently used canyon street models. The present paper discusses the geometry and orientation aspects of the canyon street climate and how these aspects are affected and can be reconciled in the presence of shade trees. Some consequences of environmental design of urban spaces and their effects on outdoor thermal comfort are also considered.

Prediction of urban air temperature variations using the analytical CTTC model

Abstract The impact of buildings on the thermal climate in built-up environments is considered. Urban geometry, construction details and thermal characteristics of typical urban fabrics are investigated in connection with their role in the evolution and intensity of urban-rural and intra-urban thermal differences. In consequence, the cluster thermal time constant (CTTC) analytical model for predicting air temperature variations in the urban canopy layer (UCL) is developed. The CTTC parameter, which expresses the thermal inertia of urban landscapes, is virtually proportional to the urban surface area within the UCL relative to the plot area of the neighbourhood. This model simulates, with good agreement, air temperature measurements conducted over a fair-weather summer period in selected clusters at the city centre of Jerusalem (c. 750 m above sea-level, 32 °N, 34 °E). Consistent intercluster differences of up to 4 K were observed, and consequently calculated by CTTC model simulation. Neighbourhoods characterized by extensive shaded area and high CTTC parameter exhibited negative heat-island (cool-island) intensities over most of the day and positive intensities at night.

The Green CTTC model for predicting the air temperature in small urban wooded sites

Abstract An analytical model, the Green CTTC (cluster thermal time constant) model, for predicting diurnal air temperature inside an urban wooded site, is the object of this study. The proposed model is based on the same principles as the CTTC model, developed earlier by M.E. Hoffman and colleagues, with the addition of vegetation effects. It is shown that the tree thermal effect can be evaluated either as the shade effect partly offset by the convection component of the tree radiation balance or, equivalently, as the combined effect of evapotranspiration and the change in the plant heat storage. In this paper, the former approach is adopted. Simulations for testing the validity of the Green CTTC model were carried out on summer data of 11 small urban wooded sites in the Tel-Aviv metropolitan area near the Mediterranean sea coast. Results show a satisfactory fit, with average root-mean-square-error K for all studied sites and time intervals at 09:00, 15:00, and 18 : 00 h (summer time). The CTTC values and the convection parameters were estimated from the empirical data, using a novel procedure. The proposed model, which can be enlarged to encompass the cases of groves and lawns, is an appropriate tool for assessment of the climatic impact of trees and other greeneries on urban design alternatives.

Climatic impacts of urban design features for high- and mid-latitude cities

Abstract Three aspects of the analytical cluster thermal time constant (CTTC) model developed earlier by the authors for predicting urban air temperature variations under Mediterranean mid-latitude (31–32°N) climatic conditions are considered here: its possible adaptability to serve as a tool for predicting high-latitude urban climates, its applicability to the assessment of climatic impacts of urban design alternatives, and the sensitivity of calculated temperature values to variations in the input parameters of the model. Simulations made by means of the CTTC model yield air temperature variations in good agreement with those measured (by others) in the city centre of Essen (c. 52°N, 7°E) during 30 days of a clear and calm-weather summer period. The model is also applied to predict diurnal air temperature variations in two similar hypothetical clusters composed of repeated uni-directional semi-infinite street canyons differing only in their longitudinal axis orientations. Both clusters exhibit a “cool island” during most of the daylight hours and a “heat island” at night, but intensities differ in each case. It is clearly shown that the maximum nocturnal urban heat island intensity is not related solely to the mean aspect ratio of street canyons (H/W), but also to other features of the urban texture, such as the street pattern orientation. Sensitivity tests were also conducted in order to explore the response of urban thermal climates to changes in such planning details of urban layouts as radiative, flow, and geometrical characteristics.

Calculation of the thermal response of buildings by the total thermal time constant method

Abstract This paper describes the theoretical development and experimental proof of the total thermal time constant ( TTTC ) method for calculation of the thermal response of buildings. The output is obtained in the form of time sequences of temperature, under given time-variation of internal heating load, or in the form of time dependent heating (or cooling) loads, under given patterns of internal temperature variation in time. TTTC method considers ventilation conditions, internal heating, metabolic heat production, cooling, solar radiation absorption on, and longwave i.r. radiation loss from, the external surfaces, solar radiation penetration through windows and the external air temperature and humidity variations in time. The main feature of this method is that each component of the building is represented here, as a heat transfer path, only by two easy to calculate numbers: the thermal resistance and the TTTC (this includes thermal resistances and heat capacities and their relative position in the heat transfer path, including partitions and ceilings) [ 1–3 ]. The two parameters characterize the influence of the element on the thermal response of a building as a whole. Experimental demonstration of the accuracy of the TTTC method in computing the thermal response of buildings is presented and compared with measured temperature time patterns both in models and actual buildings under various external conditions. The method is useful not only for the thermal design of buildings and the selection of building materials, but also for the design of passive methods of climatization, e.g. by the use of solar radiation for heating, and conversely, the cooling of a structure by longwave radiation loss (to the outer space through the atmosphere) and by ventilation. Thermal storage and insulation properties are also considered.

