Dew Measurements Research Articles

The different effects of regional and local winds on dew formation in the Negev desert

Abstract With dew serving as an important water source for various small organisms and plants in deserts, knowledge regarding the spatial distribution of dew (which constitutes an important fraction of the non-rainfall water, NRW) is of prime importance. This is also the case for the Negev dew desert. According to the classical model, local nocturnal katabatic winds that descend down the slopes during the night to the wadi beds are responsible for the accumulation of cold air, subsequently triggering dew formation in the wadis. Nevertheless, NRW measurements that were conducted in a one-order drainage basin in the Negev during the dewy season (late summer and fall) yielded half the amount in the wadi bed in comparison to the hilltop, attributed to the sheltered position of the wadi from the cooling effect of the regional (sea-breeze) winds, which are not considered by the classical model. Hypothesizing that the classical model may however take place at wide wadi where the sea breeze winds are not sheltered, measurements of dew and temperatures were periodically carried out at the beds of a 5 m-wide narrow (NW) and a 200 m-wide (WW) wadi beds and at the hilltop (HT). The findings did not fully support our hypothesis. In comparison to the hilltop, and despite the mutual effect of the katabatic and the sea breeze winds on the wide wadi bed, also the wide wadi exhibited lower amounts of dew than that of the hilltop, with NRW following the pattern HT > WW > NW. The overwhelming effect of the sea-breeze winds was also supported indirectly by periodic NRW and temperature measurements during the winter during which the sea breeze does not commonly take place. Evidence suggests that whereas the classical model takes place during the winter during which the katabatic winds may play a central role in dew formation, the occurrence of the sea breeze (regional wind) during the late summer and fall overshadows the effect of the local katabatic winds. Our findings point to the possibility that the classical model may not adequately predict dew formation in regions subjected to sea-breeze winds.

Wet–dry cycles on sandy and loessial Negev soils: Implications for biocrust establishment and growth?

AbstractLong‐term field campaigns in the Negev observed wet–dry cycles (WDCs) following rain events, principally on sand. Aiming to assess the relevant mechanism (whether dewfall, water capillarity or distillation derived by temperature‐induced vapor flux [TIVF] which stems from the soil), a set of outdoor experiments was conducted on sand and loess. They included nonrainfall water (NRW) and temperature measurements during the rainless dry seasons (summer, fall) and during the winter, which was divided to a wet (following rain) and dry periods. Dew measurements were conducted on cloths, whether directly attached to the soil or located at 0.5 cm above ground, aiming to eliminate capillarity and assessing the possible contribution of dewfall and TIVF‐induced distillation. Our findings show that (a) WDCs were only observed following rainstorms, particularly on sand, (b) WDCs could not have been explained by capillarity (they continued at the presence of an air gap), (c) significantly lower minimum surface temperatures (averaging 0.8°C) characterized the sand, which following rainstorms resulted in higher NRW due to TIVF, that is, visibly noted WDCs. With sand being characterized by a lush cover of biocrusts in the Negev dunefields, while being largely absent from the loessial playas scattered within the dunefield, it is suggested that WDCs may play an important role in the establishment and growth of biocrusts over sand dunes of the Negev and possibly in other arid dunes.

Atmospheric humidity is unlikely to serve as an important water source for crustose soil lichens in the Tabernas Desert

