Search

It is shown that the greenhouse gases carbon dioxide and water vapour reflect back to the surface, all IR radiation originating at the surface within their respective spectral bands. This reflection occurs in a very thin layer at the surface, not much over 12cm in thickness. Heat is lost from the surface by heat exchange with the atmosphere and by loss of radiation. About 52% of radiation leaves the surface in two principal window regions but this is not enough to account for the earth's equilibrium temperature. This window radiation seems to disappear quite quickly and is replaced by black body radiation. It is this which eventually contributes to the earth's radiation balance, and has to originate approximately between 40 and 50km altitude where the temperature is about correct, near 255K. Doubling the CO2 concentration increases the surface temperature by about 0.9°C and this need not have any influence higher up in the atmosphere. The surface temperature seems indeed to have no direct influence on the earth's external radiation balance.

This study examines the concept of ‘greenhouse gases’ and various definitions of the phenomenon known as the ‘Atmospheric Radiative Greenhouse Effect’. The six most quoted descriptions are as follows: (a) radiation trapped between the Earth’s surface and its atmosphere; (b) the insulating blanket of the atmosphere that keeps the Earth warm; (c) back radiation from the atmosphere to the Earth’s surface; (d) Infra Red absorbing gases that hinder radiative cooling and keep the surface warmer than it would otherwise be – known as ‘otherwise radiation’; (e) differences between actual surface temperatures of the Earth (as also observed on Venus) and those based on calculations; (f) any gas that absorbs infrared radiation emitted from the Earth’s surface towards free space. It is shown that none of the above descriptions can withstand the rigours of scientific scrutiny when the fundamental laws of physics and thermodynamics are applied to them.

Terrestrial planet atmospheres must be in long-term radiation balance, with solar radiation absorbed matched by thermal radiation emitted. For hot moist atmospheres, however, there is an upper limit on the thermal emission which is decoupled from the surface temperature. If net absorbed solar radiation exceeds this limit the planet will heat uncontrollably, the so-called \runaway greenhouse". Here we show that a runaway greenhouse induced steam atmosphere may be a stable state for a planet with the same amount of incident solar radiation as Earth has today, contrary to previous results. We have calculated the clear-sky radiation limits at line-by-line spectral resolution for the first time. The thermal radiation limit is lower than previously reported (282 W/sq m rather than 310W/sq m) and much more solar radiation would be absorbed (294W/sq m rather than 222W/sq m). Avoiding a runaway greenhouse under the present solar constant requires that the atmosphere is subsaturated with water, and that cloud albedo forcing exceeds cloud greenhouse forcing. Greenhouse warming could in theory trigger a runaway greenhouse but palaeoclimate comparisons suggest that foreseeable increases in greenhouse gases will be insufficient to do this.

Water-rich planets such as Earth are expected to become eventually uninhabitable, because liquid water does not remain stable at the surface as surface temperatures increase with the solar luminosity over time. Whether a large increase of atmospheric concentrations of greenhouse gases such as CO$_2$ could also destroy the habitability of water-rich planets has remained unclear. We show with three-dimensional aqua-planet numerical experiments that CO$_2$-induced forcing as readily destabilizes the climate as does solar forcing. The climate instability is caused by a positive cloud feedback. The climate does not run away, but instead attains a new steady state with global-mean sea-surface temperatures above 330 K. The upper atmosphere is considerably moister in this warm steady state than in the reference climate, implying that the planet would be subject to substantial loss of water to space. For either a certain range of elevated CO$_2$ concentrations or solar irradiation, we find both cold and warm equilibrium states, implying that the transition to the warm state may not simply be reversed by removing the additional forcing., 30 pages, 8 figures

The earth's natural climate has always been and still constantly changing. Greenhouse effect is warming of the surface of the earth by a complex process involving sunlight, trace gases and particles in the atmosphere. Some of the heat energy escapes to the space but much of it is trapped by the layers of atmospheric greenhouse gases to outer space and get absorbed. Thus the earth's lower atmosphere heats up. The greenhouse effect is a natural phenomenon and vital to life. Without the greenhouse effect the earth's average temperature would be -180C, instead of current average temperature of 150C. However, problem arise when the atmospheric concentration of greenhouse gases such as CO2, CH4, N2O, O3, halocarbons and water vapour increases.

Estimates are made for the exchange of radiative fluxes between the Earth's surface, its atmosphere, and free space on the basis of their average thermal structure and estimated values for the emissivity and absorptivity of those individual entities. The radiative fluxes obtained reflect nighttime conditions but also indicate the components of those fluxes that must be subtracted from the input solar flux for daytime conditions. The estimates show that infrared radiant energy is lost to free space from both the Earth's surface and its atmosphere. Under the normal atmospheric temperature structure there is no net radiation from the atmosphere that reaches the Earth's surface to be absorbed by it, and thus the so-called “greenhouse effect”, as it is traditionally depicted, is devoid of physical reality. The only exception occurs during atmospheric inversion conditions, but even in the presence of such rare inversions, the radiant power lost to free space from the atmosphere as a whole overwhelms the radiant power absorbed by the Earth's surface from the warmer inversion layer above it. Inversion conditions are thus the only ones in which the so-called “greenhouse effect” can possibly have any form of physical reality. But such inversion conditions, however, are present over only a small fraction of the Earth's surface for limited periods of time. In addition, since the recent increases in atmospheric CO2 concentrations have a trivial effect on the atmosphere's total emissivity, the effect of those recent CO2 increases on the overall infrared radiative flux balance is insignificant. The analysis also suggests a cloud cooling mechanism for the Earth's surface and its atmosphere that is significantly different from the one traditionally cited. Cloud blockage such as that reflected in the Earth's albedo acts symmetrically with respect to both the input heating by solar radiation and the output cooling by infrared radiation to free space. On the other hand, it is shown that the increasing infrared emissivity of the atmosphere caused by the presence of clouds provides an effective cooling mechanism that is asymmetric.

