Inside Clouds Research Articles

Temperature Inversion on the Surface of Externally Heated Optically Thick Multigrain Dust Clouds

It was recently discovered that the temperature in the surface layer of externally heated optically thick gray dust clouds increases with the optical depth for some distance from the surface, as opposed to the normal decrease in temperature with distance in the rest of the cloud. This temperature inversion is a result of efficient absorption of diffuse flux from the cloud interior by the surface dust exposed to external radiation. Grains of size 1 μm or bigger experience this effect when the external flux is of stellar spectrum. We explore what happens to the effect when dust is a mixture of grain sizes (multigrain). Two possible boundary conditions are considered: (1) a constant external flux without constrains on the dust temperature, and (2) the maximum dust temperature set to the sublimation temperature. We find that the first condition allows small grains to completely suppress the temperature inversion of big grains if the overall opacity is dominated by small grains. The second condition enables big grains to maintain the inversion even when they are a minor contributor to the opacity. In reality, the choice of boundary condition depends on the dust dynamics. When applied to the physics of protoplanetary disks, the temperature inversion leads to a previously unrecognized disk structure in which optically thin dust can exist inside the dust destruction radius of an optically thick disk. We conclude that the transition between the dusty disk and the gaseous inner clearing is not a sharp edge, but rather a large optically thin region.

Supersaturation, dehydration, and denitrification in Arctic cirrus

Abstract. A polar cirrus case study is discussed with the help of a one-dimensional model with explicit aerosol and ice microphysics. It is demonstrated that continuous cooling of air in regions with small amounts of ice and slow ice deposition rates of water vapor drives significant in-cloud supersaturations over ice, with potentially important consequences for heterogeneous halogen activation. Radiatively important cloud properties such as ice crystal size distributions are investigated, showing the presence of high number concentrations of small crystals in the cloud top region at the tropopause, broad but highly variable size spectra in the cloud interior, and mostly large crystals at the cloud base. It is found that weakly forced Arctic cirrostratus are highly efficient at dehydrating upper tropospheric air. Estimating nitric acid uptake in cirrus with an unprecedented treatment of diffusion-limited trapping in growing ice crystals suggests that such clouds could also denitrify upper tropospheric air masses efficiently, but a closer comparison to suitable observations is needed to draw a definite conclusion on this point. It is also shown that low temperatures, high ice supersaturations, and the absence of ice above but close to the cloud top region cause efficient uptake of nitric acid in background aerosol particles.

Spatial distribution of cloud droplets in a turbulent cloud‐chamber flow

AbstractWe present the results of a laboratory study of the spatial distribution of cloud droplets in a turbulent environment. An artificial, weakly turbulent cloud, consisting of droplets of diameter around 14µm, is observed in a laboratory chamber. Droplets on a vertical cross‐section through the cloud interior are imaged using laser sheet photography. Images are digitized and numerically processed in order to retrieve droplet positions in a vertical plane. The spatial distribution of droplets in the range of scales, l, from 4 to 80 mm is characterized by: the clustering index CI(l), the volume averaged pair correlation function η(l) and a local density defined on a basis of correlation analysis. The results indicate that, even in weak turbulence in the chamber that is less intense and less intermittent than turbulence observed in clouds, droplets are not spread according to the Poisson distribution. The importance of this deviation from the Poisson distribution is unclear when looking at CI(l) and η(l). The local density indicates that in small scales each droplet has, on average, more neighbours than expected from the average droplet concentration and gives a qualitative and intuitive measure of clustering. Copyright © 2005 Royal Meteorological Society.

MOUNT FUJI [CI] LINE SURVEY

We have constructed the Mount Fuji submillimeter-wave telescope at Nishiyasugawara (alt. 3725 m) near the summit of Mt. Fuji (alt. 3774 m). Thanks to the excellent condition of Mt. Fuji, we have successfully carried out the [CI] survey toward more than 40 square degrees of sky, including qrion MC, Taurus MC, Rosetta MC, DR 15, DR 21, NGC 1333, NGC 2264, W 3, W 44, W 51, L 134, p-Oph. Our [CI] survey have revealed that the [CI] 492 GHz emission widely extends to the molecular clouds. The spatial and velocity structures of the [CI] 492 GHz emission resemble those of 13CO J=l-0 in many molecular clouds, implying that [CI] 492 GHz and <TEX>$^{13}CO$</TEX> J=1-0 are emitted from the same gas. The column density of <TEX>$C^o$</TEX> linearly correlates with that of CO up to high Av, suggesting that <TEX>$C^o$</TEX> exist in the deep interior of molecular clouds. In several regions, we have found that the distributions of <TEX>$C^o$</TEX> and CO are different from each other. The <TEX>$C^o$</TEX>-rich area is found in the Hieles' cloud 2. The C+/CO/<TEX>$C^o$</TEX> configuration is found in DR 15, p-Oph, M 17, Orion KL, and NGC 1333. These results indicate that an origin of <TEX>$C^o$</TEX> is unrelated with the photodissociation process. We discuss the observed <TEX>$C^o$</TEX> distributions in relation to the non-equilibrium chemistry.

