Large Diurnal Temperature Variations Research Articles

Effect of double-layer formwork curing on the early temperature field and properties of precast concrete components in environments with large diurnal temperature variations

An efficient double-layer formwork curing method using an “inner supporting formwork + outer insulation formwork” was proposed in this study to address the early cracking of precast concrete components in high altitude regions. Steel and plywood formwork were designed as inner support formwork, while polystyrene (PS) and polyurethane (PU) were used as outer insulation formwork. Indoor experiments and two finite element methods (The complete simulation method focuses on computational accuracy, and the equivalent simulation method emphasizes computational efficiency) were employed to analyze the evolution of the concrete temperature field under different double-layer formwork curing methods throughout the curing period, combined with compressive strength and pore structures testing. The results show that steel + 5-mm-thick PU insulation formwork curing method can significantly mitigate the impact of large diurnal temperature variations on the internal temperature of concrete. Unlike traditional steam-curing, this method does not deteriorate the pore structure or compressive strength of the concrete. This study is of great significance in addressing the problem of early cracking of precast concrete components exposed to large diurnal temperature vriations in high altitude regions.

Damage and fracture in in-service concrete structures subjected to large diurnal temperature variations based on phase-field approach

In-service concrete structures are exposed to the natural environment for extended periods, making them susceptible to performance degradation and cracking due to large diurnal temperature ranges (DTR). This paper presents the development of a novel thermo-mechanical-fatigue coupling phase-field model to investigate the cracking process in in-service concrete structures under varying DTR conditions. The results indicate that concrete walls exposed to different single-day DTRs (20–60 ℃) exhibit phase-field damage without cracking. The phase-field damage is negligible at a 20 ℃ DTR but increases to 0.41 at a 60 ℃ DTR. Considering temperature fatigue, the concrete wall initiates cracking when the DTR exceeds 40 ℃ within 100 cycles. With an increase in DTR from 40 ℃ to 50 ℃ and 60 ℃, the time for crack nucleation decreases from 70 cycles to 29 and 25 cycles, respectively. Concrete walls subjected to 40 ℃ and 50 ℃ DTR exhibit a single crack on the top surface, whereas those exposed to a 60 ℃ DTR have two cracks on the top surface and one crack on each side. Furthermore, the cracks induced by the DTR are surface cracks that exhibit rapid expansion up to a specific length of approximately 140 mm before stabilizing.

Effects of alternating positive and negative temperature curing on the mechanical properties of ultra-high performance concrete

The Tibetan region is characterized by long winters and large diurnal temperature variations. Using prefabricated bridges combined with a small amount of cast-in-place concrete joints can address the challenging construction environment. To extend the bridge construction window for concrete in the joint, the performance of Ultra-High Performance Concrete (UHPC) under alternating positive and negative temperature environments was investigated. The effects of curing conditions (−10 to 10 °C and natural curing), different proportions of sulfoaluminate cement (0% / 6%), different concrete materials (UHPC/NC), and the presence of antifreeze admixtures on the macro-strength and late-stage microstructure of UHPC were explored. The results indicated that the slump expansion of the prepared UHPC in this study exceeds 600 mm. Alternating temperature environments significantly degrade the mechanical properties of UHPC without antifreeze admixtures. However, for self-made UHPC with antifreeze admixtures, the formed early structure can prevent freeze-thaw damage as long as the critical antifreeze strength is reached. After the temperature rebounded, the later-stage strength of UHPC can continue to develop and reach the compressive strength of samples that had not been exposed to low-temperature curing. The appropriate addition of sulfate aluminate cement positively affected the early compressive strength (1 day) and elastic modulus of UHPC. It also reduces the drying shrinkage of UHPC.