Model calculations and the few data that are available show that over 100 L water condense yearly on each square meter of a greenhouse cover. It is known that the presence of condensate reduces light transmission. This effect is suppressed to some extent by adding film-forming (anti-drop) additives to plastic film covers and surface structures or coatings on hard cover materials. There is a need, therefore to assess the effect of the surface treatment on the loss of light. On the other hand, condensation releases the energy that was used for evaporation, thereby warming-up the cover and somewhat decreasing the heating requirement of the greenhouse. The amount of condensation energy that is recovered may be expected to depend on the external and internal climate conditions. In this work we analysed the effect of condensation on light transmission and energy budget of a greenhouse, with seven different cover materials. Various internal vs external conditions were created by placing the model greenhouse (about 3¿4 m) in a large climate chamber. Each experiment was repeated for two temperature differences between inside and outside (10 and 20°C) and two air movements in the greenhouse (7.5 and 15 cm s-1). Light transmissivity was reduced by 9% on average, with large differences among materials. Anti-drop coatings did suppress this effect, as did a surface structure meant to increase light diffusivity of the material. As far as energy is concerned, the overall heat transfer coefficient (U-value) of the greenhouse increased by an average of 16% (single layers) or 12% (double layer covers) when wet. Obviously there was an effect of the temperature difference on the U-value, which was found to be consistent with the heat transfer theory, whereas little effect was found of the air movement within the house.

The Earth's climate, as well as planetary climates in general, is broadly regulated by three fundamental parameters: the total solar irradiance, the planetary albedo and the planetary emissivity. Observations from series of different satellites during the last three decades indicate that these three quantities are generally very stable. The total solar irradiation of some 1,361 W/m 2 at 1 A.U. varies within 1 W/m 2 during the 11-year solar cycle (Frohlich 2012). The albedo is close to 29 % with minute changes from year to year but with marked zonal differences (Stevens and Schwartz 2012). The only exception to the overall stability is a minor decrease in the planetary emissivity (the ratio between the radiation to space and the radiation from the surface of the Earth). This is a consequence of the increase in atmospheric greenhouse gas amounts making the atmosphere gradually more opaque to long-wave terrestrial radiation. As a consequence, radiation processes are slightly out of balance as less heat is leaving the Earth in the form of thermal radiation than the amount of heat from the incoming solar radiation. Present space-based systems cannot yet measure this imbalance, but the effect can be inferred from the increase in heat in the oceans where most of the heat accumulates. Minor amounts of heat are used to melt ice and to warm the atmosphere and the surface of the Earth. The reverse is happening when there is a volcanic eruption that emits particles that reflect solar radiation. In this case less heat is entering the Earth's system than leaving it and a cooling take place, as the system is moving toward another equilibrium. In reality the Earth's system is never in equilibrium as it is continuously exposed to disturbances in its energy balance. In addition to external and anthropogenic effects there are natural pro- cesses in the Earth's system caused by variations in the cloud cover, in the exchange of heat between ocean and atmosphere, in the surface conditions such as snow and ice cover, and in changes in the three-dimensional distribution of water vapor. Other changes in the energy balance include land vegetation that affects both the surface albedo and the surface fluxes of heat and water. Some of the processes now affecting the Earth's climate system are relatively well understood, and others less so. The radiative effect of the greenhouse gases added to the

Long-wave energy emitted by the Earth-atmosphere into space is characterized by changes in power over time that always lag behind the changes in power of the absorbed solar radiation due to slow variation in enthalpy of the Earth-atmosphere system. Long-term variation of the solar energy radiation absorbed by the Earth remains uncompensated by the energy radiated into space over the interval of time that is determined by the thermal inertia. The basic state of the climate system is when the debit and credit sides in the Earth’s global annual mean energy budget (including the air and water envelopes) are almost always unbalanced. The annual mean balance of the heat budget of the Earth-atmosphere over a long time period will reliably define the behavior and magnitude of the energy excess accumulated by the Earth or energy deficit to allow us to determine adequately and to predict beforehand the trend and amplitude of the forthcoming climate change using the prognosis of variations in the total solar irradiance (solar constant). The decrease in solar constant has been observed since the early 1990s. The Earth as a planet will have a negative balance in the energy budget in the future as well, because the Sun is entering the decline phase of the bicentennial luminosity changes. This will lead to a drop in temperature in approximately 2014. The increase in albedo and decrease in greenhouse gas concentration in the atmosphere will result in the additional decrease in absorbed portion of the solar energy and reduced greenhouse effect. The additional drop in temperature exceeding the effect of decreased solar constant can occur as a result of successive feedback effects. A deep bicentennial minimum in solar constant is to be anticipated in 2042 ± 11 and the 19th Little Ice Age (for the last 7500 years) may occur in 2055 ± 11.

The “natural greenhouse effect” is one of the fundamentals of traditional climatology. To maintain the outgoing thermal radiation from the Earth's surface in equilibrium with solar irradiation requires the mathematical construct of back-radiation emitted by some infrared-active constituents in the atmosphere; otherwise, the convection and evapotranspiration flows from the surface and the corresponding back flows cannot be explained in view of the net radiation budget. To surmount the biased view of all-dominating radiative processes in the atmosphere, an alternative approach should take into account the effects of gravity on the atmosphere. An atmospheric temperature effect is completely determined by the mean global temperature at the surface, the radiative-equilibrium temperature of the earth/atmosphere system (in an altitude of some kilometres) and the pseudoadiabatic temperature lapse rate. There is no separate conservation law for radiation at the Earth's surface but only one for energy. In the unequal competition with atmospheric thermal radiation, solar radiation has an unique position relating to the transformation into heat in the absorbing material.

Solar energy is generated by nuclear fusion reactions within the Sun. The energy that radiates from the Sun is a mixture of ultraviolet, visible, and infrared radiation. The intensity of this radiation when it reaches the Earth is 1361 W/m2. When it passes through the atmosphere, some of this radiation is scattered and some absorbed. Even so, the amount of energy that reaches the surface of the Earth is so large that the amount falling in one hour could meet all the electricity demands of the planet. Solar thermal power plants can only utilize direct radiation from the Sun, but solar cells, like plants, can absorb both direct and scattered, diffuse radiation. The distribution of solar energy across the globe is not even, and some regions have much higher insolation levels than others. These might become major sources of solar-generated electricity in the future.

Icy surfaces like the polar caps of Mars, comets, Edgeworth–Kuiper belt objects or the surface areas of many moons in the outer Solar System behave different than rock and soil surfaces when irradiated by solar light. The latter ones absorb and reflect incoming solar radiation immediately at the surface. In contrast, ices are partially transparent in the visible spectral range and opaque in the infrared. Due to this fact it is possible for the solar radiation to reach a certain depth and increase the temperature of the sub-surface layers directly. This internal temperature rise is called “solid-state greenhouse effect,” in analogy to the classical greenhouse effect in an atmosphere. It may play an important role in the energy balance of various icy bodies in the Solar System. Within the scope of a project conducted at the Space Research Institute of the Austrian Academy of Sciences in Graz the solid-state greenhouse effect was investigated experimentally and theoretically. A number of experiments with diverse materials, focussing mainly on layered samples with a surface cover consisting of transparent H 2 O-ice, were performed. The samples were irradiated under cryo-vacuum conditions by a solar simulator. The temperature distributions inside the samples were measured and compared with the results of numerical model calculations. We found that the predicted sub-surface temperature maximum is very clearly measurable in glass beads samples with various particle size distributions, but can also be detected in transparent compact surface ice layers. However, in the latter case it is less distinct than originally expected. Measuring the effect by laboratory methods turned out to be a difficult task due to the shallow depth where the temperature maximum occurs.