Evaluation of the Dropsonde Humidity Sensor Using Data from DYCOMS-II and IHOP_2002

Abstract The dropsonde humidity data have not been fully utilized due to lack of knowledge on performance of the dropsonde humidity sensor. This study evaluates the performance of the dropsonde humidity sensor using data collected from two field experiments, the Dynamics and Chemistry of Marine Stratocumulus Phase II: Entrainment Studies (DYCOMS-II) and the International H2O Project (IHOP)_2002. During DYCOMS-II, 63 dropsondes were dropped above marine stratocumulus clouds. It provides a unique opportunity to evaluate the performance of the dropsonde humidity sensor within clouds. Relative humidity (RH) inside clouds did not reach 100% all the time, but the maximum RH reached 100% for 28% of soundings and was within the sensor accuracy range (94%–100%). This suggests that the dropsonde humidity sensor displays no systematic dry bias near saturation. The dropsonde humidity sensor experienced large time-lag errors when it descended from a dry environment above clouds into clouds. The mean estimated time constant of the sensor is 5 s at 15°C, which is much larger than 0.5 s at 20°C given by the manufacturer. The humidity sensor still reported near-saturation RH after it exited clouds because of water on the sensor. The approximately coincident dropsonde and aircraft temperature data during DYCOMS-II show that the dropsonde underestimates temperatures inside and below clouds by averages of 0.21° and 0.93°C, respectively. Seventy-one pairs of dropsonde and radiosonde soundings during IHOP_2002 were launched within a half hour and 50 km and sampled the same air mass based on the visual examination. The comparisons show that the dropsonde and radiosonde RH data agree with each other within ±2% RH, suggesting no systematic dry bias in dropsonde humidity data. However, dropsonde-measured temperature is consistently colder than that by radiosonde by ∼0.4°C.

Chemistry ahead of Herbig-Haro objects

There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are ‘illuminated’ due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds.

Inference of Cloud Optical Depth from Aircraft-Based Solar Radiometric Measurements

A method is introduced for inferring cloud optical depth τ from solar radiometric measurements made on an aircraft at altitude z. It is assessed using simulated radiometric measurements produced by a 3D Monte Carlo algorithm acting on fields of broken boundary layer clouds generated from Landsat imagery and a cloud-resolving model. The method uses upwelling flux and downwelling zenith radiance measured at two solar wavelengths where atmospheric optical properties above z are very similar but optical properties of the surface–atmosphere system below z differ. This enables estimation of cloud reflectance into nadir for upwelling diffuse flux and, finally, τ above z. An approximate one-dimensional radiative Green's function is used to roughly account for horizontal transport of photons in all, even broken, clouds. This method is compared to its surface-based counterpart and shown to be superior. Most notably, the aircraft-based approach deals easily with inhomogeneous land surfaces, is less susceptible to poor sampling, and need not account for aerosol below z. The algorithm appears as though it will have little difficulty inferring high-resolution time series of τ ≲ 40 for most (single layer) clouds. For larger values of τ, biases emerge; particularly, underestimation for the statistically infrequent interiors of cumuliform clouds as photon leakage through cloud sides is not addressed. For the cumuliform and stratiform clouds used here, mean bias errors for retrieved τ are ∼1 (or ∼15%) and ∼0.3 (or ∼3%), respectively. For stratiform clouds with textured bases, performance is likely to improve slightly for flights just up from mean cloud base.

