Moderate Surface Temperature Research Articles

Detection and characterization of heavy drizzle cells within subtropical marine stratocumulus using AMSR-E 89-GHz passive microwave measurements

Abstract. This empirical study demonstrates the feasibility of using 89-GHz Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) passive microwave brightness temperature data to detect heavily drizzling cells within subtropical marine stratocumulus. For the purpose of this paper, we define heavily drizzling cells as areas ≥ 6 km × 4 km with C-band Z > 0 dBZ; equivalent to > 0.084 mm h−1. A binary heavy drizzle product is described that can be used to determine areal and feature statistics of drizzle cells within the major marine stratocumulus regions. Current satellite liquid water path (LWP) and cloud radar products capable of detecting drizzle are either lacking in resolution (AMSR-E LWP), diurnal coverage (MODIS LWP), or spatial coverage (CloudSat). The AMSR-E 89-GHz data set at 6 km × 4 km spatial resolution is sufficient for resolving individual heavily drizzling cells. Radiant emission at 89 GHz by liquid-water cloud and precipitation particles from drizzling cells in marine stratocumulus regions yields local maxima in brightness temperature against an otherwise cloud-free background brightness temperature. The background brightness temperature is primarily constrained by column-integrated water vapor for moderate sea surface temperatures. Clouds containing ice are screened out. Once heavily drizzling pixels are identified, connected pixels are grouped into discrete drizzle cell features. The identified drizzle cells are used in turn to determine several spatial statistics for each satellite scene, including drizzle cell number and size distribution. The identification of heavily drizzling cells within marine stratocumulus regions with satellite data facilitates analysis of seasonal and regional drizzle cell occurrence and the interrelation between drizzle and changes in cloud fraction.

Infrared Continental Surface Emissivity Spectra and Skin Temperature Retrieved from IASI Observations over the Tropics

AbstractLand surface temperature and emissivity spectra are essential variables for improving models of the earth surface–atmosphere interaction or retrievals of atmospheric variables such as thermodynamic profiles, chemical composition, cloud and aerosol characteristics, and so on. In most cases, emissivity spectral variations are not correctly taken into account in climate models, leading to potentially significant errors in the estimation of surface energy fluxes and temperature. Satellite infrared observations offer the dual opportunity of accurately estimating these properties of land surfaces as well as allowing a global coverage in space and time. Here, high-spectral-resolution observations from the Infrared Atmospheric Sounder Interferometer (IASI) over the tropics (30°N–30°S), covering the period July 2007–March 2011, are interpreted in terms of 1° × 1° monthly mean surface skin temperature and emissivity spectra from 3.7 to 14 μm at a resolution of 0.05 μm. The standard deviation estimated for the surface temperature is about 1.3 K. For the surface emissivity, it varies from about 1%–1.5% for the 10.5–14- and 5.5–8-μm windows to about 4% around 4 μm. Results from comparisons with products such as Moderate Resolution Imaging Spectroradiometer (MODIS) low-resolution emissivity and surface temperature or ECMWF forecast data (temperature only) are presented and discussed. Comparisons with emissivity derived from the Airborne Research Interferometer Evaluation System (ARIES) radiances collected during an aircraft campaign over Oman and made at the scale of the IASI field of view offer valuable data for the validation of the IASI retrievals.

The spatial distribution of surface fCO2 in the Southwestern East Sea/Japan sea during summer 2005

Fugacity of CO2 (fCO2), temperature, salinity, nutrients, and chlorophyll-a were measured in the surface waters of southwestern East Sea/Japan Sea in July 2005. Surface waters were divided into three waters based on hydrographic characteristics: the water with moderate sea surface temperature (SST) and high sea surface salinity (SSS) located east of the front (East water); the water with high SST and moderate SSS located west of the front (West water); and the water with low SST and SSS located in the middle part of the study area (Middle water). High fCO2 larger than 420 μatm were found in the West water. In the Middle water, CO2 was undersaturated with respect to the atmosphere, with values between 246 and 380 μatm. Moderate fCO2 values ranging from 370 to 420 μatm were observed in the East water. For the East and West waters, estimates of temperature dependency of fCO2 (12.6 and 15.1 μatm °C−1, respectively) were rather similar to a theoretical value, indicating that SST is likely to be a major factor controlling the surface fCO2 distribution in these two regions. In the Middle water, however, the estimated temperature dependence was somewhat lower than the theoretical value, and relatively high concentrations of surface chlorophyll-a coincided with the low surface fCO2, implying that biological uptake may considerably affect the fCO2 distribution. The net sea-to-air CO2 flux of the study area was estimated to be 0.30±4.81 mmol m−2 day−1 in summer, 2005.