The presence of atmosphere can appreciably warm a planet’s surface above the temperature of an airless environment. Known as a natural Greenhouse Effect (GE), this near-surface Atmospheric Thermal Enhancement (ATE) as named herein is presently entirely attributed to the absorption of up-welling long-wave radiation by greenhouse gases. Often quoted as 33 K for Earth, GE is estimated as a difference between planet’s observed mean surface temperature and an effective radiating temperature calculated from the globally averaged absorbed solar flux using the Stefan-Boltzmann (SB) radiation law. This approach equates a planet’s average temperature in the absence of greenhouse gases or atmosphere to an effective emission temperature assuming ATE ≡ GE. The SB law is also routinely employed to estimating the mean temperatures of airless bodies. We demonstrate that this formula as applied to spherical objects is mathematically incorrect owing to Hölder’s inequality between integrals and leads to biased results such as a significant underestimation of Earth’s ATE. We derive a new expression for the mean physical temperature of airless bodies based on an analytic integration of the SB law over a sphere that accounts for effects of regolith heat storage and cosmic background radiation on nighttime temperatures. Upon verifying our model against Moon surface temperature data provided by the NASA Diviner Lunar Radiometer Experiment, we propose it as a new analytic standard for evaluating the thermal environment of airless bodies. Physical evidence is presented that Earth’s ATE should be assessed against the temperature of an equivalent airless body such as the Moon rather than a hypothetical atmosphere devoid of greenhouse gases. Employing the new temperature formula we show that Earth’s total ATE is ~90 K, not 33 K, and that ATE = GE + TE, where GE is the thermal effect of greenhouse gases, while TE > 15 K is a thermodynamic enhancement independent of the atmospheric infrared back radiation. It is concluded that the contribution of greenhouse gases to Earth’s ATE defined as GE = ATE – TE might be greater than 33 K, but will remain uncertain until the strength of the hereto identified TE is fully quantified by future research. Electronic supplementary material The online version of this article (doi:10.1186/2193-1801-3-723) contains supplementary material, which is available to authorized users.

The Earth’ climate is a complex system consisting of the atmosphere, the ocean and the land surface. This system receives energy from the sun. Latitudinal variations in insolation and the rotation of the Earth lead to a complex turbulent geophysical flow which induces important transports of heat and mass. Part of the solar radiation is reflected, the rest is radiated as long wave radiation. The radiative balance is complicated by atmospheric water vapour and other trace gases, clouds, heat transport and other factors.

The antigreenhouse effect results from a high altitude haze which blocks sunlight while allowing transmission of thermal infrared. An antigreenhouse is observed on Titan and has been proposed for the early Earth. Here, we derive an analytic solution useful for computing the atmospheric and surface temperature with both the greenhouse and antigreenhouse effects included. In agreement with detailed calculations, the analytic solution predicts that a runaway greenhouse on early Titan caused by accretional heat would collapse when the flux fell to 0.9 W m −2 . For the present value of the solar luminosity a runaway greenhouse collapses at a geothermal heat flux of 0.4 W m −2 . We show that the antigreenhouse effect may compromise attempts to solve the faint young sun problem on the early Earth with a methane–ammonia greenhouse shielded by an organic haze, more than doubling the infrared opacity needed to warm the early Earth compared to the case with no antigreen- house.

Recent studies have highlighted the important contribution made by the far i.r. component of the water vapour spectrum to present-day greenhouse forcing, and the possible influence of this region upon future climate. In order to further assess these findings at high spectral resolution a method allowing the calculation of longwave clear sky fluxes and cooling rates based on the GENLN2 line-by-line radiative transfer code has been implemented, and studies have been carried out of the sensitivity and response of the outgoing planetary radiation to changes in atmospheric composition. Comparisons with other line-by-line model show typical cooling rate agreement of within 0.05 K day −1 at all pressure levels. Taking two model atmospheres representative of tropical (TRP) and sub-arctic winter (SAW) conditions, several experiments have been performed to assess both the present-day greenhouse effect, and the impact of perturbations in carbon dioxide and water vapour amounts on the greenhouse trapping of each atmosphere. The results reinforce the importance of the far i.r. with, for the present-day conditions, one third and one quarter of the total absolute greenhouse forcing calculated to occur in this region for the SAW and TRP atmospheres respectively. The sensitivity of the normalised greenhouse effect to water vapour concentrations is greatest in the far i.r. for the SAW atmosphere, and in the window region for the TRP; this sensitivity differing most between atmospheres in the far i.r. The sensitivity to vertical variations in water vapour perturbation shows a peak response located at mid-tropospheric levels within the window region for the TRP case, and within the far i.r. at upper tropospheric levels for the SAW atmosphere. The carbon dioxide perturbation experiments indicate the increasingly important role played by the weaker carbon dioxide bands as concentrations increase, and provide a further illustration of the fallacy of the suggestion of a `CO 2 saturated' atmosphere.

The natural greenhouse effect of the Earth is strongly influenced by the radiative effects of water vapour and clouds in the atmosphere, which control the energy absorbed from the sun, and that lost through thermal emission to space. Any perturbations to the climate balance, for example through so-called ‘radiative forcing’ due to increasing CO2 amounts, variations in solar constant, or other causes, can be amplified by the feedback processes that involve water in its various phases. The radiative cooling of the Earth in the absence of clouds has recently been shown to be dominated by emission from upper-tropospheric water vapour, in the far infrared portion of the spectrum, and this is illustrated: observations of this radiative flux, and of the distribution of water vapour in the upper troposphere, are urgently needed. The role of clouds is discussed, and it is noted that their response to global warming is not presently unambiguously determined with available models, due to the complexity of competing processes: again, as in the cloud-free case, more accurate global observations are needed. The paper is illustrated by data from satellite experiments, most notably the Earth Radiation Budget Experiment sponsored by NASA.

The Sun is Earth’s primary source of energy. In this paper, we compare the magnitude of the Sun to all other external (to the atmosphere) energy sources. These external sources were previously identified in Sellers (1965); here, we quantify and update them. These external sources provide a total energy to the Earth that is more than 3700 times smaller than that provided by the Sun, a vast majority of which is provided by heat from the Earth’s interior. After accounting for the fact that 71% of incident solar radiation is deposited into the earth system, the Sun provides a total energy to Earth that is still more than 2600 times larger than the sum of all other external sources.