The prediction of impervious ground surface temperature by the surface thermal time constant (STTC) model

Abstract Temperature patterns of impervious and dry ground surfaces typical of urban and dry rural environments are considered. Ground-surface temperature depends on the thermophysical properties of the ground material between the surface in question and the constant temperature plane at the diurnal “penetration depth” (lower boundary of participating layer), on wind conditions and on solar and long-wave infrared radiation exchange and geometries. The surface thermal time constant (STTC) analytical model introduced here is an effective tool for predicting ground-surface temperature in homogeneous and layered soils. The STTC itself is defined as the energy stored in the ground participating layer per unit change of the absorbed radiation flux density on the grounds surface. An extensive series of field observations were conducted. Surface and ground temperature patterns and meteorological conditions in winter and summer were obtained continuously during a nine-month period of measurements. Calculated values of surface temperature by means of the STTC model showed good agreement with observed values. The STTCs of the impervious concrete and asphalt surfaces — typical of urban environments — were found to be 2.2 hours and 1.8 hours respectively, and that of dry bare soil — typical of rural environments — 1.3 hours. The STTC, here defined as a thermal property of the ground, compares qualitatively with the thermal inertia (admittance) of a homogeneous ground layer represented by (kϱc) 1 2 commonly used in numerical models for predicting surface temperature.

Parametric analysis of membrane characteristics and membrane structure

Abstract The work tests the applicability of two empirical equations to the analysis of membranes, and is motivated by a long recognized need for a characterization of membrane phenomenology by a physical model. Empirical relations are often used to describe the phenomenology of complicated systems and are also used as starting points for a detailed molecular analysis of systems like solutions and liquids. We apply here coefficients of the empirical Collanders equation, which is related to the physiological characteristics of membranes, and of the Barclay-Butler relation, which was successfully used as a starting point for the construction of a molecular model of solutions. These coefficients we use to analyze the phospholipid membrane. In the analysis we use three sets of solutes: noble gases, paraffins and aliphatic alcohols and test whether conclusions drawn from simple, but basic, physiochemical considerations agree with results obtained by more direct methods. We also ask ourselves what information can be obtained by the parametric analysis which is not available already by other methods. The results of our analysis agree with results obtained by direct physical methods and bear a close relation to membrane physiology. This type of analysis may therefore bridge a gap between biological phenomenology and purely physical methods.

Thermal and comfort conditions in a semi-closed rear wooded garden and its adjacent semi-open spaces in a Mediterranean climate (Athens) during summer

The cooling effect in a courtyards garden and in the adjoining ground- and first floor verandas, attached to the NNE side of a two-storey building is evaluated with measurements performed during a hot weather summer period in Athens. Results revealed a well defined and strong daytime cool island between the buildings rear garden (with about 85% canopy covering) and an air temperature reduction for the ground floor veranda, as compared with an urban square with low canopy coverage (about 15%), reaching a maximum air temperature reduction of 6.5 K during daytime. Compared with a nearby densely wooded park, the garden and the veranda were found 1–1.5 K cooler during 4 and 8 hours during daytime, respectively. Using the physiologically equivalent temperature thermal index with appropriate adjustments to local conditions, it was found that those two sites, compared with the urban square, were able to mitigate the extreme thermal stress conditions and to decrease the daily number of hours associated with strong thermal stress conditions. It is concluded that appropriately designed semi-open spaces in residential buildings, well known from vernacular architecture for their qualitative benefits, may be considered as positive bioclimatic pedestrian transitional elements in sustainable urban design for Mediterranean climates.

Solar heating using common building elements as passive systems

Abstract The paper reviews the first steps of a study on use of windows as passive solar air collectors, offsetting naturally the excess of heat in the thermal mass of the building itself, and of vertical solar collectors, with air as working fluid, and with storage systems designed as intergral parts of the building, incorporated in the concrete elements. Some examples of architectural solutions combining these principles in a modular design are suggested. Incorporated storage examples using ceilings and partitions with appropriate air transfer through them are proposed. The relevant thermal analysis on the use of windows as passive solar collectors is based on the Total Thermal Time Constant (TTTC) Method, developed by two of the authors. An analysis is also presented for a vertical flat-plate solar collector using air as working fluid and capable of integration in a blank (windowless) part of an external wall. Design and dimensioning of the fin surface are proposed for heat transfer from collector surface to fluid. The final section deals with experimental model based on the above principles and combining a vertical collector and heat storage for use in daytime and at night, respectively. The model was so dimensioned as to represent a 1:1 unit in modular building design. An overall thermal efficiency of about 18 per cent was obtained.