Effects of Hurricane Ike on the Louisiana–Texas coast from satellite and model data

The river-influenced Mississippi–Louisiana–Texas (MsLaTex) coast in the northern Gulf of Mexico was in 2008 impacted by Hurricane Ike that made landfall along the Texas coast on 13 September 2008, causing widespread damage due to storm surge and coastal inundation. We examine the effects of the hurricane on the MsLaTex coast using remote sensing, model simulation and field observations. Moderate resolution imaging spectroradiometer sea surface temperature imagery indicated an approximately 2–4°C decrease in temperature to the right of the hurricane track. The high-resolution (∼2 km) three-dimensional Navy Coastal Ocean Model simulation and tide data suggested the generation of a coastally trapped ‘barotropic Kelvin’ wave during the passage of Ike, which may have contributed to an earlier than expected surge in sea level. A distinct elevated band of suspended particulate matter (SPM) concentration extending from Louisiana to Texas (∼90.5–94.5°W) coast was observed offshore in sea-viewing wide field-of-view sensor-derived imagery of 17 and 25 September that appeared to be associated with the coastally trapped Kelvin wave that was enhanced west of the Mississippi Canyon. Two intermittent wind events on 15 and 21 September likely kept the SPM in suspension for an extended period. Subsequently, on 27 and 30 September, SPM decreased along the elevated band and near the landfall site but increased near the bays and passes due to receding flood waters. Off the Atchafalaya and Mississippi deltas a large pulse of river discharge containing elevated SPM levels caused a plume around the two river deltas and its offshore dispersion into the open Gulf of Mexico.

Effects of platinum stagnation surface on the lean extinction limits of premixed methane/air flames at moderate surface temperatures

A stagnation flow reactor was used to study the effects of platinum on the lean flammability limits of atmospheric pressure premixed methane/air flames at moderate stagnation surface temperatures. Experimental and computational methods were used to quantify the equivalence ratio at the lean extinction limit ( ϕ ext ) and the corresponding stagnation surface temperature ( T s ). A range of flow rates (57–90 cm/s) and corresponding strain rates were considered. The results indicate that the gas-phase methane/air flames are sufficiently strong relative to the heterogeneous chemistry for T s conditions less than 750 K that the platinum does not affect ϕ ext . The computational results are in good agreement with the experimentally observed trends and further indicate that higher reactant flow rates (>139 cm/s) and levels of dilution (>∼10% N 2) are required to weaken the gas-phase flame sufficiently for surface reaction to play a positive role on extending the lean flammability limits.

Divertor power load feedback with nitrogen seeding in ASDEX Upgrade

Feedback control of the divertor power load by means of nitrogen seeding has been developed into a routine operational tool in the all-tungsten clad ASDEX Upgrade tokamak. For heating powers above about 12 MW, its use has become inevitable to protect the divertor tungsten coating under boronized conditions. The use of nitrogen seeding is accompanied by improved energy confinement due to higher core plasma temperatures, which more than compensates the negative effect of plasma dilution by nitrogen on the neutron rate. This paper describes the technical details of the feedback controller. A simple model for its underlying physics allows the prediction of its behaviour and the optimization of the feedback gain coefficients used. Storage and release of nitrogen in tungsten surfaces were found to have substantial impact on the behaviour of the seeded plasma, resulting in increased nitrogen consumption with unloaded walls and a latency of nitrogen release over several discharges after its injection. Nitrogen is released from tungsten plasma facing components with moderate surface temperature in a sputtering-like process; therefore no uncontrolled excursions of the nitrogen wall release are observed. Overall, very stable operation of the high-Z tokamak is possible with nitrogen seeding, where core radiative losses are avoided due to its low atomic charge Z and a high ELM frequency is maintained.

10.1007/s11455-008-1018-6

Disturbance propagation processes in supersonic laminar and turbulent boundary layers and in channels have been studied. Analysis of the corresponding subcharacteristics leads to an integral relation determining the velocity of disturbance propagation. The study of flows in a turbulent boundary layer described by a simple model leads to a conclusion that the upstream propagation of disturbances is possible only provided that the surface temperature (or the thickness of the region of a subsonic near-wall flow) is sufficiently large. For moderate surface temperatures, the upstream propagation of disturbances is restricted to distances comparable with the boundary layer thickness.