[1] Solar shortwave and thermal longwave radiation at the Earth's surface and at the top of the atmosphere is commonly measured at surface stations, from airplanes and from satellites. Here we show radiative flux profiles measured with radiosondes ascending from the Earth's surface to 35 km into the stratosphere. During two-hour flights solar shortwave and thermal longwave radiation are measured both downward and upward with four individual sensors. Daytime solar and thermal radiation is compared to nighttime measurements and 24-hour average radiation budget profiles are shown through the atmosphere. However, of even greater importance with regard to climate change are measured upward and downward longwave greenhouse radiation profiles. Their changes with temperature and water vapor enable direct measurement of radiative forcing through the atmosphere. Measurements during two cloud-free nights with different temperature and different water vapor amount, show an almost equal but opposite net longwave radiation change, or water vapor greenhouse forcing, downwards to the surface and upward into space. Radiative flux profiles clearly illustrate the Earth's atmospheric greenhouse effect, and allow important investigations of clouds and other atmospheric constituents and their effects on shortwave reflection, as well as longwave emission towards the surface and into space.

We generalize the problem of the semi-gray model to cases in which a non-negligible fraction of the stellar radiation falls on the long-wavelength range, and/or that the planetary long-wavelength emission penetrates into the transparent short wavelength domain of the absorption. Second, applying the most general assumptions and independently of any particular properties of an absorber, we show that the greenhouse effect saturates and any Earth-like planet has a maximal temperature which depends on the type of and distance to its main-sequence star, its albedo and the primary atmospheric components which determine the cutoff frequency below which the atmosphere is optically thick. For example, a hypothetical convection-less planet similar to Venus, that is optically thin in the visible, could have at most a surface temperature of 1200-1300K irrespective of the nature of the greenhouse gas. We show that two primary mechanisms are responsible for the saturation of the runaway greenhouse effect, depending on the value of the wavelength above which the atmosphere becomes optically thick. Unless this wavelength is small and resides in the optical region, saturation is achieved by radiating the thermal flux of the planet through the short wavelength tail of the thermal distribution. This has the observational implication, the radiation from such a planet should be skewed towards the NIR. Otherwise, saturation takes place by radiating through windows in the FIR., 13 pages 14 figures

The transfer of thermal energy by radiation is discussed in the context of the Earth's surface and its atmosphere. When considering what happens as the Sun is warming the surface each morning, it is noted that its radiation is being directed onto the land surfaces and some distance below the surface of the oceans. So, additional radiation supposedly transferring further thermal energy from the cooler atmosphere to the warmer surface would violate the Second Law of Thermodynamics. This law must apply (on a macro scale) between any two points at any particular time. An apparent violation cannot be excused on the basis of "net" radiation, because "net" radiation has no corresponding physical entity and is meaningless and useless for determining heat flow in situations when other processes are also involved. It may be deduced that none of the radiation from a cooler body (and only a portion of the radiation from a warmer body) has any thermodynamic effect on the other body. All such radiation from a cooler source is rejected in some way, and it can be deduced that resonance and scattering occurs without any conversion to thermal energy. The radiation continues in another direction until it strikes a cooler target, which could be in space. Furthermore, the stability of sub-surface temperatures will tend to maintain the observed close thermal equilibrium at the interface between the surface and the atmosphere. Hence other heat loss mechanisms are likely to adjust, in order to compensate for any reduced radiation. Some commonly raised questions are answered in the Appendix, where there is also discussion of temperature trends and climate cycles, as well as counter arguments for several possible objections to matters raised herein.

The cooling rates for a thin upper layer of impact-melted material on the surface of the growing Earth were calculated using the experimental data for convective heat transfer coefficient. The presence of an atmosphere on the Earth embryo leads to very high cooling rates of the surface layer of impact crater. We find that during Safronov's type of accretion more than 90% of the Earth's surface was below the freezing point of water and the blanketing effect of greenhouse gases was unable to maintain a global magma ocean on Earth.

The greenhouse effect on the Earth is identified by the difference between the effective radiating temperature of the planet and its surface temperature. The difference between the energy emitted by the surface and that emitted upward to space by the upper atmosphere quantifies it; it can therefore be defined as the long wave energy trapped in the atmosphere. Climate forcing and the response of the climate system within which climate feedback mechanisms are contained, will be defined in this review. Quantitative examples will illustrate what could happen if the greenhouse effect is perturbed by the human activities, in particular if atmospheric CO2 concentrations would double in the future. Recent measurements by satellites of the greenhouse effect will be given. The net cooling effect of clouds on the Earth and whether or not there will be less cooling by clouds as the planet warms, are discussed following a series of papers recently published by Ramanathan and his collaborators.

In this paper, we propose an analysis of the greenhouse effect on the basis of a net‐exchange formulation for clear sky atmospheres. This formulation allows access to exchanges beetwen the differents elements of the atmosphere (gas layers, the ground and space). When the greenhouse gas concentration increases, we first use a simple configuration to analyse the variations of analytic monochromatic net exchange rates. The same type of analysis is then applied to the Earth atmosphere for a clear‐sky middle latitude summer configuration with an increase in water vapour of 20% at all altitudes.

There are many parallels between the atmospheric thermal structure of the Saturnian satellite Titan and the terrestrial greenhouse effect; these parallels provide a comparison for theories of the heat balance of Earth. Titan's atmosphere has a greenhouse effect caused primarily by pressure-induced opacity of N2, CH4, and H2. H2 is a key absorber because it is primarily responsible for the absorption in the wave number 400 to 600 cm-1 "window" region of Titan's infrared spectrum. The concentration of CH4, also an important absorber, is set by the saturation vapor pressure and hence is dependent on temperature. In this respect there is a similarity between the role of H2 and CH4 on Titan and that of CO2 and H2O on Earth. Titan also has an antigreenhouse effect that results from the presence of a high-altitude haze layer that is absorbing at solar wavelengths but transparent in the thermal infrared. The antigreenhouse effect on Titan reduces the surface temperature by 9 K whereas the greenhouse effect increases it by 21 K. The net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature (82 K). If the haze layer were removed, the antigreenhouse effect would be greatly reduced, the greenhouse effect would become even stronger, and the surface temperature would rise by over 20 K.