Abstract Dew is commonly regarded as an important water source for lichens. This is also the case for crustose lichens that are attached to the substrate, whether rocks or soil. While being verified during ample research on rock-dwelling lichens in the Negev, the findings from soil-dwelling lichens (lichen biocrusts) are not conclusive. In the Tabernas Desert, the soil surface is characterized by a lush cover of crustose lichens. These soil biocrusts (biological soil crusts) were reported to use dew for photosynthesis while, at the same time, it was also observed that these crustose chlorolichens are relatively non-wettable. In an attempt to explore the apparent controversy, two year-long meteorological data (minimum air temperature and relative humidity, RH), during which chlorolichens were thought to utilize dew for photosynthesis (2006–2007) were analyzed. The analysis includes a comparison to the meteorological conditions that prevailed in the Negev during 135 days of manual dew measurements. As found for the Negev, net photosynthesis by the chlorolichens is expected once the RH, as measured at the meteorological station, is ≥90% while vapor condensation (dew) is expected once RH is ≥95%. RH in the Negev was substantially higher than the average RH of 75.0–87.2% registered during the rainless days of 2006–2007 in the Tabernas, implying that RH in the Tabernas is too low to facilitate frequent dew formation and net photosynthesis by the lichens. Photosynthesis in the Tabernas is mainly confined to rainy periods, taking place either due to direct wetting by rain, or following vapor condensation from the subsurface (distillation). Our findings do not support the view that dew is an important water source for the establishment and growth of crustose soil lichens in the Tabernas. Moreover, the low RH in the Tabernas may also imply that dew may only have a very limited role in providing water to lithobionts in this ecosystem.

LANFEX: A Field and Modeling Study to Improve Our Understanding and Forecasting of Radiation Fog

AbstractFog is a high-impact weather phenomenon affecting human activity, including aviation, transport, and health. Its prediction is a longstanding issue for weather forecast models. The success of a forecast depends on complex interactions among various meteorological and topographical parameters; even very small changes in some of these can determine the difference between thick fog and good visibility. This makes prediction of fog one of the most challenging goals for numerical weather prediction. The Local and Nonlocal Fog Experiment (LANFEX) is an attempt to improve our understanding of radiation fog formation through a combined field and numerical study. The 18-month field trial was deployed in the United Kingdom with an extensive range of equipment, including some novel measurements (e.g., dew measurement and thermal imaging). In a hilly area we instrumented flux towers in four adjacent valleys to observe the evolution of similar, but crucially different, meteorological conditions at the different sites. We correlated these with the formation and evolution of fog. The results indicate new quantitative insight into the subtle turbulent conditions required for the formation of radiation fog within a stable boundary layer. Modeling studies have also been conducted, concentrating on high-resolution forecast models and research models from 1.5-km to 100-m resolution. Early results show that models with a resolution of around 100 m are capable of reproducing the local-scale variability that can lead to the onset and development of radiation fog, and also have identified deficiencies in aerosol activation, turbulence, and cloud micro- and macrophysics, in model parameterizations.

Roof-integrated dew water harvesting in Combarbalá, Chile

Dew harvesting can be a supplementary source of freshwater in semiarid and arid areas. Several experiments on small-scale dew condensers (usually of 1 m 2 ) have been carried out in many places in the world; however, few experiments have been conducted on large-scale collectors integrated into buildings. This work aims to assess one year of dew water harvesting in Combarbala (Chile) using a painted galvanised steel roof as collecting surface. The roof (36 m 2 ) was coated with a high-infrared-emissivity paint containing aluminosilicate minerals (OPUR, France). Dew measurements were conducted daily from September 2014 to August 2015. The dew yield and its relationship with meteorological variables were analysed. The results show that despite the low nocturnal relative humidity throughout the year (average: 48%), dew collection occurred on 56.1% of the recorded days. The daily average collection rate was 1.9 L d −1 , with a maximum of 15 L d −1 . The maximum daily dew yield is correlated strongly with relative humidity and correlated weakly with air temperature and wind speed. Considering the same rooftop can collect dew and rain, it was estimated that over one year dew water could contribute to roughly 8.2% of the total water collected, considering both sources.

Dew measurement and estimation of rain-fed jujube (Zizyphus jujube Mill) in a semi-arid loess hilly region of China