Jellyfish blooms:Crambionella orsini(Scyphozoa: Rhizostomeae) in the Gulf of Oman, Iran, 2002–2003

In 2002, a jellyfish bloom occurred throughout the Gulf of Oman and the Persian Gulf causing decreased catch in artisanal and industrial fisheries, damage to fishing gear, interrupted operation of desalination plants, and damage to the seawater cooling systems of coastal power plants. The cause and source of the bloom and the species of medusa were unknown. Here, the temporal and spatial distribution of the medusa are described using data from incidental catches in Iranian fisheries surveys, and morphological and molecular evidence are used to identify the medusa asCrambionella orsini(Vanhöffen). This species is endemic to the western Indian Ocean and adjacent basins which dismisses species introduction as the cause of this bloom. Furthermore, because the bloom was geographically widespread, it probably was not caused by local coastal degradation. Although human activities with large-scale effects cannot be ruled out, large-scale oceanographic or ecosystem conditions, perhaps part of natural climate cycles, that yielded moderate sea surface temperatures (in terms of the annual mean monthly anomaly for each year since 1982) were the most likely causes of the bloom of this endemic jellyfish.

Land surface response to precipitation events using MODIS and NEXRAD data

We have developed a wavelet‐based information theoretic approach to examine the interaction between precipitation (PPT) forcing events and the land surface response. Combining Next Generation Weather Radar (NEXRAD) PPT with Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation (NDVI) and surface temperature (T s) data over the Missouri Basin in the north‐central USA, we are able to address the spatial and temporal fluctuations surrounding the hydrometeorology of grassland ecosystems. Information theory metrics of entropy and mutual information content are combined with a wavelet multi‐resolution analysis to examine to what extent the observed PPT signal directly determines the spatial distribution of the land surface temperature and vegetation and how this relationship varies with spatial scale. Results indicate that (1) there is a reduction in the temporal variance of the wavelet coefficients as the signal is transferred from the PPT into the surface temperature and finally the vegetation signal, (2) there are significant correlations as a function of spatial resolution between PPT–NDVI and PPT–T s signals which generally increase with spatial resolution, while there is little correlation between the NDVI and T s signals as a function of resolution, and (3) the scale‐wise entropy and the mutual information content of the signals increase for all fields as the spatial resolution increases. This provides a methodology for determining the relative impact of regional climatology and local land–atmosphere interactions as a function of spatial scale.

Propagation of disturbances in supersonic laminar and turbulent boundary layers

Disturbance propagation processes in supersonic laminar and turbulent boundary layers and in channels have been studied. Analysis of the corresponding subcharacteristics leads to an integral relation determining the velocity of disturbance propagation. The study of flows in a turbulent boundary layer described by a simple model leads to a conclusion that the upstream propagation of disturbances is possible only provided that the surface temperature (or the thickness of the region of a subsonic near-wall flow) is sufficiently large. For moderate surface temperatures, the upstream propagation of disturbances is restricted to distances comparable with the boundary layer thickness.

Lake surface water temperature retrieval using advanced very high resolution radiometer and Moderate Resolution Imaging Spectroradiometer data: Validation and feasibility study

In this study we prove the feasibility of the advanced very high resolution radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer sea surface temperature algorithms to derive operational lake surface water temperature (LSWT). A validation study covering 2 years was done using data from the AVHRR on NOAA 12, 15, 16, and 17, the MODIS on TERRA, and AQUA and with different method‐ingested in situ data from different sized lakes. Best results were found for NOAA 16 nighttime data at Lake Geneva (bias of 0.18 K and standard deviation of 0.73 K) and TERRA nighttime data at Lake Constance (satellite‐buoy bias of −0.08 K and standard deviation of 0.92 K). For all sensor families an overall scatter ranging from 0.9 to 1.6 K was found. Bias of MODIS is larger, −1.73 to 1.9 K, than the one of the AVHRR (−0.28 to 1.5 K). The current orbital configuration of the platforms used revealed the diurnal evolution of the lake surface temperature amplitude from space. The damped mixing found for a typical calm and clear‐sky regime is different from open ocean conditions. As the main error source, we found undetected cloudy pixel. Furthermore, the physical difference between skin and bulk temperature, especially its relation to the diurnal thermocline, solar insolation, and wind stress contributes to the bias and scatter within the match‐up data set. The data sets have been validated to allow further application for LSWT climatology and assimilation into numerical weather prediction models.