Increasing levels of certain greenhouse gases (GHGs), most importantly CO2 in the earths atmosphere, lead to climate change and global warming as a result of these gases interacting with thermal infrared (TIR) radiation from earth to space. Here, the option of modifying this radiation is analyzed which would result in modified TIR radiation that would interact less with atmospheric CO2. This alleviates the enhanced greenhouse effect, and at the same time would allow for energy recovery as heat and/or power. Power production is, of course, limited by thermodynamics Second Law. It is shown that various options exist for TIR radiation modification which may be used to generate temperature gradients or temperature differences between volumes of (gases containing) CO2 of sufficient optical thickness. This may be further exploited for power generation: a first, simple case shows power generation of ∼1 W per m2 surface at a Carnot efficiency of ∼7%, using the sky and ground level surroundings as heat reservoirs.

Publisher Summary This chapter provides an overview of the optical properties, composition, and vertical structure of the Earth's atmosphere; the major wind systems; and the climatological-mean distribution of precipitation. The chapter introduces some of the terminology and conventions that will be used in performing calculations involving amounts of mass and rates of movement. The Earth's atmosphere is relatively transparent to incoming solar radiation and opaque to outgoing radiation emitted by the Earth's surface. The blocking of outgoing radiation by the atmosphere, popularly referred to as the “greenhouse effect,” keeps the surface of the Earth warmer than it would be in the absence of an atmosphere. Much of the absorption and reemission of outgoing radiation are due to air molecules, but cloud droplets also play a significant role. The radiation emitted to space by air molecules and cloud droplets provides a basis for remote sensing of the three-dimensional distribution of temperature and various atmospheric constituents using satellite borne sensors. The atmosphere is composed of a mixture of gases in different proportions, where fractional concentration by volume is the same as that based on numbers of molecules, or partial pressures exerted by the gases.

The sun is the battery that drives processes on earth, including evaporation, transpiration, and the ascent of water in plants. Therefore, it is essential to understand the sun's energy. The chapter studies solar radiation and the laws associated with a black body, because the sun can be considered to be a black body. It calculates the heat budget at the surface of the earth and defines the radiation balance. A comparison of solar and terrestrial radiation is given in the chapter. All materials at temperatures above absolute zero are continually emitting radiation. As the temperature of a solid is increased, the energy radiated from the solid increases rapidly. The amount of radiant power emitted by a solid depends significantly on the character of the surface of the solid. In beginning, the discussion of radiation is helpful to define a “perfect radiator,” whose rate of radiation is the maximum possible for its temperature. Such a maximum rate exists can be shown by considering the inverse process, absorption. The radiation balance consists of two radiation streams of different spectral ranges. There is a short-wavelength part only as long as the sun shines, that is, during the daytime.

A common view is that the current global warming rate will continue or accelerate. But we argue that rapid warming in recent decades has been driven mainly by non-CO(2) greenhouse gases (GHGs), such as chlorofluorocarbons, CH(4), and N(2)O, not by the products of fossil fuel burning, CO(2) and aerosols, the positive and negative climate forcings of which are partially offsetting. The growth rate of non-CO(2) GHGs has declined in the past decade. If sources of CH(4) and O(3) precursors were reduced in the future, the change in climate forcing by non-CO(2) GHGs in the next 50 years could be near zero. Combined with a reduction of black carbon emissions and plausible success in slowing CO(2) emissions, this reduction of non-CO(2) GHGs could lead to a decline in the rate of global warming, reducing the danger of dramatic climate change. Such a focus on air pollution has practical benefits that unite the interests of developed and developing countries. However, assessment of ongoing and future climate change requires composition-specific long-term global monitoring of aerosol properties.

Publisher Summary Of all the factors that control the weather, the sun is by far the most powerful. Practically, everything that occurs on the Earth is controlled, directly, or indirectly, by the sun. The sun affects the places humans inhabit, in the kind of homes that are built, the work that is done, and the equipment that is used. Less than one-millionth of the energy emitted from the sun's surface travels ninety-odd million miles to reach this planet. The sun's energy crosses those miles in the form of short electromagnetic radio waves, identical in nature to those used in broadcasting, which pass through the atmosphere and are absorbed by the Earth's surface. These waves warm the Earth's surface and are then re-radiated back to space. The wavelength of the energy emitted by the Earth is much longer than that emitted by the sun, because the Earth is much cooler than the sun. Radiation from the sun consists of three differing wavelengths: heat rays, actinic rays, and light rays. Heat rays and actinic rays are intercepted by solid bodies and produce peculiar effects in varying degrees according to the nature of the surface on which they fall. The light rays are responsible for daylight, and both light rays and actinic rays are necessary for the life processes of plants. The heat ray's most important manifestation is temperature. However, radiant energy can also damage equipment as heat rays can warm the material or its surrounding environment to dangerous levels and photochemical degradation of materials can be caused by the ultraviolet content of solar radiation.

Climate is a region's general pattern and synthetic atmospheric or weather conditions over a long period. Average temperature and precipitation are the two main factors which determine a region's climate and its effect on people. Small amounts of certain gases play a key role in determining the average temperature and its climate. These gases include water vapour, carbon dioxide and synthetic chlorofluorocarbon. Together these gases are known as greenhouse gases, which allow mostly visible light and some infrared and ultraviolet radiation from the sun to pass through the troposphere. The earth's surface absorbs much of solar energy. This transforms it to longer wavelength infrared radiation which then rises into the troposphere. Some of the heat escapes into space and some is absorbed by molecules of greenhouse gases and emitted into the troposphere in the form of longer wave infrared radiation, which warms the air. This natural warming effect of troposphere is called the greenhouse effect. Human activities such as burning fossil fuels, clearing forests and growing crops release carbon dioxide, methane and nitrous oxide into the atmosphere. It is of great concern that large input of these gases into troposphere can enhance the earth's natural greenhouse effect and lead to global warming.

The climate of Earth and its global mean surface temperature are the consequence of a balance between the amount of solar radiation absorbed by Earth's surface and atmosphere and the amount of outgoing longwave radiation emitted by the system. The former is governed by the albedo (reflectivity) of the system, whereas the latter depends strongly on the atmospheric content of gases and particles (such as clouds and dust). Although the theory of absorption of infrared radiation by gases in the atmosphere (1) is well accepted and embodied in climate models, the observational and theoretical treatments of albedo, aerosols, and clouds are still under development. One brevium (2) and two reports (3, 4) in this issue report estimates of Earth's albedo and of solar radiation reaching the surface, but the uncertainties remain large. The buildup of CO_2 (5), CH_4, and other greenhouse gases during the past century has led to an increased absorption of infrared radiation in the atmosphere (enhanced greenhouse effect) and a consequent warming (“positive forcing”) of the climate. But human-made changes in aerosols and clouds can cause enhanced albedo and hence cooling (“negative forcing”), and they may already have offset a substantial part of the enhanced greenhouse effect. Present trends suggest that by 2050, the magnitude of the enhanced greenhouse effect will be so large that the net anthropogenic forcing will be unequivocally positive and substantial in magnitude (6).

