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To study Arctic stratus cloud properties and their effect on the surface radiation balance during the spring transition season, analyses are performed using data taken during three cloudy and two clear days in May 1998 as part of the First ISCCP Regional Experiment (FIRE) Arctic Cloud Experiment (ACE). Radiative transfer models are used in conjunction with surface- and satellite-based measurements to retrieve the layer-averaged microphysical and shortwave radiative properties. The surface-retrieved cloud properties in Cases 1 and 2 agree well with the in situ and satellite retrievals. Discrepancies in Case 3 are due to spatial mismatches between the aircraft and the surface measurements in a highly variable cloud field. Also, the vertical structure in the cloud layer is not fully characterized by the aircraft measurements. Satellite data are critical for understanding some of the observed discrepancies. The satellite-derived particle sizes agree well with the coincident surface retrievals and with the aircraft data when they were collocated. Optical depths derived from visible-channel data over snow backgrounds were overestimated in all three cases, suggesting that methods currently used in satellite cloud climatologies derive optical depths that are too large. Use of a near-infrared channel with a solar infrared channel to simultaneously derive optical depth and particle size appears to alleviate this overestimation problem. Further study of the optical depth retrieval is needed. The surface-based radiometer data reveal that the Arctic stratus clouds produce a net warming of 20 W m(exp -2) in the surface layer during the transition season suggesting that these clouds may accelerate the spring time melting of the ice pack. This surface warming contrasts with the net cooling at the top of the atmosphere (TOA) during the same period. All analysis of the complete FIRE ACE data sets will be valuable for understanding the role of clouds during the entire melting and refreezing process that occurs annually in the Arctic.

Current algorithms for the determination of cloud top height and cloud fraction in Polar Regions tend to provide unreliable results, particularly in the presence of isothermal conditions within the atmosphere. Alternative methods to determine cloud top heights in such regions effectively, from space borne sensors, are currently limited to stereo-photogrammetry and active sensing methods, such as LiDAR. Here we apply the modified census transform to one month of AATSR stereo data from June 2008. AATSR is unique in that it is the only space borne stereo capable instrument providing data continuously in both the visible, near infrared and thermal channels. This allows for year round imaging of the poles and therefore year round cloud top height and cloud fraction estimation. We attempt a preliminary validation of the stereo retrieved cloud top height measurements from AATSR against collocated cloud height measurements from the CALIOP LiDAR instrument. CALIOP provides an excellent validation tool due to its excellent height resolution of between 30-60 meters. In this validation, a pair of collocated swaths is assessed with a total of 154 inter-comparisons; the results show that AATSR correlates well with CALIOP cloud base layers with an R2 score of 0.71. However, in all cases AATSR appears to be underestimating the cloud top height compared to CALIOP, the causes for this are currently not fully understood and more extensive inter-comparisons are required. Once validation is completed a processing chain is in place to process the entire ATSR time-series data generating a 20 year cloud top height dataset for Greenland.

A cloud microphysical model and a model of airflow over hills are used to examine the microstructure of hill cap clouds. The lifetime of the clouds and the distance travelled by the clouds are both assumed to be sufficiently small so that condensation alone is considered to be important; the coalescence of drops is neglected. The effects of changing (i) the height of cloud base, (ii) humidity fluctuations below cloud base and (iii) incloud turbulence, are described. Process (i) is found to be important, its effect on the cloud microstructure being as great as the effect of changes in the CCN distribution or in the mean wind. Droplet loss to ground, radiative cooling at cloud top and entrainment of dry air are also discussed; these processes may all significantly affect the cloud droplet distribution.