Accurate atmospheric correction of ASTER thermal infrared imagery using the WVS method

The water vapor scaling (WVS) method involves an atmospheric correction algorithm for thermal infrared (TIR) multispectral data, designed mainly for the five TIR spectral bands of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on the Terra satellite. First, this method is improved for better applicability to ASTER/TIR imagery. The major improvement is the determination of a water vapor scaling factor on a band-by-band basis, which can reduce most of the errors induced by various factors such as algorithm assumptions. Next, the WVS method is validated by assessing the surface temperature and emissivity retrieved for a global-based simulation model (416 448 conditions), 183 ASTER scenes selected globally, and ASTER scenes from two test sites, Hawaii Island and Tokyo Bay. In situ lake surface temperatures measured in 13 vicarious calibration experiments, Moderate Resolution Imaging Spectroradiometer sea surface temperature products, and a climatic lake temperature are also used in validation. All the results indicate that although the ASTER/TIR standard atmospheric correction algorithm performs less well in humid conditions, the WVS method will provide more accurate retrieval of surface temperature and emissivity in most conditions including notably humid conditions. The expected root mean square error is about 0.6 K in temperature. Since the WVS method will be degraded by errors in gray pixel selection and cloud detection, these processing steps should be applied accurately.

Reproductive strategies of the spiny lobster Panulirus interruptus related to the marine environmental variability off central Baja California, Mexico: management implications

The reproductive strategies of the spiny lobster, Panulirus interruptus, in relation to environmental factors (temperature and upwelling) were determined in Baja California during 1989–1999. Two reproductive indices, R1 and R2, are proposed, based on the rate of occurrence of females with spermatophores and ovigerous females, respectively. P. interruptus spawned one brood annually between April and August. The size at which 50% of females were sexually mature was 72.6 mm carapace length (CL), which is well below the current minimum legal size of 82.5 mm CL. Mating and deposition of spermatophores began under moderate sea surface temperature (SST) and low upwelling conditions, but higher rates of deposition were coincident with low SST and strong upwelling. A high value of R1 generally occurred between 12.8 and 16.8 °C, and in general was inversely correlated with winter–spring temperature. Egg-bearing females were present in waters with a temperature range of 12.5–24 °C, and peaked at 14–19.2 °C. Embryonic development and larval hatching coincided with weakening Ekman transport and seasonal ocean warming. Timing of breeding was strongly temperature dependent. El Niño and La Niña condition impacts were particularly evident at the onset and duration of breeding. Warmer temperatures during El Niño periods accelerated breeding significantly, while the contrary happened under cold temperatures during La Niña. Females responded rapidly to declining winter temperatures and began to mate as early as December or as late as February/March. Similarly, oviposition and hatching were accelerated in response to increasing summer temperatures and finished as early as July and as late as September. The relationship between reproduction and the environment suggests that reproductive pattern and larval recruitment are coupled to dominant physical features of the California Current. Since environmental variability impacts on the onset and duration of breeding, the use of flexible management criteria is recommended, in order to guarantee that the opening and closing dates of the fishing season can be adjusted to take account of this unpredictability. This approach, in conjunction with monitoring of reproductive activity, will benefit the fishery by maximising the species’ annual reproductive success.

Simulated warm polar currents during the middle Permian

During Permian stage 6 (Wordian, Kazanian) the Pangaean supercontinent was surrounded by a superocean: Panthalassa. An ocean general circulation model has been coupled to an atmospheric energy balance model to simulate the sensitivity of the Wordian climate (∼265 million years ago) to changes in greenhouse gas concentrations, high‐latitude geography, and Earth orbital configurations. The model simulates significantly different circulation features with different levels of greenhouse gas forcing, ranging from a strong meridional overturning circulation in the Southern Hemisphere at low CO2concentration (present level) to more symmetric overturning circulation cells with deep water formation in polar latitudes of both hemispheres at high CO2concentration (8 times present level). The simulated climate with 4 times present level CO2concentration agrees generally well with climate‐sensitive sediments and phytogeographic patterns. The model simulates strong subtropical gyres with similarities to the modern South Pacific circulation and moderate surface temperatures on the southern continent Gondwana, resulting from a strong poleward heat transport in the ocean. An even more moderate climate is generated if high‐latitude land is removed from the southern continent so that ocean currents can penetrate into the polar regions of Gondwana.