Novel metal-organic framework (MOF) based phase change material composite and its impact on building energy consumption

Space cooling (including dehumidification) is the fastest-growing use of energy in buildings. Materials that have high thermal and moisture buffer capacities can passively mitigate indoor temperature and humidity fluctuations, thus reducing the demand for air conditioning and improving building energy efficiency. Here, we report a novel metal–organic framework (MOF) based microencapsulated phase change material (MPCM) composite. The new MOF-MPCM is a dual-function material that can simultaneously absorb/release heat and moisture from surrounding air and passively regulate the indoor hygrothermal environment. MIL-160(Al), a novel green and biomass-derived MOF material with excellent sorption performance and large-scale production potential, was prepared for moisture buffering. MPCM containing an n-octadecane core and polymethylmetracrylate shells were synthesized for temperature control. Physicochemical and hygrothermal properties of MOF-MPCM composite were characterized by SEM, XRD, DVS, DSC, and TGA techniques, etc. A HAM-Enthalpy model was developed to study the impact of MOF-MPCM on the indoor hygrothermal environment and building energy performance in different climates. Seven cities around the world (i.e., Singapore, Hong Kong, Phoenix, Denver, Barcelona, London, and Beijing) were selected as the representative climate locations. The simulation indicates that MOF-MPCM can effectively mitigate indoor temperature and moisture variations and cut down the energy consumption of air conditioning systems, especially in hot-dry, temperate, and continental climates with large diurnal air temperature and humidity variations. The maximum energy-saving potential could reach 35.2% in Phoenix. The study provides guidance for the further improvement and application of dual-function MOF-MPCM composite in different climates.

Material characterisation of heavy-weight and lightweight adobe brick walls and in-plane strengthening techniques

Adobe housing is used in developed countries and much more in less developed countries. Such construction is expected to rise with the increase in world population and the worldwide need of more sustainable and eco-friendly solutions. The large mass associated with earthen buildings is favourable for thermal comfort in regions with large diurnal temperature variations, but it causes these buildings to experience high inertial loading in the event of an earthquake, thus posing a major risk to their inhabitants. Lightweight adobe bricks have been developed and adopted in New Zealand to mitigate the seismic risk, and provide better thermal insulation for sustained periods of hot or cold. There is still a lack in research and national standard literature regarding their material characteristics and earthquake performance. The experimental campaign described herein aims to determine the material properties and in-plane performance of the lightweight adobe walls and compare them to the traditional heavy-weight adobe. Different mesh materials have also been investigated to be used as horizontal reinforcement laid in the mortar joints to enhance the shear characteristics of the adobe wall. Diagonal compression tests were undertaken in as-built panels and in walls strengthened with biaxial polypropylene geogrid.

A Review on Shape-Stabilized Phase Change Materials for Latent Energy Storage in Buildings

Many countries in the Global South have hot and dry climates with large diurnal temperature variations, which leads to large demand for space cooling—which is likely to increase with climate change. A common approach to dampen the indoor temperature fluctuations and thus reduce cooling energy demand is the use of thermal mass. However, the use of lightweight structures in many cities (e.g., high-rise structures, or for earthquake protection) precludes the use of traditional forms of thermal mass. Therefore, phase change materials (PCMs) are being widely developed as thermal energy storage systems for building applications. However, challenges such as leakage of PCMs in liquid state and their low thermal conductivity, still limit their applications in buildings. In this paper, we review the potential of Form or Shape-Stabilized Phase Change Materials (SSPCMs), which are developed by incorporating the PCM into a supporting matrix to prevent leakage in liquid state whilst improving thermal conductivity. We review different methods of preparation and the resultant thermal properties and chemical stability. We find good evidence in the literature for SSPCMs to reduce PCM leakage in liquid state, dampen indoor temperature fluctuations, and potentially alleviate peak energy demand by shifting peak loads to off-peak periods.

Lower atmosphere and pressure evolution on Pluto from ground-based stellar occultations, 1988–2016

Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons spacecraft. Aims. The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. Methods. Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of ~5 and ~380 km (i.e. pressures from ~ 10 μbar to 10 nbar). Results. (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of ~0.3 are present at 4–7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of ~20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.