Dew is an important water source for plants in arid and semi-arid regions. However, information on dew is scarce in such regions. In this study, we explored dew formation, amount, and duration of rain-fed jujube (Zizyphus jujube Mill) trees in a semi-arid loess hilly region of China (i.e., Mizhi County). The data included dew intensity and duration, relative humidity, temperature, and wind speed measured from 26 July to 23 October, 2012 and from 24 June to 17 October, 2013 using a micro-climate system (including dielectric leaf wetness sensors, VP-3 Relative Humidity/Temperature Sensor, High Resolution Rain Gauge, and Davis Cup Anemometer). The results show that atmospheric conditions of relative humidity of >78% and dew point temperature of 1°C–3°C are significantly favorable to dew formation. Compared with the rainfall, dew was characterized by high frequency, strong stability, and long duration. Furthermore, heavy dew accounted for a large proportion of the total amount. The empirical models (i.e., relative humidity model (RH model) and dew point depression model (DPD model)) for daily dew duration estimation performed well at 15-min intervals, with low errors ranging between 1.29 and 1.60 h, respectively. But it should be noted that the models should be calibrated firstly by determining the optimal thresholds of relatively humidity for RH model and dew point depression for DPD model. For rain-fed jujube trees in the semi-arid loess hilly regions of China, the optimal threshold of relative humidity was 78%, and the optimal upper and lower thresholds of dew point depression were 1°C and 5°C, respectively. The study further demonstrates that dew is an important water resource that cannot be ignored for rain-fed jujube trees and may affect water balance at regional scales.

Wind as a cooling agent: substrate temperatures are responsible for variable lithobiont‐induced weathering patterns on west‐ and east‐facing limestone bedrock of the Negev

AbstractSpatial variability in lithobiont‐induced weathering patterns on desert rocks is aspect‐dependent. While differences between the northern and southern aspects have been extensively studied, little is known concerning the differences between east‐facing (EF) and west‐facing (WF) aspects in deserts, including the Negev Desert. Whereas cobbles on both slopes are inhabited by endolithic lichens, epilithic lichens, which render the bedrock a smooth appearance, and free‐living cyanobacteria, which give the bedrock a rugged microrelief, predominate on WF and EF bedrock, respectively. Following previous research that regarded dew as the principal factor that determines lithobiont distribution, measurements of radiation, temperature, wind and dew were carried out during 2008–2009 in the Negev Desert. The data indicated that albeit slightly higher midday surface temperatures that characterize WF surfaces (cobbles and bedrock), nocturnal temperatures on these surfaces were significantly lower, therefore facilitating higher dew condensation. High amounts of dew result from the relatively rapid drop in temperatures (14:00–20:00) due to the afternoon northwesterly sea‐breeze wind (with a cooling rate of the WF bedrock being 52.9% higher than on EF bedrock, 2.6 °C h−1 in comparison to only 1.7 °C h−1), and facilitate the growth of high‐chlorophyll dew‐fed (and rain‐fed) epilithic lichens, which may act as bio‐protectors on WF bedrock. Lack of condensation on EF bedrock results in turn in the growth of rain‐fed free‐living cyanobacteria, responsible for high rock dissolution and subsequently for a rugged microrelief. By affecting the nocturnal bedrock temperatures, wind acts as a cooling agent, impacting in turn the amount of dew, and subsequently lithobiont composition and weathering patterns in the Negev Desert. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Estimating dew yield worldwide from a few meteo data

There is a general need to determine dew yield in any place of the world. However, its determination requires systematic measurements that are not available everywhere. In addition, it depends on the particularities of the dew collector. A simple analytical formula valid for planar dew collectors is elaborated here. It is based on laboratory experiments where it is established that heat loss with surrounding air is the dominant parameter that limits dew condensation. A simple analytical formulation is then derived, which only needs cloud coverage, wind velocity, air and dew point temperature data to be collected, at least once in a day before sunrise. The formulation is tested in several places in the world with different climates. Agreement within typically 30% is found with dew measurements. Such an analytical relationship can thus provide a useful tool to obtain a worldwide estimation of the dew potential.

Microlysimeter station for long term non-rainfall water input and evaporation studies

Non-rainfall atmospheric water input (NRWI), which is comprised of fog, dew and soil water vapour adsorption (WVA), has been proven to be an important water source in arid and semiarid environments. Its minor contribution to the water balance and the difficulty in measuring it have resulted in a wide variety of measurement methods (duration and quantification), especially for dew. Microlysimeters seem to be the most realistic method for dew measurement on natural surfaces and they can also detect WVA. This paper presents an automated microlysimeter that enables accurate studies of NRWI and evaporation on soil and small plants. Furthermore, we have developed a field strategy for their long term placement and installation which prevents damage from rainfall, soil movement or other field conditions, keeping the microlysimeters balanced and dry. This design allows the measurement of evaporation and NRWI on different cover types, including small plants. By monitoring the surface temperatures, dewfall and water vapour adsorption can be distinguished and the relative contribution of dew and WVA on the NRWI can also be found. Our automated microlysimeter design, construction and field installation have proven to be an useful and effective tool in an NRWI study.