Cloud base height (CBH) is an important parameter for many applications such as aviation, climatology or solar irradiance nowcasting (forecasting for the next seconds to hours ahead). The latter application is of increasing importance for the operation of distribution grids and photovoltaic power plants, energy storage systems and flexible consumers. To nowcast solar irradiance, systems based on all-sky imagers (ASIs), cameras monitoring the entire sky dome above their point of installation, have been demonstrated. Accurate knowledge of the CBH is required to nowcast the spatial distribution of solar irradiance around the ASI's location at a resolution down to 5 m. To measure the CBH, two ASIs located at a distance of usually less than 6 km can be combined into an ASI pair. However, the accuracy of such systems is limited. We present and validate a method to measure the CBH using a network of ASIs to enhance accuracy. To the best of our knowledge, this is the first method to measure the CBH with a network of ASIs which is demonstrated experimentally. In this study, the deviations of 42 ASI pairs are studied in comparison to a ceilometer and are characterized by camera distance. The ASI pairs are formed from seven ASIs and feature camera distances of 0.8…5.7 km. Each of the 21 tuples of two ASIs formed from seven ASIs yields two independent ASI pairs as the ASI used as the main and auxiliary camera, respectively, is swapped. Deviations found are compiled into conditional probabilities that tell how probable it is to receive a certain reading of the CBH from an ASI pair given that the true CBH takes on some specific value. Based on such statistical knowledge, in the inference, the likeliest actual CBH is estimated from the readings of all 42 ASI pairs. Based on the validation results, ASI pairs with a small camera distance (especially if

Stratiform clouds constitute ~40% of global cloud cover and play a key role in determining the planetary radiation budget. Electrification remains one of the least understood effects on their microphysical processes. Droplet charging at the top and bottom edges of stratiform clouds arises from vertical current flow through clouds driven by the Global atmospheric Electric Circuit. In-cloud charge data are central in assessing the role of charge in droplet growth processes, which influence droplet size distributions and associated cloud radiative properties and precipitation. This study presents the first high vertical resolution electrical measurements made in multiple layer clouds. Of the 22 clouds sampled, all were charged at their edges, demonstrating unequivocally that all stratiform clouds can be expected to contain charge at their upper and lower boundaries to varying extent. Cloud base and cloud top are shown to charge asymmetrically, with mean cloud top space charge +32 pCm-3 and base space charge -24 pCm-3. The larger cloud top charges are associated with strong temperature inversions and large vertical electrical conductivity gradients at the upper cloud boundary. Greater charging was observed in low altitude ( 2km) cloud layers (7.0 pCm-3), consistent with the smaller air conductivity at lower altitudes caused by reduced cosmic ray ionisation. Taken together, these measurements show that the greatest cloud droplet charges in extensive stratiform clouds occur at cloud tops for low altitude (

Ice particles in the summer mesosphere – such as those connected to noctilucent clouds and polar mesospheric summer echoes - have since their discovery contributed to the uncovering of atmospheric processes on various scales ranging from interactions on molecular levels to global scale circulation patterns. While there are numerous model studies on mesospheric ice microphysics and how the clouds relate to the background atmosphere, there are at this point few studies using comprehensive global climate models to investigate observed variability and climatology of noctilucent clouds. In this study it is explored to what extent the large-scale inter-annual characteristics of noctilucent clouds are captured in a 30-year run - extending from 1979 to 2009 - of the nudged and extended version of the Canadian Middle Atmosphere Model (CMAM30). To construct and investigate zonal mean inter-seasonal variability in noctilucent cloud occurrence frequency and ice mass density in both hemispheres, a simple cloud model is applied in which it is assumed that the ice content is solely controlled by the local temperature and water vapor volume mixing ratio. The model results are compared to satellite observations, each having an instrument-specific sensitivity when it comes to detecting noctilucent clouds. It is found that the model is able to capture the onset dates of the NLC seasons in both hemispheres as well as the hemispheric differences in NLCs, such as weaker NLCs in the SH than in the NH and differences in cloud height. We conclude that the observed cloud climatology and zonal mean variability are well captured by the model.

Using the satellite-infrared-based Simple Convective Aggregation Index (SCAI) to determine the degree of aggregation, 5 years of CloudSat–CALIPSO cloud profiles are composited at a spatial scale of 10 degrees to study the relationship between cloud vertical structure and aggregation. For a given large-scale vertical motion and domain-averaged precipitation rate, there is a large decrease in anvil cloud (and in cloudiness as a whole) and an increase in clear sky and low cloud as aggregation increases. The changes in thick anvil cloud are proportional to the changes in total areal cover of brightness temperatures below 240 K [cold cloud area (CCA)], which is negatively correlated with SCAI. Optically thin anvil cover decreases significantly when aggregation increases, even for a fixed CCA, supporting previous findings of a higher precipitation efficiency for aggregated convection. Cirrus, congestus, and midlevel clouds do not display a consistent relationship with the degree of aggregation. Lidar-observed low-level cloud cover (where the lidar is not attenuated) is presented herein as the best estimate of the true low-level cloud cover, and it is shown that it increases as aggregation increases. Qualitatively, the relationships between cloud distribution and SCAI do not change with sea surface temperature, while cirrus clouds are more abundant and low-level clouds less at higher sea surface temperatures. For the observed regimes, the vertical cloud profile varies more evidently with SCAI than with mean precipitation rate. These results confirm that convective scenes with similar vertical motion and rainfall can be associated with vastly different cloudiness (both high and low cloud) and humidity depending on the degree of convective aggregation.

