Clear Summer Days Research Articles

The influence of meteorological conditions and topographic parameters on the beech forest microclimate of Simbruini Mountains, central Italy

The relation between vegetation surface temperature and remotely sensed spectral vegetation indices has been examined by a number of authors. The observed linear decrease in surface temperature with the increase in vegetation cover density has generally been explained in terms of the increase in latent heat flux associated with greater amounts of transpirationally active vegetation. However, these investigations have initially concentrated in spatially uniform crop or pasture targets on level terrain, excluding more complex forested environments with variable Sun-sensor-surface geometry. In irregular terrains, the vegetation surface temperature may be strongly influenced by topographic parameters, such as altitude and insulation angle, so that the actual forest microclimate is often difficult to evaluate. Moreover, in the thermal regime, the emission of radiative flux within the canopy element is very tightly coupled to the environment through driving mechanisms such as meteorological conditions. In fact, the allocation of absorbed solar radiation into sensible heat flux and latent heat flux is dominated by the availability of water at the Earth's surface and thus by precipitations and air temperature conditions. In this paper, which uses remotely sensed inputs of surface temperature and vegetation fractional cover, the effects of topographic parameters and vegetation cover density on surface temperature of vegetation are investigated based on Landsat 5 satellite images obtained in the daytime of two clear summer days with different antecedent meteorological conditions. For both scenes analysed, results indicate that altitude as well as the orientation of the surface relative to the Sun were the most important factors controlling surface temperatures of beech forests of Simbruini Mountains, in central Italy.

Solar Radiation Is Shown to Select for Pigmented Bacteria in the Ambient Outdoor Atmosphere

Abstract— The percentage of pigmented to total bacteria in the outdoor atmospheric population was studied in the field and in controlled laboratory experiments to evaluate the effects of solar radiation (SR) on bacterial survival. The field experiments showed that the percentage of pigmented bacteria positively correlated with SR activity during clear summer days. The percentage was lowest during darkness before dawn and around midnight (ca 33%) and as the SR increased during the day, gradually increased to a maximum of ca 50–60% at noontime to early afternoon and decreased thereafter. In the laboratory the ambient outdoor atmospheric bacteria impacted on culture plates were exposed to simulated SR and a germicidal light. With increased exposure, more nonpig‐mented bacteria were killed and the percentage of pigmented bacteria gradually increased. These observations suggest an inverse relationship between the atmospheric bacterial survival and the percentage of pigmented bacteria contained therein, thus supporting the notion that pigmentation might provide protection for outdoor atmospheric bacteria from sunlight damage. As a consequence, viable pigmented bacteria (and other UV‐resistant forms) in the atmosphere could be enriched under areas of stratospheric ozone depletion.

Seasonal shading of vertical south-facades with prismatic panes

Using prismatic panes, a seasonal shading responsive to the Sun's position can be realized. Thus the solar gains from direct radiation can be closely matched to the annual pattern of the heating energy requirements of buildings. The panes are suitable for window or facade elements, preferably in applications which do not need a clear view, and especially in front of transparently insulated solar walls. The direct radiation received on a vertical south-facade is reduced to 10% on clear summer days, while 90% is transmitted on clear winter days.

Response in chlorophyll a fluorescence of six New Zealand tree species to a step-wise increase in ultraviolet-B irradiance

Abstract A laboratory protocol to quantify damage from a sudden increase in ultraviolet-B irradiance (UV-B, waveband 280–320 nm) to cucumber (cv. Poinsett) seedlings, including saturating photosynthetically active irradiance (400–700 nm, 1200 μmol/m2/s), was determined using leaf chlorophyll a fluorescence measurements to estimate photoinhibition. Seedlings of six native New Zealand tree species, grown in a common garden, were irradiated for 4 h, receiving a total biologically-effective UV-B dose of 17 kJ/m2 or about twice that on a clear summer day in New Zealand. There was a wide range of responses to an increase in UV-B irradiance including a 20% increase in photoinhibition for two shade tolerant species and 10% increase for the shade intolerant red beech (Nothofagus fusca). The other species were not affected. Mountain beech (N. solandri var. cliffortioides) was particularly tolerant of high UV-B doses. Leaf and epidermal thickness, and flavonoid concentration did not correspond well with species resp...