J123 高圧水素中の微量水分の露点測定および露点推算(一般講演(2))

J123 高圧水素中の微量水分の露点測定および露点推算(一般講演(2))

The effect of substrate properties, size, position, sheltering and shading on dew: An experimental approach in the Negev Desert

Affected by the interrelationships between the air mass and the substrate properties, dew measurements entail numerous difficulties. Inattentiveness to the complexities involved may result in inconsistent and consequently unreliable data and thus in misleading conclusions. This is especially important in dew deserts such as the Negev where dew may be regarded as an important water source. In an attempt to highlight some of the factors that may affect dew condensation, an experimental approach was adapted during which dew condensation at different surfaces (smooth and ragged Plexiglas, bright limestone and dark flint cobbles), at various size substrata with identical axes (of 2.5, 5, 10, 20, 30 and 40cm) or different size axes (of 10×20, 10×30 and 10×40cm), and substrata at different heights and settings was measured. Some of the settings explored were also sheltered by a shrub, a cave, an 80-cm-tall cliff and neighboring cobbles.No significant differences were found in the dew amounts condensing on the limestone and the flint cobbles. Smooth Plexiglas surfaces yielded significantly higher amounts than a ragged Plexiglas surface. As for the substratum height and size, the dew amounts increased with height (from 0.7 to 10.0cm) and size (from 2.5×2.5cm to 40×40cm). However, an increase in the long axis (a-axis) to 20, 30 and 40cm while leaving the b-axis constant (at 10cm) resulted in only a slight increase in the dew amounts in comparison to a 10×10cm substratum. While confined to non-sheltered habitats, dew at partially sheltered habitats (such as the margin of a shrub or a cave, the vicinity of a cliff and on 7-cm-high cobble surrounded by 15-cm-high cobbles) was significantly lower than at the exposed habitats. The differences in dew amounts coincided with minute differences in the surface temperatures, explained as resulting from differences in the outgoing nocturnal longwave radiational cooling. The findings may shed light on the dew regime of lithobionts at different microhabitats and upon inconsistent values reported in the literature.

Lichen Colonization on Cobbles in the Negev Desert Following 15 Years in the Field

Rock slabs were placed from 1991 through 2006 at the bottom of a north- (NFS) and a south-facing (SFS) slope of a first order drainage basin in the Negev Desert Highlands, Israel (P = 95 mm) where natural cobbles have >95% cover of endolithic and epilithic lichens. Periodic measurements of rain, dew and chlorophyll content took place. Lichen cover and the regeneration time were aspect-dependent. Lichen cover was 98% and 61% at NFS and SFS, respectively, while the lichen's recovery time was respectively ∼45 and ∼80 years. The fast regeneration time may have important implications regarding weathering and soil formation.

Method Exploring on Dew Condensation Monitoring in Wetland Ecosystem

Dew is a crucial factor of water cycle in wetland ecosystem. This study aims to set up a series of parameters about dew characterization, to build the system of monitoring dewfall and make a model of predicting dew in mire wetland ecosystem. The experiment was in situ carried out during the growing season from mid-May through mid-October in a mire wetland where Carex lasiocarpa is the dominant plant. Dew was measured by using four different kinds of collectors which were glass, poplar woodstick, sunflower stick and filter paper, respectively. Statistical analysis showed that, compared to other collectors, poplar woodstick was the optimal collector for dew measurement considering the ideal surface idiosyncrasy and its convenience for material and operation. Dew began to condense from one and a half hour after sunset and Dew Intensity reached the peak at about half an hour before sunrise. The factual Dewfall in unit field area was calculated by the factors of leaf area index, Dew Days, Dew Intensity, etc. Based on the synchronous meteorological factors, a stepwise linearmulti-variation regression model was established to predict Dew Intensity. Furthermore, the model successfully interpreted the relationship between simulated and measured Dew Intensity. This method which used poplar woodstick, as well as the formula for calculating Dewfall offered a reliable way to analyze dewfall datas in wetland ecosystem, and most importantly, the parameters of dew, which were defined for the first time, provide a more precise way to describe and study dew.