Snowfall-related deterioration behavior of the Ming Great Wall in the eastern Qinghai-Tibet Plateau

The Ming Great Wall in the Qinghai Province is an important section of the Great Wall of China. Most of this section is fabricated with rammed earth. The preserving environment of the Ming Great Wall is characterized by frequent snowfalls in winter, strong evaporation, and large diurnal temperature variations. Nearly all types of deterioration, including crazing, gully erosion, surface crust erosion, bottom sapping, and collapses, are evident in the Ming Great Wall in the Qinghai Province. Based on field investigations, the snowfall in winter and the presence of soluble salts are the primary factors causing the deterioration of the rammed earth. To investigate the coupled effect of freeze–thaw cycles and salinization on this deterioration process, a series of geotechnical tests are performed at Lanzhou University using samples collected from the collapse accumulation near the earthen sites. The test results showed that the microstructure damage in the rammed-earth materials caused by the freeze–thaw cycles and salinization is the primary controlling factor for the deterioration process of rammed-earth structures.

Estimating yield potential in temperate high-yielding, direct-seeded US rice production systems

Accurate estimation of a crop’s yield potential (Yp) is critical to addressing long-term food security via identification of the exploitable yield gap. Due to lack of field data, efforts to quantify crop yield potential typically rely on crop models. Using the ORYZA rice crop model, we sought to estimate Yp of irrigated rice for two widely used rice varieties (M-206 and CXL745) in three major US rice-producing regions that together represent some of the highest yielding rice regions of the world. Three major issues with the crop model had to be addressed to achieve acceptable simulation of Yp; first, the model simulated leaf area index (LAI) and biomass agreed poorly for all direct-seeded systems using default settings; second, cold-induced sterility and associated yield losses were poorly simulated for environments with a large diurnal temperature variation; lastly, simulated Yp was sensitive to the specified definition of physiological maturity. Except for the simulation of cold-induced sterility, all issues could be remedied within the existing model structure. In contrast, simulation of cold-induced sterility posed a continuing challenge to accurate simulation—one that will likely require changes to ORYZA's formulation. Estimates of Yp from the modified model were validated against large multi-year data sets of experimental yields covering the majority of US rice production areas. Validation showed the adjusted model simulated Yp well, with most top yields falling within 85% of Yp for both varieties (77% and 78% observed yields within 15% of Yp for CXL745 and M-206 respectively). Maximum estimated Yp was 14.3 (range of 8.2–14.5) and 14.5 (range of 8.7–15.3)tha‐1 for the Southern US and CA, respectively.

Experimental Research on Effect of Curing Methods on the Strength and Resistance to Chloride Ion Permeability of Concrete in Simulated Arid and Large Diurnal Temperature Variation Climates

Curing techniques and curing conditions have crucial effects to the strength and durability of concrete. The objective of this experimental study is to examine the strength and resistance to chloride ion permeability of high performance concrete under various curing methods in different simulated arid and large diurnal temperature variation climates. Laboratory experiments were conducted to investigate the strength and electric flux when different curing methods were used. Three curing methods and three simulated climates were applied to concrete specimens. The results show that the difference of measured experiment data are not significant under standard curing (SC), moisture insulation curing (MIC) and thermal and moisture insulation curing (TMIC) in an arid climate. However, the compressive strength and resistance to chloride ion permeability are worse under MIC and TMIC compared with SC in an arid and large diurnal temperature variation due to frost heaving and thermal stress. Moreover, compressive strength and resistance to chloride ion permeability decrease significantly when the large diurnal temperature variation increases and the lowest temperature reduces. The insulation material can effectively reduce concrete temperature variation between near surface and core region. Therefore, TMIC is a better curing method than MIC in an arid and large diurnal temperature variation climate.

Assessing the influence of pronounced diurnal temperature variations in nontemperate zones on the denitrification/nitrification rate using the COST Benchmark activated sludge model no. 1 simulation

AbstractActivated sludge plants are usually designed using formulae based on fixed wastewater temperatures. In some nontemperate zones, these procedures may need altering, particularly for areas that experience large diurnal and seasonal temperature variation such as in desert or high‐altitude environments. This study investigates these temperature effects on the denitrification/nitrification rate by utilising the COST Benchmark activated sludge model no. 1 (ASM1) simulation model. The COST Benchmark was developed under the European Cooperation in Science and Technology programme Action 624 which specifically focused on the Optimal Management of Wastewater Systems. Various simulations and associated control strategies were carried out using gPROMS software package (Process Systems Enterprise Limited, London, UK) to ascertain the following: any likely nutrient removal implications; use of a nitrate and/or ammonia sensor coupled to a strategy to change the solids loading inventory to minimise falls in nitrification rate during night time hours; and changes to controller set point levels during the day and night to reflect diurnal load, flow and temperature variations using a temperature‐dependent control algorithm. It was found that inclusion of temperature dependency in simulation models may prove beneficial when designing plants located in these regions.