The Greenhouse Effect is the term applied to the warming of the earth’s surface that results from the fact that atmospheric gases have a greater transparency for incoming solar radiation than for outgoing longwave thermal radiation emitted by the earth’s surface. Ultraviolet (UV) radiation contributes little directly to this whole global radiation balance. First, UV radiation is a small fraction of extraterrestrial solar radiation, and second, much of it is absorbed in the stratosphere in comparison with other radiation components. However, ozone (O3) and chlorofluorocarbons (CFCs) are greenhouse effect gases. Ozone is important because it absorbs radiation in the middle of the “atmospheric window,” within the 8 to 12 micrometer wavelength bandwidth where many of the other standard greenhouse-effect gases (water vapor, CO2, CH4, N2O) have little effect. Tropospheric and stratospheric ozone play a role in the greenhouse effect. CFCs are important because 1) they have been recently introduced into the earth’s atmosphere, 2) they have a strong warming potential for each increment added to the atmosphere, 3) they will continue to be released to the atmosphere from emissions from closed cell foams and fugitive refrigerant gases. CFC substitutes may have their own greenhouse effect impacts.

The calculations show that the absorption of the direct solar radiation in the surface 1-metre layer of the oceandepending on the transparence of the water can result in he change of the heat-and-mass exchange in it. Two new regimes of the heat and mass exchange are described, found experimentally in situ with the help of micro-thermoprobe. Data on the intensity of heat-and-mass exchange of the ocean and atmosphere are given. Formation of climate and weather on the Earth is basically determined by heat and massexchange between the ocean and the atmosphere which occurs by mean of processes of water evaporation, effective long-wave radiation and contact heat transfer. Being the initial cause ofglobal circulation of atmospheric and water masses on the Earth these processes themselves take place on the very surface of the ocean in a layer of 40 1um thin.Radiation characteristics of the ocean and the atmosphere are different and due to radiationexchange between the Sun, the Earth and Space are in thermal disbalance —

Over 100 years ago, the Swedish chemist Svante Arrhenius first pointed out that the release of carbon dioxide into the atmosphere by the burning of fossil fuels was warming the Earth. At the time, neither he nor anybody else was particularly concerned. Greenhouse gases, such as CO2, water vapour and methane, absorb infrared radiation emitted from the Earth's surface and radiate the heat back towards the planet. The natural concentrations of these gases increase the temperature of the Earth by about 35 °C, and arc thus essential for a habitable planet.

In this paper the exact solution of a realistic physical model of the thermal radiation exchange inside a single span greenhouse is offered which yields the net radiation flux density at ground level. A similar but approximate model is also offered for a multispan grenhouse. The rate of radiative heat loss was computed for the particular situation when the inside ground, the cladding material and the outside air and ground are all at freezing temperature, and the sky is clear. It turns out that the radiative heat loss rate is nearly the same for the single and for the multispan greenhouse, and is mainly determined by the transmittance of the cladding material, being little dependent on the relative magnitudes of its emittance and reflectance. Finally, the coefficient of radiative heat transfer for the inside ground surface was obtained as well.

The vertical thermal structure of a primitive terrestrial atmosphere is investigated with a radiative-convective-photochemical model. The radiative code includes the short wave contribution from water vapor and ozone, and long wave contribution from methane, carbon dioxide, water vapor and ozone. Calculations for an oxygen level of 10 −3 PAL and different CO 2 levels shows that the water vapor content, and consequently the odd hydrogen concentration, in the stratosphere is controlled by the temperature which is strongly reduced from present values due to the lower ozone content. As a result, depending on the assumed mechanism for controlling the H 2 O mixing ratio, a considerable feedback is introduced on the ozone columnar density. The same model is used to parameterize the infrared outgoing flux as a function of surface temperature to be used in a two-mode energy balance climate model. This computation is addressed to the question of whether a large amount of carbon dioxide in the primitive atmosphere could be effective in producing a greenhouse effect able to compensate for the Sun's lower luminosity. It is found that with 25 times the present carbon dioxide mixing ratio, due to the ice-albedo feedback mechanism, a decrease of 9% in the solar constant could be enough to produce an ice-covered Earth.

The principal features which distinguish the atmosphere on Venus from that of the Earth are the slow rotation of the planet, the large mass of the atmosphere, and the opacity of the atmosphere to long-wave radiation. The slow rotation of the planet gives rise, first of all, to nongeostrophuc dynamics (the atmosphere gas has a tendency to move along the pressure gradient), with the result that the region of the main influx of solar energy is located on one side of the planet, and the region of maximum cooling on the other. These considerations lead to a much simpler scheme of circulation than that in the Earth's atmosphere. The large mass of the atmosphere is the cause of a high thermal and mechanical inertia, which explains why the atmospheric circulation is asymmetrical relative to the solar-antisolar axis. The daily center of circulation is displaced to the second half of the Cytherean solar day, i.e., to the line of zero budget of thermal energy corresponding to a height of the Sun abobe the horizon of about 20°. The notions of cold and warm regions are very relative for Venus. While the horizontal temperature differences on the Earth may reach 100°, a mean horizontal temperature drop as small as 3° in the Cytherean atmosphere may be looked upon as an exceptional phenomenon. This high thermal homogeneity is due to a very large thermal inertia, with cooling at the poles never manifesting itself in the temperature fields obtained. The opacity of the Cytherean atmosphere to long-wave radiation results in vertical heat transfer by turbulence, mesoscale convection, and large-scale currents. This produces adiabatic stratification in the troposphere and a high temperature in the lower layers. These phenomena were studied in a general manner using two- and three-level models. Steps have recently been undertaken to investigate in greater detail the vertical structure of the troposphere on Venus using ten-level models. It appeared that the vertical dynamic structure of the troposphere is very much dependent on the distribution in height of the solar energy influx. In the greenhouse model, the entire atmosphere is affected by circulation. Pronounced velocity maxima are observed in the lower and upper layers. In a model with adsorption of solar radiation in the upper layer, the velocity is small in the lower layers, but it rapidly increases and changes its direction several times in the upper layers. The mean kinetic energy of the atmosphere proves to be two to three times smaller than in the greenhouse model. Attempts have been made in the calculations to find the principal modes of the statistical fluctuations. The results obtained show that atmospheric circulation may be represented by a global mean basic state following the rotation of the planet with deviations from that basic state which are indeterminate disturbances. The mean basic state exhibits a high degree of symmetry relative to the equator. On account of nonlinearity, the disturbances were observed in all the models independently of space and time resolution. This phenomenon appears to reflect the actual properties of the Cytherean atmosphere and has no bearing on the details of the numerical scheme.