Root-zone soil temperature variation associated with microsite characteristics in high-elevation forest openings in the interior of British Columbia

Low soil temperatures have been suggested as a factor contributing to poor rooting of planted conifer seedlings and hence a cause for failure of some high-elevation reforestation efforts. Few studies have attempted to document soil temperature variation and its causal factors in post-logged, high-elevation openings during the growing season. A field survey of afternoon soil temperature (at a depth of −10 cm from the mineral soil surface) was conducted in six high-elevation clear-cuts located in the Engelmann Spruce-Subalpine Fir (ESSF) biogeoclimatic zone in central British Columbia, Canada. On a clear summer day, a difference of 8°C in afternoon soil temperatures was measured between the warmest microsites (bare mineral soil, 17.6 ± 2.5°C) and the coolest microsites (covered by forest floor and vegetation, 9.3 ± 0.7°C). Heavily covered microsites showed little diurnal or spot-to-spot (microsite) variation in soil temperature. Conversely, microsites with little surface cover exhibited the greatest spatial and temporal variability in soil temperature. Variation in afternoon root-zone soil temperature ( T) was most strongly associated with the combination of thickness of duff (forest floor) ( D), cumulative % vegetation cover ( C), and % soil moisture ( M). Comprised of these three microsite attributes, a multiple regression model, T = 24.06 − [1.05(ln( C) + 1.64(ln( D)) + 1.38(ln( M))] ( R 2 + 0.73) was created which predicts high-elevation afternoon soil temperatures to within ±2.0°C. Multiple regression models including relative surface irradiance (as a percentage of above canopy light), cumulative % vegetation height, micro-slope and micro-aspect were less predictive. The relationship between afternoon soil temperature and each microsite attribute generally showed a strongly negative curvilinear pattern, described by double exponential decay functions. Microsites associated with suboptimally low afternoon soil temperatures (< 12°C) during the growing season were widespread (70 to 80% of the sampled areas) in the surveyed high-elevation openings.

Modeling Photosynthetically Active Radiation in Water of Tampa Bay, Florida, with Emphasis on the Geometry of Incident Irradiance

Field studies that compare the spatial and temporal variation in light attenuation often neglect effects of solar elevation angle, yet these effects can be significant. To approximately correct for these angular effects, we developed a model that uses a simplified geometric description of incident direct solar beam and diffuse skylight. The model incorporates effects of solar elevation angle and cloudiness on the amount of in-air photosynthetically active radiation (PAR) that passes through the air-water interface and onK0in waters of relatively low turbidity. The model was calibrated with 3266 5-min averages of scalar PAR measured in air and at two depths in water and permits the value ofK0to be adjusted approximately for the effects of time of day, season and cloudiness. The model was then used with 255 days of in-air PAR data (15-min averages) to evaluate irradiance that entered the water and attenuation in the water. On an annual basis, 49% of the incident scalar irradiance, or 380μmol m−2s−1, was estimated to enter the water of Tampa Bay. The value ofK0was estimated to vary as much as 41% on a clear summer day due to changes in solar elevation angle. The model was used to make estimates of the depth to which sea-grasses might receive adequate light for survival for a range of values ofK0. This approach should be useful for projecting the effect of changes in water clarity on the depth of sea-grass survival and for comparing values ofK0collected at different times of day and in different seasons.

Photochemical formations of lower aldehydes and lower fatty acids under long-range transport in central Japan

Long-range transport (LRT) of photochemical air pollution from the coastal area with large emission sources to the inland mountainous region occurs very frequently on clear summer days in central Japan. Photochemical formations of lower aldehydes (C 1–C 3) and lower fatty acids (C 2–C 6) were investigated in the cooperative field observation of meteorology and chemistry in summer, 1983, 1984 and 1986. Measurements were made at 3-h intervals at inland sites along the LRT route. Aldehydes and fatty acids were produced by the photochemical oxidation of anthropogenic hydrocarbons during the LRT. The concentrations of aldehydes and fatty acids increased in the daytime and decreased at night. The ratios of aldehydes and fatty acids to CO and to acetylene were similar to that of O 3. The increase of the aldehyde concentrations during the LRT compared well with those calculated from a gas-phase chemistry model (Chang, 1987, Pr.D. Thesis; Atkinson, 1990, Atmospheric Environment 24A, 1–41). However, model-predicted concentrations of acetic acid were smaller than the observed result. The model prediction was improved with the addition of the production mechanism of acetic acid suggested by Moortgat et al. (1989, Chem. Phys. Lett. 160, 443–447) and Madronich et al. (1990, Geophys. Res. Lett. 17, 2361–2364).