강화지역의 기후특성 분석을 통한 '강화약쑥'의 생육 환경 연구

Eupatilin, one of representative medical components of mugwort, can be efficiently extracted from the `Gangwha Sajabalssuk`. The Eupatilin content may depend on environmental factors such as soil and regional climate in addition to a genetic factor and Gangwha region has a profitable environmental condition for the mugwort growth. In this study, the climatological characteristics of Gangwha was analyzed in order to find the environmental condition of mugwort containing high Eupatilin in term of atmospheric, oceanographic and land variables. The climate of Gangwha is characterized by the relatively low daily temperature and large diurnal variation with plenty of solar radiation, long sunshine duration and less cloudiness. According to our correlation analysis, the long sunshine duration and the large diurnal temperature variation are highly correlated with the Eupatilin contents. The result implies that Gangwha has the favorable conditions for the cultivation and the habitat of the high-Eupatilin concentrated mugwort. Because of the sea surrounding Gangwha Island with low salinity and moderate wind, the salt contained in sea breeze is relatively low compared to other regions. Furthermore, Gangwha has clean atmospheric environment compared to other regions because the concentrations of toxic gases harmful to crop growth such as nitrogen dioxide (), sulfite gas () and fine dust (PM-10) are lower in the air. The ozone () concentration is moderate and within the level of natural production. It is also found that moderately coarse texture or fine loamy soils known as good for water drainage and for the growth and cultivation of the `Gangwha-mugwort` are distributed throughout the areas around mountainous districts in Gangwha, coinciding with those of mugwort habitat.

The Bio-Climatic Analysis and Thermal Performance of Residential Building in Upper Egypt A Case Study, “East El-Owauinat Region”

ABSTRACTAs a result of the change and development of Egyptian society, Egyptian governmenthas focused its attention of comprehensive development to various directio ns. One of theseattentions is housing, construction and land reclamation in Upper Egypt to go out from thenarrow valley which is about 5% of Egypt area. East El- Owauinat is one of these areas. It isan agriculture project and must be valid all the facto rs needed for sustainability. East El -Owauinat region is located at 220 28` N latitude and 280 42` E longitude. The climate of thisregion is caricaturized by; aridity, high summer daytime temperature, large diurnaltemperature variation, low relative humidity and high solar radiation reaches to about1000W/m2 on horizontal surface in summer seasons .In such environments, man losses his ability to work and to contribute effectively in thedevelopment planning due to the high thermal stress affected on him. The strategy of buildingin this regioy to understand the needs of the people but to create an indoor environment whichis suitable for healthy, and comfortable to live and work in it. Also, reduce or if possibleeliminate the energy expenditure for environmental control. In order to achieve this, attentionhas to be focused on building design which is a function of building form, orientation,location, and materials used. This study deals with the bio-climatic analysis of East El-Owauinat region and the thermal performance of building in this region to valid the EgyptianEnergy Efficiency Residential Code. The results show that, the air catcher, court and Passivecooling systems (evaporative cooling), maintained the indoor climate in the thermal huma ncomfort zone during the hottest period under the effect of climatic conditions of East El -Owauinat. Orientation, size, shape, building materials and outdoors -climatic factors influencein energy auditor of the building envelope. Also shading devices, ins ulating materials, andsmart glass achieve a harmony building with environment , and save energy by about 60%.Thermal insulation is needed for all exposed roofs and the required thermal resistance hasa minimum R-value ≥2.5 m2K/W. The required thermal resistance for walls must bebetween 0.9 and 1.3 m2K/W.