SUMMARY The convergence of heat below subsidence inversions was estimated from radiosonde ascents on fine summer days. Sensible heat input from the ground was estimated from measurements of heat fluxes and net radiation over a wheat field. Absorption and back scattering of solar radiation by aerosol throughout the atmosphere were calculated from measurements of solar radiation at the ground. Radiation absorbed by aerosol in the thermal boundary layer heated the lower atmosphere at an average rate of 3.3degC day-’ (60Wrn-’), about twice as fast as estimated for gaseous constituents. An equation is derived to show how the net effect of aerosol on the lower atmosphere depends on the reflection coefficient of the surface and on the aerodynamic and surface resistances to vapour transfer. Over the wheat, the net effect was almost zero when the crop was transpiring fast, but when the crop was mature, aerosol caused net cooling. I. INTRODUCTION As part of a wider attempt to identify possible causes of climatic change, attention has recently been directed to the absorption and scattering of solar radiation by aerosol, processes which modify the heat balance of the atmosphere and the radiation budget at the earth’s surface. Calculations by Mitchell (1971), and by others, demonstrate that aerosol may be responsible either for cooling or for heating the lower atmosphere depending on factors such as the height, thickness and optical properties of the layer and the relative inputs of sensible and latent heat from the ground. The main sources of uncertainty in predicting thermal effects of aerosol in the atmosphere are an incomplete knowledge of (i) optical properties and distribution of aerosol, and (ii) the relationship between net radiation and sensible heat flux at the earth’s surface. We have made direct measurements, during summer anticyclonic conditions in Britain, of heat storage in the lower atmosphere, coupled with measurements of surface radiation and heat fluxes. In the first instance, the measurements were analysed to show the importance of radiant energy absorbed by aerosol in determining the energy budget of the lower atmosphere. The analysis was then extended to estimate the influence of atmospheric aerosol on the input of sensible heat to the atmosphere from the ground. It appears that aerosol may be responsible for a net heating or cooling of the lower atmosphere at a rate which depends on surface reflectivity and on the ratio of aerodynamic and surface resistances to vapour transfer. 2. THEORY During anticyclonic weather in Britain, inversions formed by subsidence are common at heights between 1 and 2km. Below the main inversion, the atmosphere is heated by radiation and by an input of sensible heat from the ground. Heat may also be entrained from the warmer air above the inversion as discussed by Cattle and Weston (1975). For simplicity, we shall consider the heat budget of a ‘thermal boundary layer’ defined as that layer of the atmosphere above which there was no measurable change of temperature due 95

Weather is driven by the sun’s energy input and the difference between insolation per unit area of the poles and the equator. The energy flux of the Earth is in long‐term balance—as much is radiated away by the Earth as is absorbed, or the mean temperature would have to increase or decrease steadily (and, of course, this is not observed). CO2 and other ‘‘trace gases’’ can cause the Earth’s mean temperature to rise through the Greenhouse Effect. The mean temperature in the Little Ice Age was only 1 °C cooler, but large effects were felt, especially toward the poles. The CO2 which stays in the atmosphere will raise Earth’s mean temperature, with effects which are relatively certain: a lot of warming at the poles, and a very small amount of warming at the equator.

The present state of our knowledge of the greenhouse effect is discussed and the limitations of existing atmospheric models that are used to predict changes in the greenhouse effect are pointed out. In particular the possibly dominant effect of the cloud/radiation feedback process is mentioned. Current models predict about a 2 K rise in surface temperature for a doubling of the atmospheric CO 2 concentration and a 6 K rise if the CO 2 increased by a factor of 8 as is possible in the future if the increase in the burning of fossil fuels continues. Possible greenhouse effects due to the release of chlorofluoromethanes (c.f.ms) and aerosols into the atmosphere from man-made sources are small. Analysis of the surface temperature changes which have occurred in the Northern Hemisphere during the last 100 years are consistent with most of the changes being due to increased CO 2 and decreased stratospheric aerosol.

Computer simulations of the evolution of the earth's atmospheric composition and surface temperature have been carried out. The program took into account changes in the solar luminosity, variations in the earth's albedo, the greenhouse effect, variation in the biomass, and a variety of geochemical processes. Results indicate that prior to two billion years ago the earth had a partially reduced atmosphere, which included N2, CO2, reduced carbon compounds, some NH3, but no free H2. Surface temperatures were higher than now, due to a large greenhouse effect. When free O2 appeared the temperature fell sharply. Had earth been only slightly further from the sun, runaway glaciation would have occurred at that time. Simulations also indicate that a runaway greenhouse would have occurred early in earth's history had earth been only a few percent closer to the sun. It therefore appears that, taking into account the possibilities of either runaway glaciation or a runaway greenhouse effect, the continuously habitable zone about a solar-type star is rather narrow, extending only from roughly 0.95 to 1.01 AU.

There is general agreement that certain surface features on Mars are indicative of the presence of liquid water at various times in the geologic past. In particular, the valley networks are difficult to explain by a mechanism other than the flow of liquid water. It has been suggested in several studies that a thick CO2 atmosphere on Mars early in its history could have provided a greenhouse warming that would have allowed the flow of water either on the surface or just below the surface. However, this effect was examined with a detailed radiation model, and it was found that if reduced solar luminosity early in the history of the solar system is taken into account, even three bars of CO2 will not provide sufficient greeenhouse warming. The addition of water vapor and sulflur dioxide (both plausible gases that may have been emitted by Martian volcanoes) to the atmosphere also fail to warm the surface above 273 K for reduced solar luminosity conditions. The increase in temperature may be large enough, however, for the formation of these features by brines.

A model is offered through which the solar irradiation can be computed inside a single span hemicylindrical tunnel greenhouse. The model is mathematically exact. From the physical point of view it is founded on the assumption that the cladding surface is fully diffusive and that the radiation diffused through the atmosphere and by the ground is fully isotropic. It is found that the solar irradiation inside the greenhouse depends upon its orientation. A greenhouse with its longitudinal axis aligned along the north-south direction collects more radiation in summer and less in winter as compared with a greenhouse with its axis aligned along the east-west direction. On the other hand the east-west oriented greenhouse is more efficient in collecting solar radiation in winter than in summer, while the north-south oriented greenhouse exhibits a constant efficiency the year round. Finally, it is shown that the collected radiation depends upon the optical properties of the cladding surface being mainly determined by its transmittance.

