Cloud Extinction Research Articles

Total Surface Area Estimates for Individual Ice Particles and Particle Populations

Abstract Representations for the surface area of ice particles in terms of the projected area have been developed using two different methods. The first method uses ice particles that are imaged in situ and geometric calculations that are based on the outline of the two-dimensional image of the particle. The second method uses computer-generated ice particle shapes and calculates the total surface area analytically. The results of the second method compare reasonably well with the results of the first method. Surface area estimates for individual particles were combined with particle size distribution and projected area measurements from the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL)–Florida Area Cirrus Experiment (FACE) field project to give total surface area estimates for observed ice particle populations. Population surface area estimates were also made from balloon-borne replicator data collected during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment, phase II (FIRE-II). A relationship between the particle population surface area and projected area (cloud extinction) has been derived. The total particle surface area for particle populations is estimated to be between 8 and 10 times the projected area or between 4 and 5 times the extinction and has a small dependence on the properties of the particle size distribution for particles observed in random orientations.

Towards cloud detection and cloud frequency distributions from MIPAS infra-red observations

Historically, clouds have been observed principally using nadir sounding but more recently, limb sounding observations have begun to reveal details of clouds in the upper troposphere, mainly sub-visible cirrus, and in the polar lower stratosphere. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), launched on ENVISAT in March 2002, provides an unprecedented opportunity to examine the infra-red emission spectra of clouds at high spectral resolution (0.025 cm −1 unapodised) and vertical resolutions of 3 km. In this paper, we examine the efficiency of a cloud index which is used operationally to detect clouds in MIPAS data and show that water vapour, the chief gaseous influence on the cloud index, does not affect the detection at altitudes of 12 km and above. We show further that it is possible to map the cloud index to an effective cloud extinction coefficient provided that either water vapour concentrations are known or else: (i) at 18 km and above in the tropics; (ii) at 15 km and above in mid-latitudes; (iii) between 15 and 27 km in polar winter. We apply a conservative cloud detection limit of 2.2 to available MIPAS level 1b off-line data (calibrated emission spectra) versions 4.61 and 4.62, to derive seasonal cloud frequency distributions. For altitudes between 12 and 30 km, higher cloud frequencies are found in the tropics and in the region of the Antarctic ozone hole, whilst relatively low cloud frequencies are observed over mid-latitudes and in the Arctic stratospheric vortex of 2002–2003.

Subtropical cirrus cloud extinction to backscatter ratios measured by Raman Lidar during CAMEX‐3

The NASA/GSFC Scanning Raman Lidar was stationed on Andros Island, Bahamas for the third Convection and Moisture Experiment (CAMEX 3) held in August–September, 1998 and acquired an extensive set of cirrus cloud measurements [Whiteman et al., 2001]. Distinct differences in the optical properties of the clouds are found when the cirrus are hurricane‐induced versus non‐hurricane‐induced. Hurricane‐induced cirrus clouds are found to generally possess lower values of extinction‐to‐backscatter ratio (S) than non‐hurricane‐induced clouds. Comparison of the S measurements made here with those of other studies reveal at times large differences. Given that S is often a required parameter for space‐based retrievals of cloud optical depth using backscatter lidar, these large differences in S measurements imply difficulties in developing a parameterization of S for use in space‐based lidar retrievals.

Extinction of multi-species char clouds in pulverised fuel combustors

The extinction characteristics of multi-species char clouds in pulverised fuel combustors were theoretically investigated using an adaptation of Semenov's thermal explosion theory. The critical sizes of char particles in the cloud were found to be strongly affected by the particle temperature, shape, and reactivity, as well as the mass fraction of each species, and ambient conditions. The cloud shape, cloud particle number density, fuel mass fraction, and char reactivity were identified as the major parameters impacting upon the cloud extinction potential. Our analysis indicate that blending of a base fuel with a less reactive fuel generally increases the extinction potential of the cloud (i.e. likelihood of extinction) while addition of a more reactive secondary fuel to the base fuel minimises the probability of cloud extinction.