Planetary habitability: is Earth commonplace in the Milky Way?

Is there life beyond planet Earth? This is one of the grand enigmas which humankind tries to solve through scientific research. Recent progress in astronomical measurement techniques has confirmed the existence of a multitude of extra-solar planets. On the other hand, enormous efforts are being made to assess the possibility of life on Mars. All these activities have stimulated several investigations about the habitability of cosmic bodies. The habitable zone (HZ) around a given central star is defined as the region within which an Earth-like planet might enjoy the moderate surface temperatures required for advanced life forms. At present, there are several models determining the HZ. One class of models utilises climate constraints for the existence of liquid water on a planetary surface. Another approach is based on an integrated Earth system analysis that relates the boundaries of the HZ to the limits of photosynthetic processes. Within the latter approach, the evolution of the HZ for our solar system over geological time scales is calculated straightforwardly, and a convenient filter can be constructed that picks the candidates for photosynthesis-based life from all the extra-solar planets discovered by novel observational methods. These results can then be used to determine the average number of planets per planetary system that are within the HZ. With the help of a segment of the Drake equation, the number of "Gaias" (i.e. extra-solar terrestrial planets with a globally acting biosphere) is estimated. This leads to the thoroughly educated guess that there should exist half a million Gaias in the Milky Way.

Habitable zone for Earth-like planets in the solar system

We present a new conceptual Earth system model to investigate the long-term co-evolution of geosphere and biosphere from the geological past upto 1.5 billion years into the planet's future. The model is based on the global carbon cycle as mediated by life and driven by increasing solar luminosity and plate tectonics. As a major result of our investigations we calculate the “terrestrial life corridor”, i.e. the biogeophysical domain supporting a photosynthesis-based ecosphere during planetary history and future. Furthermore, we calculate the behavior of our virtual Earth system at various distances from the Sun, using different insolations. In this way, we can find the habitable zone as the band of orbital distances from the Sun within which an Earth-like planet might enjoy moderate surface temperatures and CO 2-partial pressures needed for advanced life forms. We calculate an optimum position at 1.08 astronomical units for an Earth-like planet at which the biosphere would realize the maximum life span. According to our results, an Earth-like planet at Martian distance would have been habitable upto about 500 Ma ago while the position of Venus was always outside the habitable zone.

Dynamic Moisture Vapor Transfer Through Textiles

This study investigates new techniques for measuring the influence of fiber, type on fabric surface vapor pressure and temperature, which are related to sensations of cloth ing comfort. Microfine hygrometry and thermometry are used to measure dynamic changes in moisture vapor pressure and temperature on both surfaces of a two-layer fabric assembly under simulated body-clothing conditions. Findings show that fiber type significantly influences transient, comfort-related variables such as changes of inner surface vapor pressure and temperature. Polyester double layer assemblies exhibit the lowest inner surface temperature, but a much higher concentration of moisture vapor. Cotton assemblies show the slowest rate of inner vapor pressure accumulation with the highest inner surface temperature rise. Mixed fabric layers of cotton and polyester reveal moderate inner surface temperature peaks and moderate vapor pressure changes. Using a completely different methodology, this study reinforces other findings that fiber type and finish are indeed important to understanding transient changes in moisture vapor and temperature within fabric and clothing assemblies and their interior microclimates.

The role of excited species in UV-laser materials ablation

UV-laser ablation is described in terms of a two-level system in which the excitation energy is dissipated via stimulated emission, thermal relaxation, and activated desorption of excited species. For thermal relaxation times t T > 10 -9 s and ΔE * << ΔE (activation energies for excited-state and ground-state species) the model predicts high ablation rates at moderate surface temperatures, typically below 2000°C