Effects of Dew on the Radiometric Signal of a Grass Field at L-Band

The future Soil Moisture and Ocean Salinity satellite time of overpass is 6 A.M. and 6 P.M. at the equator. In many regions, morning dew is expected at the time of the satellite overpass and might play a role in soil moisture retrievals. The aim of this study was to assess the effects of dew on L-band measurements. Radiometric, biomass, and dew measurements were performed over a natural grass field. Our results show that at the diurnal scale, vegetation internal water content changes play a major role in emission. A direct impact of dew on measurements was not identified. However, a brightness temperature increase of 0.5 and 1 K was observed at vertical and horizontal polarizations, respectively. This increase was detected later after dew, suggesting that it was not directly caused by the presence of dew on the grass blades. Our hypothesis is that the observed increase in brightness temperatures is due to the absorption of dew water by the litter layer.

Dew measurement by Eddy covariance and wetness sensor in a semiarid ecosystem of SE Spain

Summary Information regarding dew precipitation is scarce, both because it is considered a minor component of the water balance and because it is difficult to measure. Nevertheless, although yielding relatively low amounts of water, dew can be of great importance for the local water balance in semiarid and arid environments. The aim of this study was to explore the adequacy of the Eddy Covariance technique, combined with qualitative methods (such as wetness sensors), as a tool to measure actual dew in semiarid conditions. This technique was used to assess the relative contribution of dew to the local water balance in a dry river bed representative of semiarid environments (Rambla Honda, Tabernas) located in the Almeria province (SE Spain). Results show that an accurate filtering and management of data from Eddy Covariance (EC) combined with the records of the wetness sensors (WS), allowed us to generate a data base which can be used to develop and parameterize theoretical or empirical dew deposition models. We found that a simple equation for potential dew predicted adequately actual dew for the study area, and permitted us to estimate a dew contribution of 13 mm from February to June 2003, which accounts for 12% of the rainfall for the same period. Dew deposition was higher in late winter and early spring, when vegetation starts to grow actively. In consequence, this additional input of water can be of great relevance to the local water balance especially during dry years.

Application of passive radiative cooling for dew condensation

Dew water was collected from several passive foil-based radiative condensers established in a variety of geographic settings: continental (Grenoble, in an alpine valley, and Brive-la-Gaillarde, in the Central Massif volcanic area, both in France), French Atlantic coast (Bordeaux), eastern Mediterranean (Jerusalem, Israel), and the island of Corsica (Ajaccio, France) in the Mediterranean Sea. In Ajaccio two large 30 m 2 condensers have been operating since 2000. Additional semi-quantitative dew measurements were also carried out for Komiža, island of Vis (Croatia) in the Adriatic Sea, and in Mediterranean Zadar and Dubrovnik (both in Croatia). Dew potential was calculated for the Pacific Ocean island of Tahiti (French Polynesia). The data show that significant amounts of dew water can be collected. Selected chemical and biological analyses established that dew is, in general, potable. Continued research is required for new and inexpensive materials that can enhance dew condensation.

Contribution of dew to the water budget of a grassland area in the Netherlands

The annual amount of dew input to the water budget in the midlatitudes is mostly neglected, possibly because direct dew measurements are very difficult and time‐consuming. As the Netherlands has a very high frequency of dew events, a grassland area was selected to determine whether dew input could be significant. The study site is situated within the Wageningen University meteorological station. Dew measurement experiments were carried out in 2004. Data were used to verify a surface energy dew model, which was then applied to an 11‐year data set. A mean annual dew amount of 37 mm was obtained with a standard deviation of 8 mm, while the mean annual precipitation was 830 mm with a standard deviation of 200 mm. Dew contributed about 4.5% of the mean annual precipitation. The average number of dew nights per year was 250 (70%) with a standard deviation of 25 nights. This frequency significantly affects leaf wetness and possible vegetation diseases.