Temperature-dependent thermal inertia of homogeneous Martian regolith

[1] Past studies of the thermophysical properties of the Martian surface layer have assumed temperature-independent thermal inertia, which is a function of the material density, specific heat, and bulk conductivity. In this paper, we evaluate the temperature-driven variations of these quantities for particulated and cemented material under Martian conditions of atmospheric pressure and temperature. Temperature-driven density variations are negligible. The specific heat of a basaltic material is strongly influenced by the temperature (∼75% increase from 150 to 315 K), inducing significant variations of the thermal inertia. The thermal conductivity of uncemented Martian regolith is weakly controlled by the solid phase conductivity and strongly controlled by the gaseous phase conductivity. As a result, the conductivity of the solid phase (i.e., composition, temperature) is unimportant, whereas medium to large variations (30–50%) of the bulk conductivity are associated with temperature-induced fluctuations of the pore-filling gas conductivity. Overall, the thermal inertia of uncemented Martian soils is predicted to vary significantly (∼80%) throughout the range of the observed surface temperatures. In the case of cemented soils, the contribution of the gas conductivity is generally small, and the solid phase (i.e., grains and cement) conductivity (i.e., composition, temperature) becomes more important. Consequently, the magnitude of the thermal inertia change for cemented soils is variable, and smaller than that predicted for uncemented materials (10–50%). Large diurnal and seasonal temperature variations only occur within the top material, and most of the near-surface regolith does not experience large thermal inertia variations. The shapes of modeled diurnal temperature curves are not significantly modified (e.g., the 0200 LT (Martian local time) apparent thermal inertia of uncemented regolith is up to ∼15% lower than the average daily inertia of the top material). Thermally derived grain sizes are usually based on conductivity measurements operated at room temperature or above, where the thermal inertia and specific heat are higher than on Mars, implying that grain size predictions for Mars are currently underestimated. However, in situ observations by the Mars Exploration Rovers and thermal conductivity modeling suggest that this underestimation is not significant.

Role of Diurnal Warm Layers in the Diurnal Cycle of Convection over the Tropical Indian Ocean during MISMO

Abstract The role of air–sea interaction in the diurnal variations of convective activity during the suppressed and developing stages of an intraseasonal convective event is analyzed using in situ observations from the Mirai Indian Ocean cruise for the Study of the Madden–Julian oscillation (MJO)-convection Onset (MISMO) experiment. For the whole period, convection shows a clear average diurnal cycle with a primary maximum in the early morning and a secondary one in the afternoon. Episodes of large diurnal sea surface temperature (SST) variations are observed because of diurnal warm layer (DWL) formation. When no DWL is observed, convection exhibits a diurnal cycle characterized by a maximum in the early morning, whereas when DWL forms, convection increases around noon and peaks in the afternoon. Boundary layer processes are found to control the diurnal evolution of convection. In particular, when DWL forms, the change in surface heat fluxes can explain the decrease of convective inhibition and the intensification of the convection during the early afternoon.

Correcting Dual‐Probe Heat‐Pulse Readings for Changes in Ambient Temperature

We examined three methods of analyzing dual‐probe heat‐pulse (DPHP) data for estimating volumetric water content of near‐surface materials where diurnal temperature variations can approach ± 20°C. The three methods of analyzing thermal responses include: (i) the single‐point method (SPM); (ii) a method that uses a Levenburg–Marquardt optimization of thermal conductivity, volumetric heat capacity, and drift in ambient temperature with subsequent calculation of water content (LM‐Uncorrected); and (iii) an expansion of the LM‐Uncorrected method that incorporates temperature dependencies of soil thermal conductivity and optimizes on volumetric water content, apparent needle spacing, and drift in ambient temperature during the measurement (LM‐Corrected). Ambient temperature measurements were made with either a soil thermistor installed away from the DPHP sensor (SPM) or from the temperature response of the DPHP sensor itself (LM‐Uncorrected and LM‐Corrected). Methods were tested based on measurements made in April and August 2006 from sensors installed at ground surface in the Mojave Desert. The SPM results showed that large (summertime) diurnal temperature variations led to significant oscillations of water content, recorded as high as ± 0.10 m3 m−3 The LM‐Uncorrected method showed a significant reduction in water content oscillations, but variations were still large. The LM‐Corrected method showed that the time series of water content was significantly smoother than uncorrected water contents (i.e., ± 0.005 m3 m−3 in August). The DPHP‐derived water contents compared favorably to data from water content reflectometers installed nearby. The results show that soil water content estimates from the LM‐Corrected method were substantially less affected by large ambient temperature variations.