The influence of the expected rise of CO 2 content in our atmosphere upon terrestrial temperature is uncertain. A significant increase in temperature could be threatening to certain aspects of terrestrial biology. On the other hand, it is a general consensus among paleobiologists that the Earth possessed a CO 2 atmosphere in the past billion years, without dramatic temperature variations endangering the continuity of life. In order to clarify this problem, and to contribute to the understanding of the CO 2 greenhouse effect on Venus we have computed the absorption spectrum of CO 2 for a wide range of atmospheric concentrations. More than 2500 spectral lines of the 15 micron band were taken into account in our line-by-line calculation. We have used an empirical exponential line-shape function at the line edges. Our results agree with the experimental data of F.W. Taylor. The estimated increase in surface temperature does not reach the boiling point of water even for CO 2 concentrations thousands of times larger than the present concentrations. Higher energy (> 666 cm −1 CO 2 bands and/or an increase in atmospheric H 2 O may, however, amplify the greenhouse effect.

The study of the radiative balance in the atmosphere is very complex and difficult. The factors which determine the equilibrium are known in general terms, but the problem of developing a detailed understanding, particularly if the aim is to assist meteorological prediction, is another matter. Only part of the incoming solar radiation reaches ground level without being affected by the atmosphere; the remainder is either reflected back into space or absorbed or scattered by the atmosphere. The incident radiant energy absorbed in this way is degraded to longer wavelengths and is radiated from the atmosphere as infra-red radiation. Part of this reaches the surface and is absorbed there, while the remainder is re-radiated to outer space. Some heat also reaches the surface from the atmosphere by eddy-current transport. Again, energy leaves the surface in the form of infra-red radiation. Most of this is absorbed by the water vapour in the atmosphere, except in the wavelength region 8 to 13 μ . Radiation in this wave­length ‘window’ passes right through the atmosphere to outer space. Heat is also carried to the atmosphere from the surface by evaporation, condensation and precipitation phenomena.

In previous models of the greenhouse effect in the Venus atmosphere it has been assumed that infrared-absorbing atmospheric gases provide the sole contribution to the infrared opacity of the Venus atmosphere. In the present study the influence of an extensive cloud cover, opaque to infrared radiation, is also included in the greenhouse model. The magnitude of the greenhouse effect, which is defined here as the ratio of the surface temperature produced by the greenhouse to the surface temperature of an atmosphereless Venus, is computed as a function of infrared opacity of the atmosphere, and amount and height (actually ratio of cloud-top pressure to surface pressure) of clouds. It is assumed that the Venus atmosphere is gray, that the absorbing gas has a constant mixing ratio, and that the temperature variation with altitude is linear. Calculations are made for two temperature lapse rates: the adiabatic lapse rate and nine-tenths of the adiabatic lapse rate. The adiabatic lapse rate maximizes the greenhouse effect; for this case estimates of the minimum infrared opacity required to maintain the observed surface temperature can be determined. For a surface temperature of 700°K, 99 per cent cloudiness, and cloud-top temperature of 240°K, the minimum required infrared opacity is 6. Uncertainties and questionable side effects of the model are discussed. The model is quite general and can be applied to other planetary atmospheres; sample calculations are shown for the earth and Mars.

A formalism is developed for solving the radiative transfer equation in the presence of imbedded heat sources. This is accomplished by demonstrating the equivalence of an internally heated model atmosphere to the well-known problem of diffuse reflection from an isotropically scattering atmosphere. A replacement algorithm is then used to calculate the steady state limb-darkening and vertical thermal profile of a semi-infinite, plane-parallel atmosphere heated solely from above. In our model the heat source, due to direct absorption of sunlight, is a maximum at the top of the atmosphere and falls off exponentially with increasing depth. The greenhouse factor, defined as the ratio of the Planck temperature for large depth and the effective temperature of an externally viewed planet, is found as [B(∞)/B eff ] =√3H(μ 0 )/4, where μ 0 is the ratio of infrared and visible absorption coefficients. H (μ 0 is the Chandrasekhar H function, generalized for values of the argument μ 0 greater than unity. The strong dependence of the greenhouse effect on the ratio of absorption coefficients at differing spectral regions is expected. It comes as an intuitive surprise, however, to note that a pure gray atmosphere [μ 0 = 1, H(μ 0 ) = 2.9078] whose heating is concentrated at the upper levels nonetheless exhibits limb-darkening and steady state increase of temperature with depth.

Calculations of the mean surface temperature of Venus are performed with a simple nongray radiation balance model. The model is based upon a balance of net incoming solar radiation and emerging thermal radiation at the top of the atmosphere. To calculate the emerging thermal radiation, it is assumed that the shape of the vertical temperature profile is similar to that observed by Mariner 5 and Venera 4—that is, a constant lapse rate of 9°C/km from the surface to a pressure level of a few tenths of an atmosphere, above which the temperature remains constant. Given the atmospheric composition and surface pressure, the surface temperature can be determined from the balance requirement at the top of the atmosphere. Calculations are performed for a low surface pressure (20 atm) and high surface pressure (65 atm) model. For a pure carbon dioxide atmosphere, the results indicate that mean surface temperatures of 500° to 550°K can be maintained in the 20-atm model, and 600° to 650°K in the 65-atm model, if water vapor mixing ratios are of the order of 10 −3 .

The theory of radiative equilibrium demands that on the average the total amount of energy absorbed by the earth and its atmosphere in the form of short-wave solar radiation should be exactly equal to the total amount of energy given back to space in the form of long-wave heat radiation. From a study of the absorbing and radiating properties of the atmosphere, F. Albrecht arrived at the fundamental result that the major contribution to the long-wave heat radiation into outer space originates from a layer of some three to four kilometres thickness in the upper troposphere, which he designates as the “Emission Layer”. The emission layer is thus a portion of the upper atmosphere which is continually cooling due to radiative loss of heat. The height of the emission layer is a function of the water vapour content of the atmosphere; it is more when the atmosphere is hot and humid and less when the atmosphere is cold and dry. The author has made a detailed study of the thermal structure of the atmosphere over Agra based on the results of sounding balloon ascents over a period of ten years. A number of interesting features find a ready explanation on the assumption that the emission layer over Agra is located approximately between 11 and 14 gkm. in the monsoon months and between 8 and 11 gkm. during the remaining months, —an assumption in conformity with Albrecht’s work. The observed seasonal variations in the thermal structure of the atmosphere over Agra thus lend, strong evidence for the existence of the emission layer in the atmosphere and the variation of its altitude depending upon the moisture content of the atmosphere.