Leaf and stem maximum water storage capacity of herbaceous plants in a mountain meadow

Leaf and stem maximum water storage capacities of nine dominant species (three graminoids and six herbs) from a mountain meadow were investigated employing two different methods—submersing of and spraying at phytoelements, respectively. The resulting maximum water storage capacities were in overall good agreement with literature. The submersing method yielded significantly lower values as compared to the spraying method, but, using in situ dew measurements, it could not be conclusively determined which method is to be preferred. Significant correlations between maximum water storage capacity and several morphological parameters could be found both across species and within species, yet these relationships usually possessed relatively little predictive power and were not consistent across species. It is concluded that other plant characteristics than those investigated in the present study are responsible for determining differences between species and that the dependence of the maximum water storage capacities on morphological factors is highly species-specific.

Angle and aspect dependent dew and fog precipitation in the Negev desert

Knowledge regarding dew and fog precipitation in relation to angle and aspect may be of great importance to plant photosynthesis and to rock dwelling microorganisms, especially in arid land where water is highly limited. Measurements of dew and fog were carried out periodically during the fall of two years (1989, 1992) at a hilltop in the Negev Desert Highlands, Israel. Measurements were carried out using velvet-like cloths (6×6×0.15 cm) attached to the center of different angle sides of 5 pairs of wooden boxes (50×50×10 cm) and to the top of an additional pair (10×10×5 cm) that served as a control (horizontal surface). The sides of each pair of boxes had angles of 30, 45, 60, 75 and 90° facing north, east, south and west. Whereas fog showed an advective pattern of precipitation, with windward aspects receiving higher amounts, that was not the case with dew. When the dew values received in the different aspects within each angle were grouped together, surfaces at 30, 45, 60, 75 and 90° received 99.8 (SE±11.9), 78.2 (SE±17.9), 42.0 (SE±18.3), 33.8 (SE±10.8), and 25.6 (SE±9.7) percent of the amounts condensed on the horizontal surface, respectively. A positive linear relationship characterized the dew amounts with cos ( θ). The data also showed an increase in dew duration with west>north>south>east. The data thus imply that aspect and angle may play an important role in controlling dew and fog availability.

Dew water collector for potable water in Ajaccio (Corsica Island, France)

We report on the development of an inexpensive radiative condenser for collecting atmospheric vapor. Based on the experience gained using a small working model in Grenoble (France), a prototype of 10×3 m 2 was established in Ajaccio (Corsica, France). The condensing surface is a rectangular foil made of TiO 2 and BaSO 4 microspheres embedded in polyethylene and has an angle of 30° with respect to horizontal. The hollow part of the device, thermally isolated, faces the direction of the dominant nocturnal wind. Dew measurements were correlated with meteorological data and compared to dew condensed on a horizontal polymethylmethacrylate (PMMA, Plexiglas) reference plate. The plate served as a reference standard unit and was located nearby. Between July 22, 2000 and November 11, 2001 (478 days), there were 145 dew days for the reference plate (30%), but 214 dew days for the condenser (45%). This yield corresponds to 767 l (3.6 l, on average, per dew day). The maximum yield in the period was 11.4 l/day. Dew mass can be fitted to a simple model that predicts dew production from simple meteorological data (temperature, humidity, wind velocity, cloud cover). Chemical analyses of the water collected from the plate were performed from October 16, 1999 to July 16, 2000 and from the condenser, from July 17, 2000 to March 17, 2001. The following parameters were investigated: suspended solids, pH, concentration of SO 4 2−, Cl −, K +, Ca 2+ ions. Only Cl − and SO 4 2− ions were sometimes found significant. Wind direction analyses revealed that Cl − is due to the sea spray and SO 4 2− to the combustion of fuel by an electrical plant located in the Ajaccio Gulf. Except for a weak acidity (average pH≈6) and high concentration of suspended solids, dew water fits the requirements for potable water in France with reference to the above ions.