Low-Mass Stars and Substellar Objects in the NGC 1333 Molecular Cloud

We present the results of near-infrared imaging and low-resolution near- infrared spectroscopy of low mass objects in the NGC 1333 molecular cloud. A JHK survey of an 11.4' x 11.7' area of the northern cluster was conducted to a sensitivity of K < 16 mag. Using near-infrared magnitudes and colors from this and previously published surveys, twenty-five brown dwarf candidates were selected toward the high extinction cloud core. Spectra in the K band were obtained and comparisons of the depths of water vapor absorption bands in our candidate objects with a grid of dwarf,subgiant, and giant standards were made to derive spectral types. These data were then used to derive effective temperatures and stellar luminosities which, when combined with theoretical tracks and isochrones for pre-main sequence objects, resulted in estimates for their masses and ages. The models suggest a median age for the sample of < 1 Myr with substellar masses for at least 9 of the candidates including the x-ray flare source ASR 24. Surface gravities have been estimated for the brown dwarf candidates and, for a given spectral type,found to resemble more closely dwarfs than giants. Using the near-infrared imaging data and age estimates from the spectroscopic sample, an extinction-limited sample in the northern cluster was defined. Consistent with recent studies of other young clusters, this sample exhibits an accretion disk frequency of 0.75 +-0.20 and a mass spectrum slope across the hydrogen-burning limit of alpha < 1.6 where dN/dM ~ M^-(alpha).

Ice Cloud Optical Thickness and Extinction Estimates from Radar Measurements

A remote sensing method is proposed to derive vertical profiles of the visible extinction coefficients in ice clouds from measurements of the radar reflectivity and Doppler velocity taken by a vertically pointing 35-GHz cloud radar. The extinction coefficient and its vertical integral, optical thickness t, are among the fundamental cloud optical parameters that, to a large extent, determine the radiative impact of clouds. The results obtained with this method could be used as input for different climate and radiation models and for comparisons with parameterizations that relate cloud microphysical parameters and optical properties. An important advantage of the proposed method is its potential applicability to multicloud situations and mixed-phase conditions. In the latter case, it might be able to provide the information on the ice component of mixed-phase clouds if the radar moments are dominated by this component. The uncertainties of radar-based retrievals of cloud visible optical thickness are estimated by comparing retrieval results with optical thicknesses obtained independently from radiometric measurements during the yearlong Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment. The radiometric measurements provide a robust way to estimate t but are applicable only to optically thin ice clouds without intervening liquid layers. The comparisons of cloud optical thicknesses retrieved from radar and from radiometer measurements indicate an uncertainty of about 77% and a bias of about 214% in the radar estimates of t relative to radiometric retrievals. One possible explanation of the negative bias is an inherently low sensitivity of radar measurements to smaller cloud particles that still contribute noticeably to the cloud extinction. This estimate of the uncertainty is in line with simple theoretical considerations, and the associated retrieval accuracy should be considered good for a nonoptical instrument, such as radar. This paper also presents relations between radar-derived characteristic cloud particle sizes and effective sizes used in models. An average relation among t, cloud ice water path, and the layer mean value of cloud particle characteristic size is also given. This relation is found to be in good agreement with in situ measurements. Despite a high uncertainty of radar estimates of extinction, this method is useful for many clouds where optical measurements are not available because of cloud multilayering or opaqueness.

Infrared Photometry of Starless Dense Cores

Deep JHKs photometry was obtained towards eight dense molecular cores and J-H vs. H-Ks color-color plots are presented. Our photometry, sensitive to the detection of a 1 solar mass, 1 X 10^6 year old star through approx. 35 - 50 magnitudes of visual extinction, shows no indication of the presence of star/disk systems based on J-H vs. H-Ks colors of detected objects. The stars detected towards the cores are generally spatially anti-correlated with core centers suggesting a background origin, although we cannot preclude the possibility that some stars detected at H and Ks alone, or Ks alone, are not low mass stars or brown dwarfs (< 0.3 Solar Masses) behind substantial amounts of visual extinction (e.g. 53 magnitudes for L183B). Lower limits to optical extinctions are estimated for the detected background stars, with high extinctions being encountered, in the extreme case ranging up to at least Av = 46, and probably higher. The extinction data are used to estimate cloud masses and densities which are comparable to those determined from molecular line studies. Variations in cloud extinctions are consistent with a systematic nature to cloud density distributions and column density variations and extinctions are found to be consistent with submillimeter wave continuum studies of similar regions. The results suggest that some cores have achieved significant column density contrasts (approx. 30) on sub-core scales (approx. 0.05 pc) without having formed known stars.