Recent Climatology, Variability, and Trends in Global Surface Humidity

AbstractIn situ observations of surface air and dewpoint temperatures and air pressure from over 15 000 weather stations and from ships are used to calculate surface specific (q) and relative (RH) humidity over the globe (60°S–75°N) from December 1975 to spring 2005. Seasonal and interannual variations and linear trends are analyzed in relation to observed surface temperature (T) changes and simulated changes by a coupled climate model [namely the Parallel Climate Model (PCM)] with realistic forcing. It is found that spatial patterns of long-term mean q are largely controlled by climatological surface temperature, with the largest q of 17–19 g kg−1 in the Tropics and large seasonal variations over northern mid- and high-latitude land. Surface RH has relatively small spatial and interannual variations, with a mean value of 75%–80% over most oceans in all seasons and 70%–80% over most land areas except for deserts and high terrain, where RH is 30%–60%. Nighttime mean RH is 2%–15% higher than daytime RH over most land areas because of large diurnal temperature variations. The leading EOFs in both q and RH depict long-term trends, while the second EOF of q is related to the El Niño–Southern Oscillation (ENSO). During 1976–2004, global changes in surface RH are small (within 0.6% for absolute values), although decreasing trends of −0.11% ∼ −0.22% decade−1 for global oceans are statistically significant. Large RH increases (0.5%–2.0% decade−1) occurred over the central and eastern United States, India, and western China, resulting from large q increases coupled with moderate warming and increases in low clouds over these regions during 1976–2004. Statistically very significant increasing trends are found in global and Northern Hemispheric q and T. From 1976 to 2004, annual q (T) increased by 0.06 g kg−1 (0.16°C) decade−1 globally and 0.08 g kg−1 (0.20°C) decade−1 in the Northern Hemisphere, while the Southern Hemispheric q trend is positive but statistically insignificant. Over land, the q and T trends are larger at night than during the day. The largest percentage increases in surface q (∼1.5% to 6.0% decade−1) occurred over Eurasia where large warming (∼0.2° to 0.7°C decade−1) was observed. The q and T trends are found in all seasons over much of Eurasia (largest in boreal winter) and the Atlantic Ocean. Significant correlation between annual q and T is found over most oceans (r = 0.6–0.9) and most of Eurasia (r = 0.4–0.8), whereas it is insignificant over subtropical land areas. RH–T correlation is weak over most of the globe but is negative over many arid areas. The q–T anomaly relationship is approximately linear so that surface q over the globe, global land, and ocean increases by ∼4.9%, 4.3%, and 5.7% per 1°C warming, respectively, values that are close to those suggested by the Clausius–Clapeyron equation with a constant RH. The recent q and T trends and the q–T relationship are broadly captured by the PCM; however, the model overestimates volcanic cooling and the trends in the Southern Hemisphere.

Improving night ventilation into low-rise buildings in hot-arid climates exploring a combined wall–roof solar chimney

A theoretical investigation of a combined wall–roof solar chimney to improve night time ventilation in buildings is presented. A spreadsheet computer program is used for the parametric study to find out the optimum configuration of the wall–roof chimney. It has been reported that a roof solar chimney alone can induce an air flow rate of 0.81 m 3/s when the average incident solar radiation is 850 W/m 2. The maximum air velocity induced is 1.1 m/s when the 25° inclined chimney plates are 0.25 m apart. The aim of the paper is to predict the induced air flow rate as a result of the combined effect and to find the best height. The wall chimney height is varied from 1.95 to 3.45 m to determine the optimum length in relation to the chimney inlet. The results show that the air flow rate is three times more than that of the roof solar chimney alone (0.81 to 2.3 m 3/s). The maximum air flow rate of 2.3 m 3/s occurs at 3.45 m wall height. ACTION Psychrometrics Software (Sunshine Technology, USA, 1995) is used to predict the mean cooling load corresponding to the induced ACH. An air change per hour up to 26 could be achieved for a flat volume (321 m 3). Such ACH could be utilized to improve night ventilation to reduce indoor air temperature and cool low-rise heavy buildings with large diurnal outdoor temperature variations.