H2Emission Spectra with New Formation Pumping Models

It has been predicted that H2 newly formed on the surface of dust grains should be in highly vibrationally and rotationally excited states in the ground electronic state via the formation pumping mechanism. This suggests that the rovibrational emission spectrum of H2 may be detectable even in regions without a source of UV pumping or dynamical excitation. Moreover, the infrared H2 emission arising from formation pumping can be a new probe that will directly show us the spot where the evolution from H I clouds to H2 clouds occurs. In the present work, we investigate the pure effects of formation pumping in infrared H2 emission spectra with our new formation pumping models. We build up our formation pumping models for H2 newly formed on icy mantles, carbonaceous dust, and silicate dust, on the basis of the recent progress of basic studies about the H2 formation process on the surface of dust grains. Then, we calculate the H2 emission spectra with our new formation pumping models, considering the early stage of an evolving region from H I clouds to H2 clouds dominated by the H2 formation process on the surface of dust grains, of uniform H I density nH = 103 cm-3 and the gas temperature T = 10 K, in the cold interior of interstellar clouds under a weak ultraviolet (UV) radiation field. We find that the resulting infrared H2 emission spectra dominated by formation pumping can be discriminated from those dominated by UV pumping as well as have information about the property of dust. We also find that the fluxes of the most intense emission lines arising from formation pumping should be strong enough to be detected. In the wavelength range λ 1.6 μm, if the line intensities of many transitions from higher rotational states such as 1-0 S(3), 1-0 S(5), and 1-0 S(7) become stronger than that of the 1-0 S(1) transition, it should be the evidence of H2 emission arising from formation pumping. In the wavelength range λ 1.6 μm, if many transitions from higher vibrational states are detected, the emission should be originated from H2 newly formed on icy mantles or silicate dust. On the contrary, if not, the emission should be from H2 formed on carbonaceous dust.

Gas‐Phase Production of NHD2in L134N

We show analytically that large abundances of NH2D and NHD2 can be produced by gas phase chemistry in the interiors of cold dense clouds. The calculated fractionation ratios are in good agreement with the values that have been previously determined in L134N and suggest that triply-deuterated ammonia could be detectable in dark clouds. Grain surface reactions may lead to similar NH2D and NHD2 enhancements but, we argue, are unlikely to contribute to the deuteration observed in L134N.

High-Resolution Airborne Radar Observations of Mammatus

Abstract High-resolution Doppler radar observations of mammatus clouds coupled with soundings of the preanvil and anvil environments provide a unique opportunity to examine previously reported observations of, and evaluate various hypotheses of, mammatus formation. These observations confirm the general hypothesis for mammatus formation advanced by Ludlam and Scorer, and provide detail of the cloud interior structure. Specifically, the radar observations indicate that mammatus elements are reminiscent of eddy circulations with a weak downdraft core flanked by horizontal convergence and divergence at the top and base of the cloud, respectively. Doppler spectral width measurements, however, yielded values of only 2–3 m s−1, indicating only weak turbulent motions within individual mammatus elements. Reflectivity analyses of mammatus elements indicate a firm link to the parent anvil. A dual-Doppler analysis of the parent anvil indicates that the larger-scale environment where the mammatus exist is characteriz...

A Simple Model for the Absorption of Starlight by Dust in Galaxies

We present a new model for computing the effects of dust on the integrated spectral properties of galaxies, based on an idealized description of the main features of the interstellar medium (ISM). The model includes the ionization of H II regions in the interiors of the dense clouds in which stars form and the influence of the finite lifetime of these clouds on the absorption of radiation. We compute the production of emission lines and the absorption of continuum radiation in the H II regions and the subsequent transfer of line and continuum radiation in the surrounding H I regions and the ambient ISM. This enables us to interpret simultaneously all the observations of an ultraviolet-selected sample of nearby starburst galaxies, including the ratio of far-infrared to ultraviolet luminosities, the ratio of Hα to Hβ luminosities, the Hα equivalent width, and the ultraviolet spectral slope. We show that the finite lifetime of stellar birth clouds is a key ingredient for resolving an apparent discrepancy between the attenuation of line and continuum photons in starburst galaxies. In addition, we find that an effective absorption curve proportional to λ-0.7 reproduces the observed relation between the ratio of far-infrared to ultraviolet luminosities and the ultraviolet spectral slope. We interpret this relation most simply as a sequence in the overall dust content of the galaxies. The shallow wavelength dependence of the effective absorption curve is compatible with the steepness of known extinction curves if the dust has a patchy distribution. In particular, we find that a random distribution of discrete clouds with optical depths similar to those in the Milky Way provides a consistent interpretation of all the observations. A noteworthy outcome of our detailed analysis is that the observed mean relations for starburst galaxies can be closely approximated by the following simple recipe: use an effective absorption curve proportional to λ-0.7 to attenuate the line and continuum radiation from each stellar generation, and lower the normalization of the curve, typically by a factor of 3 after 107 yr, to account for the dispersal of the birth clouds. This recipe or our full model for absorption can be incorporated easily into any population synthesis model.