Formation of convective carbon dioxide clouds near the south pole of Mars

Observations from the Mars Orbiter Laser Altimeter (MOLA) indicate a distinct difference in the nature of north and south polar clouds. The majority of north polar clouds are diffuse with relatively low optical extinction (channel 4 clouds). In contrast, a significant number of south polar clouds provide very strong laser echoes suggesting optical extinction as high as 300 km−1 (channel 1 clouds). These high extinction clouds were almost exclusively observed in the polar night between 70° and 85°S. We have previously shown that the low extinction clouds are composed of a small number of large carbon dioxide particles, which form in relatively gentle updrafts, or lee waves. A possible explanation for the high extinction clouds is that they are composed of more, but somewhat smaller, carbon dioxide particles forming in high‐velocity convective updrafts. Numerical simulations using a microphysical cloud model show that the cloud extinction depends most strongly on the availability of ice nuclei (IN), assumed to be dust, and the updraft velocity. Radiative cooling by the CO2 clouds can further destabilize the air column. Cloud extinction consistent with MOLA observations is produced for IN concentrations greater than 5 cm−3 and updraft core velocities of ∼2 m s−1. The discrepancy in north and south cloud extinction may be the result of differing IN concentrations and stability regimes between the north and south polar regions. Many channel 4 clouds may also be convective but formed in air with either low updraft velocities or low abundances of ice nuclei.

Effect of multiple scattering on depolarization measurements with spaceborne lidars

An analytical model based on the integration of the scattering-angle and light-path manifold has been developed to quantify the effect of multiple scattering on cirrus measurements obtained with elastic polarization lidars from space. Light scattering by molecules and by a horizontally homogeneous cloud is taken into account. Lidar parameter, including laser beam divergence, can be freely chosen. Up to 3 orders of scattering are calculated. Furthermore, an inversion technique for the retrieval of cloud extinction profiles from measurements with elastic-backscatter lidars is proposed that explicitly takes multiple scattering into account. It is found that for typical lidar system parameters such as those of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) instrument multiple scattering does not significantly affect depolarization-ratio measurements in cirrus clouds with small to moderate optical depths. For all simulated clouds, the absolute value of the difference between measured and single-scattering volume depolarization ratio is < 0.006. The particle depolarization ratio can be calculated from the measured volume depolarization ratio and the retrieved backscatter ratio without degradation of accuracy; thus characterization of the various cirrus categories in terms of the particle depolarization ratio and retrieval of cloud microphysical properties is feasible from space. The results of this study apply to polar stratospheric clouds as well.

A Search for Stellar Obscuration Events Due to Dark Clouds

The recent detections of a large population of faint submillimetre sources, an excess halo gamma-ray background, and the extreme scattering events observed for extragalactic radio sources have been explained as being due to baryonic dark matter in the form of small, dark, gas clouds. In this paper we present the results of a search for the transient stellar obscurations such clouds are expected to cause. We examine the Macho project light curves of 48 x 10^6 stars toward the Galactic bulge, LMC and SMC for the presence of dark cloud extinction events. We find no evidence for the existence of a population of dark gas clouds with Av > 0.2 and masses between ~ 10^-4 and 10^-2 M_solar in the Galactic disk or halo. However, it is possible that such dark cloud populations could exist if they are clustered in regions away from the observed lines of sight.

Improved mesospheric temperature, water vapor and polar mesospheric cloud extinctions from HALOE

We present a new retrieval technique for mesospheric measurements from the Halogen Occultation Experiment (HALOE). Previous HALOE retrievals did not account for polar mesospheric clouds (PMCs) and were biased whenever PMCs were in the sample volume. In the new algorithm we eliminate this bias by retrieving and correcting for PMC extinction, and we optimize the algorithm for this region of the atmosphere. We have reprocessed over ten years (1991–2002) of HALOE data with the improved algorithm, yielding nearly 75,000 vertical profiles of temperature, H2O, O3, NO and extinction at five infrared wavelengths. In this paper we describe the new algorithm in detail, present several examples of the results and discuss some initial validation and quality assessments.