Solar cooling in Madrid: Available solar energy

This paper analyzes the behaviour of an absorption chiller lithium bromide installation fed by a field of flat-plate solar collectors and condensed by swimming pool water. A method of calculation in a variable regime is developed in terms of the obtained experimental results. Starting from the meteorological variables of a clear summer day and from the project data (collector normalization curve, collector and installation mass), the minimum solar radiation level necessary to initiate the process, I[sub min], and the instantaneous available solar energy, Q[sub u] + W[sub 1] is determined. The solar radiation threshold, I[sub min], necessary to obtain the process temperature, t[sub ave], in each instant, is obtained by adding to the corrected Klein radiation threshold, I[sub k,c], the heat capacity effects of the collector, HCE[sub CO], and of the installation, HCE[sub ins], as well as the losses of heat of the pipes to the surroundings, Q[sub 1]. The instantaneous available solar energy, available useful heat, in addition to the wind collector losses to the surroundings, Q[sub u] + W[sub 1], is the difference, in each instant, between the radiation, I[sub g1T], and the radiation threshold, I[sub min].The integration during the day of the instantaneous available solar energy allowsmore » us to calculate the daily available function, H[sub T]. The value of H[sub T], measured in the swimming-pool water condensation installation reached 6.92 MJ/(m[sup 2] day ). The calculated values of H[sub T] for a conventional installation condensed by tower water, or air, have been 6.35 and 0.56 MJ/(m[sup 2] day). respectively.« less

Solar distillation of water using a V-trough solar concentrator with a wick-type solar still

A V-trough solar concentrator has been combined with an inclined flat-plate wick-type solar still. Outdoor testing was carried out with and without the solar concentrator on clear days in summer and winter. The equipment was used to investigate the enhancement of the outdoor performance of the wick-type solar still by the solar concentrator. It has been concluded that use of the solar concentrator with the inclined wick-type solar still can lead to a greater fractional increase in still efficiency and productivity on clear days in winter than on clear days in summer.

Carbon Dioxide Exchange between an Undisturbed Old‐Growth Temperate Forest and the Atmosphere

We used the eddy—correlation technique to investigate the exchange of CO2 between an undisturbed old—growth forest and the atmosphere at a remote Southern Hemisphere site on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO2 exchange, and to test whether present knowledge of leaf—level processes was sufficient to understand ecosystem—level exchange. On clear summer days the maximum rate of net ecosystem CO2 uptake exceeded 15 μmol°m—2°s—1, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between °—2 and —7 μmol°m—2°s—1. Nighttime CO2 efflux rate roughly doubled with a 10°C increase in temperature. Daytime variation in net ecosystem CO2 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature, saturation deficit, and the diffuse PPFD were of lesser, but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However, at night, the short—term exchange of CO2 between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between nocturnal CO2 exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO2 exchange. The daytime light—use efficiency of the ecosystem (CO2 uptake/incident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/mol after frosts on clear winter days. Ecosystem CO2 uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.

The behaviour of aerosol optical depth during clear summer days in Athens, Greece

The integral aerosol optical depths (kα) at the hour of 08:20 Local Standard Time (LST), are compared with those calculated previously at 11:20 and 14:20 LST, for clear days during summer in Athens over the period 1962–1988. The mean values at 08:20 LST were consistently lower than the values at 11:20 and 14:20 LST. The influence of the vertical wind profile on the values ofkα was also investigated. A comparison was made of the wind profiles at 02:00 and 14:00 LST, for days in which the 11:20 and 14:20 LST values ofkα were ⩽ 0.200 andkα ⩾ 0.350, respectively. The corresponding bulk wind shear 〈s〉 was also found for the period 1980–1988. The most significant results occurred with the first category of days. The resultant wind velocities from the surface to the 900 hPa level, in each hour were higher by 2–4 m·s−1 with respect to the corresponding values for the second category. At 02:00 LST the bulk wind shear showed a considerable difference (1.8) between the two categories of days in the surface to 700 hPa layer at 02:00 LST. Finally, the associated weather conditions that appear to initiate a period of low values ofkα (kα ⩽ 0.200) at 11:20 and 14:20 LST were examined for the period 1980–1988. Fifteen such cases were identified and it was found that they all occurred after the passage of weak cold fronts.