Climatic change on the terrestrial planets

In this paper, we review the observational data on climatic change for the terrestrial planets, discuss the basic factors that influence climate, and examine the manner in which these factors may have been responsible for some of the known changes. Emphasis is placed on trying to understand the similarities and differences in both the basic factors and their climatic impacts on Venus, the Earth, and Mars. Climatic changes have occurred on the Earth over a broad spectrum of time scales that range from the elevated temperatures of Pre-Cambrian times (∼10 9 years ago), through the alternating glacial and interglacial epochs of the last few million years, to the small but significant decadal and centurial variations of the recent past. Evidence for climatic change on Mars is given by certain channel features, which suggest an early to intermediate aged epoch of warmer and wetter climate, and by layered polar deposits, which imply more recent periodic climate variations. No evidence for climatic change on Venus exists as yet, but comparison of its present climate state with that of outer terrestrial planets offers important clues on some of the mechanisms affecting climate. The important determinants of climate for a terrestrial planet include the Sun's output, astronomical perturbations of its orbital and axial characteristics, the gaseous and particulate content of its atmosphere, its land surface, volatile reservoirs, and its interior. All these factors appear to have played major roles in causing climatic changes on the terrestrial planets. Despite a lower solar luminosity in the past, the Earth and Mars have had warmer periods in their early history. In both cases, a more reducing atmosphere may have been the responsible agent through an enhanced greenhouse effect. In this paper, we present detailed calculations of the effect of atmospheric pressure and composition on the temperature state of Mars. We find that the higher temperature period is easier to explain with a reducing atmosphere than with the current fully oxidizing one. Both the very high surface temperature and massive atmosphere of Venus may be the result of the solar flux being a factor of two higher at its orbit than at the Earth's orbit. This difference may have led to a runaway greenhouse effect on Venus, i.e., the emplacement of volatiles entirely in the atmosphere rather than mostly in surface reservoirs. But if Venus formed with relatively little or no water, it may have always had an oxidizing atmosphere. In this case, a lower solar luminosity would have led to a moderate surface temperature in Venus' early history. Quasi-periodic variations in orbital eccentricity and axial obliquity may have contributed to the alternation between Pleistocene glacial and interglacial periods in the case of the Earth and to the formation of the layered polar deposits in the case of Mars. In this paper, we postulate that two mechanisms, acting jointly, account for the creation of the laminated terrain of Mars: dust particles serve as nucleation centers for the condensation of water vapor and carbon dioxide. The combined dust-H 2O-CO 2 particle is much larger and so has a much higher terminal velocity than either a dust-H 2O or a plain dust particle. As a result, dust and water ice are preferentially deposited in the polar regions. In addition, we postulate that the obliquity variations are key drivers of the periodic layering because of their impact on both atmospheric pressure and polar surface temperature, which, in turn, influence the amounts of dust and water ice in the atmosphere. But eccentricity and precessional changes probably also play important roles in creating the polar layers. The drifting of continents on the Earth has caused substantial climatic changes on individual continents and may have helped to set the stage for the Pleistocene ice ages through a positioning of the continents near the poles. While continental drift apparently has not occurred on Mars, tectonic distortions of its lithosphere may, in some circumstances, cause an alteration in the mean value of that planet's obliquity, which would significantly impact its climate. Atmospheric aerosols can influemce climate through their radiative effects. In the case of the Earth, volcanic aerosols appear to have contributed to past climatic changes, while consideration needs to be given to the future impact of man-generated aerosols. In the case of Mars, the atmospheric temperature structure and thereby atmospheric dynamics are greatly altered by suspended dust particles. The sulfuric acid clouds of Venus play a major role in its heat balance. Cometary impacts may have added substantial quantities of water vapor and sulfur gases to Venus' atmosphere and thus have indirectly affected its cloud properties. Calculations presented in this paper indicate substantial changes in surface temperature accompany these compositional changes.

A nongrey calculation of the runaway greenhouse: Implications for Venus' past and present

Abstract We obtain the dependence of the surface temperature of a planet covered with bodies of liquid water upon the amount of incident solar flux. To determine this relationship we carry out nongrey calculations of both the thermal flux emitted to space and the planetary albedo. Water vapor is the only source of infrared opacity in our models. For most of the model atmospheres with a 50% cloud cover, the solar flux incident upon Venus at the present time is sufficiently high to cause complete evaporation of any hypothetical oceans, thus assuring that any carbon dioxide outgassed into the atmosphere remains there. However, our models with a 100% cloud cover do not achieve runaway conditions. Our calculations indicate that four and a half billion years ago, when the solar luminosity was 30% lower than its present value, Venus may have had a very moderate surface temperature and bodies of liquid water on its surface.