Near-infrared and optical emission-line structure of the Keyhole Nebula in NGC 3372

ABSTRA C T Narrow-band infrared and optical images of the Keyhole Nebula in NGC 3372 reveal which structures are caused by extinction, and show the underlying morphology of photoionized and shock-excited gas. Dark clouds conspire with ionized gas to create the apparent keyhole shape, which is prominent at blue wavelengths and less apparent in the infrared. The Pab/Ha line ratio shows the spatial distribution of foreground extinction. The wavelength dependence of this extinction indicates a reddening law with R < 4:8, different from the normal interstellar medium. This confirms previous estimates of reddening toward the Carina Nebula determined from stellar photometry, and reveals that the anomalous extinction is patchy and within the H II region. The morphology of the ionized gas is different from the extinction clouds; it shows an edge-on ionization front running NE to SW, with a limb-brightened indentation that forms the upper outline of the keyhole shape. A fast polar wind from h Carinae may have punctured the ionization front, since the indentation is directly along a projection of the polar axis of the star. This is supported by the morphology of shock-excited gas revealed by a high âS iia=Ha ratio. High-excitation gas emitting [O III] and He I has a smoother distribution. Molecular clumps in the region are also discussed.

Hubble Space Telescope/NICMOS Imaging Survey of the Ophiuchus (Lynds 1688) Cluster

We present a catalog of near-infrared photometry of young stars associated with the Ophiuchus molecular cloud based on observations made with the Hubble Space Telescope NICMOS3 camera at 1.1 and 1.6 μm. Our survey covers 0.02 deg2 centered on the dense molecular cores in Lynds 1688. We detect 165 sources at 1.6 μm and 65 sources at 1.1 μm within our estimated completeness limits of 21.0 and 21.5 mag, respectively. An analysis of the cloud extinction, based on existing molecular line maps, suggests that most of the sources lying within the 40 AV extinction contour of the cloud are probable cloud members. Approximately half (58/108) of these sources are previously unpublished.The faint embedded sources revealed by these observations are spatially concentrated in three regions of high stellar space density (N > 104 stars pc-3). While the spatial distribution of these sources reflects that of the brighter, well-known population of young stars in Ophiuchus, it is distinctly different from the distribution of cool concentrations seen in the submillimeter. Seven new brown dwarf candidates are identified based on their infrared colors and their projected locations on high column density regions of the molecular cloud. Eight new candidate binary and five new candidate triple systems, having separations between 02 and 10'' (29-1450 AU) are reported. The spatial resolution and sensitivity of these observations reveal five apparent disk/envelope systems seen via scattered light and four nebulous objects with complex morphologies.

Ongoing Massive Star Formation in the Bulge of M51

We studied the HST-WFPC2 observations of the inner kpc of the interacting galaxy M51 in six bands from 2550 to 8140 Angstrom. The images show an oval shaped area (bulge) of 11x16 arcsec or 450x650 pc around the nucleus, dominated by a smooth population of old stars with overimposed dust lanes. We found 30 bright point-like sources in the bulge of M51; many of these are bright in the UV. They are located in elongated strings which follow the general pattern of the dust lanes. The spectral energy distributions of the pointsources are compared with those of clusters or single stars. We present arguments that the pointsources are isolated massive stars or small groups of a few massive stars. The energy distributions are best fitted with those of single stars with 4000<Teff<50000 K, 4.2<log L/Lsun<7.2, and 12<Mstar<200 Msun. We have derived upper limits to the total mass of low mass stars that could be hiding within the point sources. For the bluest sources the upper limit is only a few hundred Msun. We conclude that massive stars are formed outside clusters (or in very low mass clusters) in the bulge of M51. The estimated star formation rate in the bulge is 1 to 2x10^(-3) Msun/yr. This star formation rate could be sustained for about 200 to 400 Myrs, suggesting that the ongoing massive star formation in the bulge of M51 is fed/triggered by the interaction with its companion about 400 Mrs ago. Theoretical cloud-models show that CO is destroyed in low extinction clouds near the bright nucleus of M51, so that H_2, [OI] and [CII] are the dominants coolants. This condition favors the formation of massive stars, in agreement with the observations. This mode of massive-star formation in the bulge of M51 may resemble the star formation in the early Universe, when the CO and dust contents were low due to the low metallicity.