Photosynthetic gas exchange and temperature-induced damage in seedlings of the tropical alpine species Argyroxiphium sandwicense.

The capacity of Argyroxiphium sandwicense (silverword) seedlings to acclimate photosynthetic processes to different growing temperatures, as well as the tolerance of A. sandwicense to temperatures ranging from -15 to 60° C, were analyzed in a combination of field and laboratory studies. Altitudinal changes in temperature were also analyzed in order to explain the observed spatial distribution of A. sandwicense. A. sandwicense (Asteraceae) is a giant rosette plant that grows at high elevation on two Hawaiian volcanoes, where nocturnal subzero temperatures frequently occur. In addition, the soil temperatures at midday in the open alpine vegetation can exceed 60° C. In marked contrast to this large diurnal temperature variation, the seasonal variation in temperature is very small due to the tropical maritime location of the Hawaiian archipelago. Diurnal changes of soil and air temperature as well as photosynthetic photon flux density were measured on Haleakala volcano during four months. Seedlings were grown in the laboratory, from seeds collected in ten different A. sandwicense populations on Haleakala volcano, and maintained in growth chambers at 15/5, 25/15, and 30/25° C day/night temperatures. Irreversible tissue damage was determined by measuring electrolyte leakage of leaf samples. For seedlings maintained at each of the three different day/night temperatures, tissue damage occurred at -10° C due to freezing and at about 50° C due to high temperatures. Tissue damage occurred immediately after ice nucleation suggesting that A. sandwicense seedlings tend to avoid ice formation by permanent supercooling. Seedlings maintained at different day/night temperatures had similar maximum photosynthetic rates (5 μmol m-2 s-1) and similar optimum temperatures for photosynthesis (about 16° C). Leaf dark respiration rates compared at identical temperatures, however, were substantially higher for seedlings maintained at low temperatures, but almost perfect homeostasis is observed when compared at their respective growing conditions. The lack of acclimation in terms of frost resistance and tolerance to high temperatures, as well as in terms of the optimum temperature for photosynthesis, may contribute to the restricted altitudinal range of A. sandwicense. The small seasonal temperature variations in the tropical environment where this species grows may have prevented the development of mechanisms for acclimation to longterm temperature changes.

Numerical Simulations of the Formation and Evolution of Water Ice Clouds in the Martian Atmosphere

A one-dimensional aerosol model was used to study the microphysics of water ice cloud formation and evolution on Mars. The time-dependent nature of the model permits realistic simulations of diurnal cycles. The results are sensitive to the choice of the temperature profile, the eddy diffusion coefficient, the efficiency of nucleation of the condensation nuclei, and the condensation history of the cloud. Simulations having a large diurnal temperature variation yield condensed water total optical depths of a few tenths, in good agreement with the Viking Lander opacity data. Ice clouds are typically simulated at 25-30 km altitude with mean particle radii of 0.2-5.0 μm. We also obtain good agreement with the particle extinction observed in the Viking Orbiter limb data for a temperature profile constrained by the Infrared Thermal Mapper data. In order for repeated cloud formation to occur each day, we need to include a vertical and/or horizontal resupply of water and dust to the higher altitudes, where the condensation occurs, because of loss by sedimentation. This can take the form of a micrometeoritic influx of particles at the top of the atmosphere or a latitudinal transport of material. The efficiency of the nucleation process is determined by the contact angle. Changing this angle from 0.945 to 0.999 can have the effect of doubling the total ice optical depth of the cloud. The time evolution of the cloud is important for comparison with the observations if a diurnal temperature change exists. Under these conditions, the optical thickness of the cloud is dependent on the number of condensation cycles having previously occurred and the time of day. This time dependence and the need for a resupply of material into the condensing region show that the cloud formation process cannot be considered a steady-state phenomenon. We have shown that the eddy diffusion coefficient and particle radii cannot be constrained by the available data, even if a detailed model is used for the interpretation of the observations. In our simulations there is an extensive vertical redistribution of water by clouds. However, water is only lost to the surface when the temperature is at the frost point near the ground. Precipitation across subsaturated layers is not significant.