Discrepancies between measured and modeled solar and UV radiation within polluted boundary layer clouds

Using aircraft measurements and sophisticated radiative transfer calculations, a significant gap between measured and modeled radiation inside clouds is revealed in two out of four detailed case studies of polluted boundary layer clouds. In contrast to previous aircraft studies on that subject, vertical profiles of in situ measured aerosol and drop microphysical data are included in the one‐dimensional spectral radiative transfer calculations. Instead of comparing the calculated cloud absorption with the measured one, we focus on the calculated and measured vertical profiles of solar and ultraviolet (UV) downwelling and upwelling irradiances and respective albedo profiles. These profile data are much less affected by measurement uncertainties compared to the cloud absorption, which results as residual of two net irradiances at cloud top and base. The enhanced (with respect to the calculations) solar cloud absorption is estimated to reach values up to 37 W m−2. For the UV data the discrepancy between measured and calculated down‐ and upwelling irradiances (and albedo) inside the clouds is less pronounced, although it seems to be significant as well. The reason for the discrepancies between measured and calculated radiation inside the clouds is still unresolved. Additional radiative transfer calculations show that uncertainties of surface albedo, vertical cloud inhomogeneities, and pollution of cloud drops by inclusion of strongly absorbing material are not responsible for the gap between measurements and calculations within the cloud. There is a clear need for spectrally resolved measurements and improved model calculations taking into account cloud inhomogeneities to solve this problem.

Optical Emission Lines from Warm Interstellar Clouds: A Decisive Test of the Decaying Neutrino Theory

Recently developed instruments such as the Taurus Tunable Filter and WHAM should be able to detect some or all of the optical emission lines H$\alpha$, [OI] $\lambda$6300, [SII] $\lambda$6717, [NI] $\lambda$ 5200 and [NII] $\lambda$6584 from warm interstellar clouds such as those observed by Spitzer $&$ Fitzpatrick (1993) (SF) along the line of sight to the halo star HD93521. The strengths of these lines should resolve the debate as to whether the free electrons, which SF held responsible for the observed excitation of CII in the clouds, are located mainly in the skins of the clouds or in their interiors. If the free electrons are indeed mainly located in the cloud interiors, then the substantial electron density derived by SF, and its constancy from cloud to cloud for the slow-moving clouds, when combined with their opacity to Lyman continuum radiation, lend strong support to the decaying neutrino theory for the ionisation of the interstellar medium (Sciama 1990, 1993 a, b, 1997). If the [OI] and [NI] lines are relatively strong but the [NII] line is weak, then this would lend further, decisive, support to this theory, since decay photons are unable to ionise N, although its ionisation potential is only 0.9 eV greater than that of H.

OH measurements during the First Aerosol Characterization Experiment (ACE 1): Observations and model comparisons

Airborne measurements of the hydroxyl radical, OH, performed during the First Aerosol Characterization Experiment (ACE 1), using the Selected Ion Chemical lonization Mass Spectrometry (SICIMS) technique are presented. Adaptations of the previous ground‐based technique for measurement aboard an aircraft platform are discussed, including an inlet for the straightening and slowing of the airflow, calibration, and measurement considerations at changing pressure altitudes. Steady state model calculations of the concentration of OH, [OH], throughout the entire mission were generally in good agreement, with a slight bias toward an overestimate of the measured [OH]. The largest discrepancies between measured and modeled values occurred for measurements in the boundary layer, and those in and around clouds, with the model overestimating the [OH] by ∼40% in the boundary layer or inside clouds, and underestimating it by ∼30% near clouds. The low model [OH] near clouds can be attributed to underestimating the actinic flux calculated from Eppley radiometer measurements. The model overestimates in the boundary layer and inside clouds may in part be due to a lack of heterogeneous losses of HOx species in the model. Models developed at the National Oceanic and Atmospheric Administration Aeronomy Laboratory and at Georgia Institute of Technology produce similar results with differences being attributed to the methods of calculating photolysis rates. Calculations of high noon [OH] for a flight out of Hobart using the O(1D) quantum yields from Talukdar et al. [1998] produce 17% higher values than those calculated using the currently recommended Jet Propulsion Laboratory values.