Three-Dimensional Calculation of Photolysis Frequencies in the Presence of Clouds and Impact on Photochemistry

For atmospheric photochemistry, clouds can significantly affect actinic flux distributions. In this paper, we examine the effects of convective clouds on the three-dimensional distribution of the spectral actinic flux and on photolysis frequencies for various chemical species. Three-dimensional solutions of the UV-VIS radiative transfer equation are produced using the Spherical Harmonic Discrete Ordinary Method solution technique. This solver uses as input the 3-D cloud characteristics simulated by a dynamical cloud model. The ultraviolet and visible spectra are divided into 5 intervals in order to explore the wavelength dependency of the cloud effect on the actinic flux. Results show that the distribution of the actinic flux over the cloud domain is far from homogeneous and depends primarily on the cloud extinction associated with the hydrometeors. Maximum actinic flux is found at the top edge of the cloud and is related to scattering by ice crystals. The actinic flux is enhanced by a factor of 2 to 5, compared to clear air values, above, at the top edge, and around the cloud. The 3-D actinic flux is used to calculate the photolysis rates for some chemical species (e.g. NO2, O3, and HCHO). Forcomputing photolysis rates, a discretized spectral representation of the absorption wavelengths is used in the model. The calculated photolysis rates are distributed inhomogeneously throughout the cloud, and maxima are found in regions where the actinic flux is enhancement is large. Temperature effects on absorption are found in the photolysis frequencies of some species. Finally, the potential importance of this photolysis enhancement on photochemistry is studied using box model simulations. Results show that enhanced OH concentrations are found in the upper troposphere (120–200%) overthe clouds and changes in ozone production rates (+15%) are obtained in quasi-steady state conditions.

Vertical profiles of aerosol volume from high-spectral-resolution infrared transmission measurements I Methodology

The wavelength-dependent aerosol extinction in the 800-1250-cm(-1) region has been derived from ATMOS (atmospheric trace molecule spectroscopy) high-spectral-resolution IR transmission measurements. Using models of aerosol and cloud extinction, we have performed weighted nonlinear least-squares fitting to determine the aerosol-volume columns and vertical profiles of stratospheric sulfate aerosol and cirrus cloud volume. Modeled extinction by use of cold-temperature aerosol optical constants for a 70-80% sulfuric-acid-water solution shows good agreement with the measurements, and the derived aerosol volumes for a 1992 occultation are consistent with data from other experiments after the eruption of Mt. Pinatubo. The retrieved sulfuric acid aerosol-volume profiles are insensitive to the aerosol-size distribution and somewhat sensitive to the set of optical constants used. Data from the nonspherical cirrus extinction model agree well with a 1994 mid-latitude measurement indicating the presence of cirrus clouds at the tropopause.

Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Observations of interstellar linear polarization in the spectral range 0.35-2.2??m are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining published photometry and spectral classifications with photometry from the Two Micron All Sky Survey. The polarization and extinction curves are characterized in terms of the parameters ?max (the wavelength of maximum polarization) and RV (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical properties of the dust change systematically as a function of environment, considering stars observed through progressively more opaque (and thus progressively denser) regions of the cloud. At low visual extinctions (0 3, real changes in grain properties occur, characterized by observed RV values in the range 3.5-4.0. A simple model for the development of RV with AV suggests that RV may approach values of 4.5 or more in the densest regions of the cloud. The transition between normal extinction and dense cloud extinction occurs at AV ~ 3.2, a value coincident with the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in RV are thus either a direct consequence of mantle growth or occur under closely similar physical conditions. Dust in Taurus appears to be in a different evolutionary state compared with other nearby dark clouds, such as ? Oph, in which coagulation is the dominant physical process.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Observations of Dust and Star-forming Regions in the Ocular Galaxy IC 2163 and Its Spiral Companion NGC 2207