Intraspecific variation in photosynthetic traits of Populus trichocarpa

Three experiments were conducted to evaluate net photosynthesis in black cottonwood (Populus trichocarpa Torrey &amp; Gray) from mesic and xeric regions of Washington. In 1986, six clones each from the lower Nisqually (mesic) and Yakima (xeric) river valleys and growing in a common garden were measured for their photosynthetic rates. On 2 clear days in summer, Yakima clones had significantly (p ≤ 0.10) higher rates (means: 32 and 25μmol CO2 m−2 s−1) than Nisqually clones (means: 25 and 22μmol CO2 m−2 s−1). The next year, cuttings from these clones were potted, grown separately for 2 months in a maritime (Puyallup, Wash.) and a continental climate (Wenatchee, Wash.), and then transferred to growth chambers in Seattle, Wash. Photosynthetic rates were determined at four light levels (200, 500, 800, and 1500 μmol m−2 s−1) and two temperatures (24 and 32 °C). Yakima plants had significantly (p ≤ 0.05) higher rates at high light (20μmol CO2 m−2 s−1) and also at moderate light in high temperature than Nisqually plants (16 μmol CO2 m−2 s−1 in high light). Significant acclimation (p ≤ 0.01) was also revealed: at the two moderate light levels in high temperatures the Wenatchee-grown plants, regardless of source, had higher rates than those grown at Puyallup. In 1991, eight clones from the lower elevation, xeric region of the Yakima and eight from the upper elevation, mesic region were selected in a Puyallup stoolbed, and their photosynthesis and water-use efficiency were measured on a clear summer day. Lower elevation plants had a higher photosynthetic rate than those from the upper elevations along the Yakima; values were similar to those of 1986. The pattern for water-use efficiency was reversed; values were 56 μmol CO2/mol H2O for the lower and 84 μmol CO2/mol H2O for the upper Yakima plants. These three experiments provide evidence for significant genetic variation in photosynthetic processes both between and within river valleys. Key words: Populus trichocarpa, black cottonwood, poplar, photosynthesis, water-use efficiency, adaptation.

Diurnal variation of OH radical and hydrocarbons in a polluted air mass during long-range transport in central Japan

Abstract Long-range transport (LRT) of photochemical air pollution from the coastal area, with large emission sources to the inland mountainous region, occurs frequently in central Japan on clear summer days. Hydrocarbons and other pollutants were measured together with meteorological parameters along this route, and the diurnal variations of OH radical and hydrocarbons in the polluted air mass were investigated in relation with these photochemical reactions. At inland sampling sites, hydrocarbon concentrations in a polluted air mass decreased during LRT in the daytime. In particular, olefins were destroyed significantly because of more active photochemical reactions. From these destruction rates and/or O 3 formation rates the OH radical concentration was estimated and found to increase from 0.5 × 10 −7 in the morning to a maximum of 8.0 × 10 −7 ppm at midday. These values, as well as the diurnal variation, compared well with the numerical result of the comprehensive transport/reaction/removal model (Chang et al. , 1989, Atmospheric Environment 23 , 363–393, 1749–1773). The contribution of each hydrocarbon component to the total hydrocarbon destruction was also discussed.

ATMOSPHERIC SUN PROTECTION FACTOR ON CLEAR DAYS: ITS OBSERVED DEPENDENCE ON SOLAR ZENITH ANGLE AND ITS RELEVANCE TO THE SHADOW RULE FOR SUN PROTECTION

Global irradiances measured in seven 5-nm bands of UV-B at Rockville, MD (39.1 degrees N, 77.1 degrees W) on 28 clear days near the summer solstice are convoluted with the erythemal action spectrum of human skin to determine dose rates at various hours of the day. These rates are averaged with respect to solar zenith angle to obtain the diurnal variation of mean dose rate and of the Sun Protection Factor (SPF) of the atmosphere (reciprocal of the normalized atmospheric transmissivity) on a typical clear summer day in Rockville. At a 45 degrees zenith angle the atmospheric SPF is computed to be 2.7 and increases rapidly to greater than 7 at 60 degrees. Dose rates are integrated with respect to time to obtain estimates of mean doses for various periods during clear days at Rockville in mid summer and near the autumnal equinox. In mid summer the effective erythemal UV-B exposure during the period when the solar zenith angle is less than 45 degrees is about five times greater than that during the remainder of the day. These observations provide scientific basis for a shadow rule for solar UV-B protection: when shadows are shorter than objects casting them, sunburn is much more likely than at other times.