First geometrical pathlengths probability density function derivation of the skylight from spectroscopically highly resolving oxygen A‐band observations: 1. Measurement technique, atmospheric observations and model calculations

First measurements of the probability density function of geometrical pathlengths (PDF‐GP) for the skylight transmitted from clear and cloudy skies to the terrestrial surface are reported. The measurements are performed using a novel technique, which includes spectroscopically highly resolving observations of the O2 A‐band (760–780 nm) and a Laplacian back transformation of the intensity ratios measured for the set of individual O2 A‐band rotational absorption lines. The technique provides a new and powerful tool to study the radiative transfer (RT) of the atmosphere for different kinds of atmospheric aerosol loadings and cloudiness. First measurements show, as expected, that the photons transmitted by clouds to ground generally experience longer geometrical paths inside the cloud than expected for a corresponding clear sky condition. The comparison of the measured oxygen A‐band absorption system with plane‐parallel discrete ordinate radiative transfer (DISORT) model calculations shows a reasonable agreement for direct Sun and clear sky zenith observations. For cloudy sky the same comparison shows a good agreement between the observed and the modeled weak absorption lines but systematically larger observed than modeled optical densities for strong absorption lines. Our data also suggest that this significant discrepancy is most likely caused by shortcomings in the physical description of the RT by nonstatistical models, which do not properly account for the fractal nature of the terrestrial cloud cover. In particular our study reveals that the photon path statistics involved with the multiple Mie scattering inside clouds is not properly modeled using the DISORT code. Closing this gap in our understanding of the cloud sky RT may possibly provide much of the explanation for the frequently lower modeled than measured cloudy sky absorption of the solar radiation.

On the Efficiency of Grain Alignment in Dark Clouds

A quantitative analysis of grain alignment in the filamentary dark cloud L1755 in Ophiuchus is presented. We show, that the observed decrease of the polarization to extinction ratio for the inner parts of this quiescent dark cloud can be explained as a result of the decrease of the efficiency of grain alignment.We make quantitative estimates of grain alignment efficiency for six mechanisms, involving grains with either thermal or suprathermal rotation, interacting with either magnetic field or gaseous flow. We also make semiquantitative estimates of grain alignment by radiative torques.We show that in conditions typical of dark cloud interiors, all known major mechanisms of grain alignment fail. All the studied mechanisms predict polarization at least an order of magnitude below the currently detectable levels of order of 1%. On the contrary, in the dark cloud environments where A_v<1 the grain alignment can be much more efficient. There the alignment of suprathermally rotating grains with superparamagentic inclusions, and possibly also radiative torques account for observed polarization. These results apply to L1755, which we model in detail and probably also to B216 and other similar dark clouds. Our study suggests an explanation for the difference in results obtained through polarimetry of background starlight and polarized thermal emission from the dust itself. We conjecture that the emission polarimetry selectively reveals aligned grains in the environment far from thermodynamic equilibrium as opposed to starlight polarization studie that probe the alignment of grains all the way along the line of sight, including the interiors of dark quiescent clouds, where no alignment is possible.

On the Ionization of Warm Opaque Interstellar Clouds and the Intercloud Medium

This paper uses a variety of observations to construct an integrated picture of the ionization in the interiors of quiescent warm opaque interstellar clouds and in the intercloud medium (ICM) outside dense H II regions and hot dilute bubbles. Our main conclusion is that within ~1 kpc of the Sun the ionization rate of hydrogen per unit volume both in the interiors of such clouds and in the ICM is independent of the local density of neutral hydrogen and varies with position by less than ~20%. These conclusions strongly favor the decaying neutrino hypothesis for the ionization of the interstellar medium in these regions. Our analysis is based on a variety of observations, of which the most remarkable is the 1993 discovery by Spitzer & Fitzpatrick that, in the four slowly moving clouds along the line of sight to the halo star HD 93521, the column densities of both S II and C II*, which individually range over a factor of ~4, are proportional to the column density of H I to within ~20%. This proportionality is used to show that the free electrons exciting the C II to C II* are located mainly in the interiors of the clouds, rather than in their skins, despite the large opacity of the clouds to Lyman continuum radiation. The same conclusion also follows more unambiguously from the low value of the Hα flux in this direction, which was found by Reynolds in unpublished observations. These results are then used, in conjunction with observations of three pulsar parallaxes and dispersion measures, and with data on He I, N II, and O I line emissions, to constrain the ionization of H, He, N, and O and the flux of Lyman continuum photons from O stars in the ICM. It is argued that the total density of the ICM is kept close to the electron density in the clouds by a regulation mechanism, and a possible such mechanism is suggested. This picture of the ionization of the ICM also favors the decaying neutrino theory.