Hubble Space Telescope observations in U, B, V, and I passbands of the interacting spiral galaxies IC 2163 and NGC 2207 are used to measure extinctions in the cloud and intercloud regions and ages and luminosities of the star-forming regions. The extinction in the part of NGC 2207 seen in projection against IC 2163 was determined by using the method of White & Keel. The extinctions there and elsewhere were also determined from radiative transfer models of the magnitude differences between clouds and their surroundings. The intercloud extinction in V band ranges from 0.5 to 1 mag on the line of sight, and the cloud extinction ranges from 1 to 2 mag. The measured star-forming regions in these galaxies have a power-law relation between size and luminosity and a power-law luminosity distribution function. These power laws are consistent with a fractal dimension for the star formation that is the same as that for interstellar gas, D ~ 2.2, extending over scales ranging from 20 to 1000 pc. Fifteen compact massive star clusters that are analogous to super–star clusters found in starburst regions are in the spiral arms of NGC 2207. Nothing is peculiar about these regions except for a high H I velocity dispersion (~50 km s-1). Two more super–star clusters are in the tidally compressed oval of IC 2163. These clusters have masses ranging from ~10^4 to 2 × 10^5 M⊙ and ages of a few times 10^6 yr.

TheHubble Space TelescopeKey Project on the Extragalactic Distance Scale. XXI. The Cepheid Distance to NGC 1425

The distance to NGC 1425 has been derived from Cepheid variables, as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen F555W (V) and eight F814W (I) epochs of cosmic-ray-split Wide Field Planetary Camera 2 observations were obtained. Twenty-nine Cepheids were discovered, with periods ranging from 16 to 63 days. Adopting a Large Magellanic Cloud distance modulus and extinction of 18.50 ± 0.10 mag and E(V-I) = 0.13 mag, respectively, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 31.73 ± 0.16 (random) ± 0.17 (systematic) mag was determined for NGC 1425. The corresponding distance of 22.2 ± 1.0 (random) ± 1.0 (systematic) Mpc is in satisfactory agreement with that found with an independent analysis based on the DoPHOT photometry package.

Far‐Infrared and Submillimeter Emission from Galactic and Extragalactic Photodissociation Regions

Photodissociation region (PDR) models are computed over a wide range of physical conditions, from those appropriate to giant molecular clouds illuminated by the interstellar radiation field to the conditions experienced by circumstellar disks very close to hot massive stars. These models use the most up-to-date values of atomic and molecular data, the most current chemical rate coefficients, and the newest grain photoelectric heating rates, which include treatments of small grains and large molecules. In addition, we examine the effects of metallicity and cloud extinction on the predicted line intensities. Results are presented for PDR models with densities over the range n = 101-107 cm-3 and for incident far-ultraviolet radiation fields over the range G0 = 10-0.5-106.5 (where G0 is the far-ultravioliet [FUV] flux in units of the local interstellar value), for metallicities Z = 1 and 0.1 times the local Galactic value, and for a range of PDR cloud sizes. We present line strength and/or line ratio plots for a variety of useful PDR diagnostics: [C II] 158 μm, [O I] 63 μm and 145 μm, [C I] 370 μm and 609 μm, CO J = 1-0, J = 2-1, J = 3-2, J = 6-5, and J = 15-14, as well as the strength of the far-infrared continuum. These plots will be useful for the interpretation of Galactic and extragalactic far-infrared and submillimeter spectra observable with the Infrared Space Observatory (ISO), the Stratospheric Observatory for Infrared Astronomy, the Submillimeter Wave Astronomy Satellite, the Far Infrared and Submillimeter Telescope, and other orbital and suborbital platforms. As examples, we apply our results to ISO and ground-based observations of M82, NGC 278, and the Large Magellanic Cloud. Our comparison of the conditions in M82 and NGC 278 show that both the gas density and FUV flux are enhanced in the starburst nucleus of M82 compared with those in the normal spiral NGC 278. We model the high [C II]/CO ratio observed in the 30 Doradus region of the LMC and find that it can be explained either by lowering the average extinction through molecular clouds or by enhancing the density contrast between the atomic layers of PDRs and the CO-emitting cloud cores. The ratio L[CO]/M[H2] implied by the low extinction model gives cloud masses too high for gravitational stability. We therefore rule out low-extinction clouds as an explanation for the high [C II]/CO ratio and instead appeal to density contrast in AV = 10 clouds.