On the attenuation of direct solar irradiance in Athens due to aerosols

In this study the trend of the time sequence of the “integral aerosol optical depth” (ka), as proposed by Unsworth and Monteith, was determined for clear days in summer for the period 1962–1988 in Athens. The trend was found by fitting a third degree polynomial curve and it was concluded that (ka) showed a considerable increase (i.e. from the value of 0.18 to 0.31) in the period 1962–1976 and remained approximately constant until 1979, after which it started decreasing again slowly until 1988. The increase of (ka) in the period 1962–1976 is likely attributable to the rapid development of the city in this period, while the decrease of (ka) after 1979 probably reflects the efficiency of some restrictions which were imposed on the pollutant emissions during this period. In addition, an analysis of the percentage frequency distribution found that while 95% of the values of (ka) ranged from 0.100 to 0.400 in the beginning of the period (1964–1967), in recent years (1984–1987) the same percentage of the values of (ka) ranged from 0.100 to 0.500.

Long-term effect of irradiance on growth, water relations and epidermal conductance of two cyclamen cultivais

Decrease in leaf irradiance to 50 % due to shading of plants in glasshouse only during clear summer days did not induce significant changes in growth parameters, characteristics of water relations and epidermal conductance of two cyclamen cultivars. Thus the possibility of acclimation of plants to non-stable changes in environmental conditions was not proved.

Transpiration, leaf temperature and stomatal resistance of a greenhouse cucumber crop

Transpiration, leaf temperature and stomatal resistance for a greenhouse cucumber crop were determined by simultaneously measuring solar radiation, air flow, transpiration rate, dew point, and air and leaf temperature profiles. Transpiration rate varied with solar radiation and was not uniformly distributed within the canopy. Leaf temperature was lower than air temperature on clear summer days owing to high transpiration rates. Stomatal resistance was computed and was exponentially related to solar energy. No significant correlations between stomatal resistance and other climatic variables were found.

Acid deposition in central Japan

Acid deposition at the inland mountainous regions of central Japan is investigated using the STEM-II model. Previous studies of the long range transport of pollutants in central Japan have focused on the production and transport of photochemical oxidants. Highly polluted air is found in the central mountainous areas late in the afternoon on clear summer days, resulting from the transport of pollutants from the Tokyo Metropolitan area. The impact of this LRT on acid deposition at these inland regions is evaluated by simulating a series of hypothetical storms under LRT conditions. Nitrate and SO 4 2− concentrations in the rain water in excess of 100 micromolar are predicted and the total wet deposition of NO 3 − exceeds that of SO 4 2− by a factor of 2. The wet deposition during evening storms is nearly 100% higher than that during afternoon storms. Simulations with 50% reductions in NO x and SO x emissions decrease the wet deposition of NO 3 − and SO 4 2− by ∼40 and ∼15%, respectively.

Photochemical formation of particulate dicarboxylic acids under long-range transport in central Japan

Long-range transport (LRT) of photochemical air pollution from the coastal area with large emission sources to the inland mountainous region occurs very frequently in the central Japan region on clear summer days. It is caused by local winds and the transport route is almost-fixed geographically. Along this route, behavior of dicarboxylic acids in the airborne aerosols was investigated in the cooperative field observation of meteorology and chemistry. Measurements were made every 3-h at inland sites. The concentrations of dicarboxylic acids as well as NO 3 − and total organic C increased in the daytime and decreased at night. The maximum are attained when the transported air mass was arrived at the sampling sites. These diurnal variations were similar to that of O 3. In the daytime, although the phthalates (di- n-butyl and dioctyl), n-alkanes (C 21C 32) and pinon aldehyde were abundant in the airborne aerosols, the dicarboxylic acids (C 2C 10) were the most abundant species and the total concentrations attained from 30 to 50% of the total organic particulate matter. In addition, of the dicarboxylic acids, more than 70% were estimated to be produced by photochemical reactions in the daytime. Thus, it was concluded that most of the dicarboxylic acids were produced by the photochemical oxidation of anthropogenic compounds during LRT.

Influence of canopy opening on the environment and herb layer in a northern hardwoods forest

Single- (33–37 m2) and multi-tree (51–151 m2) gaps were created in an Allegheny Plateau northern hard-woods forest to investigate environmental and herb layer response to canopy opening. After gap creation, noon light on clear summer days was brightest north of opening center. At other times of the day, and when skies were overcast, there was no difference in the light quantity beneath opened and closed canopy. Nor was the distribution of soil moisture or of soil or air temperature greatly affected by gap creation. Species establishment tended to be higher near opening centers; otherwise, there was no pronounced effect of canopy opening on plant cover or species richness during the first four years after gap creation. Biotic responses were not significantly correlated with any environmental factor.