Cloud fraction errors caused by finite resolution measurements

The standard method of deriving cloud fraction from space simply finds the fraction of total image pixels that contains some cloud. The sensitivity of this method to sensor resolution has been examined by Shenk and Salomonson [1972], assuming the cloudy pixels are detected perfectly. Their experiment is reexamined to show that the sensitivity is much more complex than they predicted. We derive the upper and lower bounds of the true cloud fraction, At, given the standard method estimate, Ae, to show that the range of possible values for At can be, in general, very wide. By including the fraction of apparent cloud edge and cloud interior pixels, the bounds can be reduced and improvements to the standard method can be obtained. However, this improvement is also resolution limited by the misidentification of partially cloudy pixels as cloud interior rather than cloud edge. A potentially better technique for estimating the true cloud fraction is therefore explored using a pattern recognition approach. A nearest neighbor classification rule is used in two sets of experiments: one using 684 simulated cloud fields as a training set, the other using 370 cloud fields based on Advanced Very High Resolution Radiometer (AVHRR) measurements. Given the underlying distribution of At, Ae overestimates At with an overall average bias of 32% and standard error of 11% for the simulated training set, and a bias of 35% and a standard error of 3% for the AVHRR training set. The pattern recognition estimate, Ap, is essentially unbiased and has a standard error of 12% for both training sets. The relevance of these training sets to new scenes and the importance of imperfect cloud detection have yet to be investigated, but the pattern recognition technique shows considerable potential advantage over the standard technique in providing unbiased estimates of cloud fraction that are less sensitive to the effects of sensor resolution.

The regulatory role of Rv in the photochemistry of dark clouds

The penetration of UV radiation inside dark clouds is discussed; and detailed calculations of R v-dependent photorates are described. Using an accurate method for solving radiative transfer in the presence of coherent anisotropic. non-conservative scattering by dust grains, the role of the parameter R v as the main regulatory agent of the penetration of radiation inside clouds is evidenced. Local variations in optical properties of dust and their possible link with the evolution of the cloud are also discussed.

Field observations in continental stratiform clouds: Partitioning of cloud particles between droplets and unactivated interstitial aerosols

The partitioning of cloud particles between activated droplets and unactivated interstitial aerosols is a primary determinant of cloud microphysical, radiative, and chemical properties. In the present study, high‐resolution aircraft measurements (1 s, ∼60 m) of the number concentrations (Namp and Ncd) of accumulation‐mode particles (AMP, 0.17 to 2.07 μm diameter) and cloud droplets (CD, 2 to 35 μm diameter), made during 10 flights in and around continental stratiform clouds near Syracuse, New York, in autumn 1984 have been used to study the local and instantaneous nature of cloud particle partitioning throughout the sampled clouds. The partitioning is defined as the activated fraction F (≡Ncd/Ntot) of all measured cloud particles (Ntot ≡ Namp + Ncd). F may be interpreted approximately as the AMP activation efficiency which is often assumed to be unity in all clouds. In the present study, F varied over its full possible range (0 to 1), being low especially in cloud edges. Even in the near‐adiabatic parts of cloud interior, its variation ranged from 0.1 to 1 over the 10 days. Statistically, its value in cloud interior exceeded 0.9 in 36% of the data but was below 0.6 in 28%. On 5 of the 10 days, stratocumulus clouds were embedded in cool, dry, and relatively clean (Ntot &lt; 600 cm−3) northerly air masses. In such cases, cloud droplet concentration increased approximately linearly with increasing total particle loading, and F in cloud interior was near unity and relatively insensitive to changes in the influencing variables. On the other days, especially in stratus clouds embedded in warm and polluted southerly air masses, F was significantly less than unity, with particles in the smallest size ranges (0.17 to 0.37 μm) activating only fractionally depending on several factors. An important feature of the clouds sampled in this study was the existence of multiple cloud layers and complex vertical thermal structure on most days. Consequently, our analysis of the dependence of F on influencing cloud variables has been based on data grouped into individual cloud layers. Besides the size of the precursor aerosol, we found total particle loading (Ntot) and the local vertical cooling rate (∼ temperature lapse rate in individual layers) to influence F the most. In particular, F decreased with increasing particle loading in excess of about 800 cm−3, and increased nearly linearly with temperature lapse rate. Evidently, the activation process can become self‐limiting in stratiform clouds under polluted conditions, in which case increasing anthropogenic aerosol loading of the atmosphere translates less and less into cloud droplet population. This observation has important implications with respect to cloud radiative forcing, precipitation formation and acidification, and for long range transport of the unactivated aerosols.