Residential photovoltaic (PV) technology is expected to have mass global deployment. With widespread PV in the electricity distribution grids, the variable nature of the solar resource must be understood to facilitate reliable operation. This research demonstrates that synthetic, 1-min resolution irradiance time series that vary on a spatial dimension can be generated based on the following inputs: mean hourly meteorological observations of okta, wind speed, cloud height and atmospheric pressure. The synthetic time series temporally validate against observed 1-min irradiance data for four locations—Cambourne, UK; Lerwick, UK; San Diego, CA USA; and Oahu, HI USA—when analysing 4 metrics of variability indices, ramp-rate size, irradiance magnitude frequency and clear-sky index frequency. Each metric is calculated for the modelled and observed data at each location and CDF profile correlation compared as well as applying the Kolmogorov-Smirnov (K–S) test with 99 % confidence limits. CDF correlation coefficients of each metric are all above R ⩾ 0.908 , and a minimum of 90.96 % of daily irradiance time series passed the K–S test. A spatial validation was performed comparing the model outputs to real observation data. The spatial correlation coefficient regression with site separation was successfully recreated with MAPE = 0.865 % , RMSE = 0.01 and R = 0.955 . The spatial instantaneous correlation was shown to behave anisotropically when using fixed cloud direction, with different correlation in along and cross wind directions. Cloud cover states of 40–60% showed the most spatial decorrelation while 0 % and 100 % had the least. The model outputs are applied to a distribution grid impact model using the IEEE-8500 node test feeder. PV scenarios of 25 % , 50 % , and 75 % uptake were modelled across a 1.5 × 1.5 km grid. The magnitude and frequency of severe tap changing events are found to be significantly higher when using a single irradiance time series for all PV systems versus individually assigning spatially decorrelating time series.

(a) Four high-velocity clouds (HVCs) with large negative velocities have been mapped at high sensitivity, with an angular resolution of 12 arcmin and a velocity resolution of 1.8 km s−1 (Cohen & Mirabel, in press). Although these are small isolated clouds they show the same two-component velocity structure that is observed in the large Northern HVC complexes at lower velocity. In addition the cool dense core of one of the clouds is found to be rotating. This rotation, and also the regular velocity patterns within the four clouds, indicates that the clouds are unlikely to be interacting with galactic material. It is interesting that similar observations of the Magellanic Stream do not show narrow velocity components, suggesting that there is a real physical difference between the Stream and other HVCs (Mirabel, Cohen & Davies, in press).

THE normal method of determining the height of the cloud base at night is to direct a vertical searchlight beam on to the cloud and measure the angle of elevation E of the spot of light from a point at a known horizontal distance L from the searchlight. The height h of the cloud base is then given by h = L tan E. The length L of the base-line is normally about 1,000 ft.

A numerical model for the brightness of polar mesospheric clouds (PMC) is described, and is compared with measurements from the Solar mesosphere Explorer (SME) satellite. These clouds occur during the summer months at polar latitudes where temperatures are known to fall below 140 K. We calculate the optical properties of a cloud by simulating the growth and sedimentation of ice particles at the cold supersaturated mesopause. Time-dependent trajectories of ice particles are calculated from their origin at the temperature minimum region to their demise at the cloud base through evaporation. We consider the effects of the removal of atmospheric water vapor by the growing particles and its restoration by ice evaporation at the cloud base. This “freeze-drying” effect is crucial in limiting the maximum size of the particles and therefore the maximum brightness of the cloud. Assuming spherical particles, Mie-scattering calculations of the directional albedo of a cloud are performed using a range of possible values for the atmospheric variables (water vapor mixing ratio, temperature, upward wind speed, atmospheric pressure, and eddy diffusion coefficient). We find that for a nominal atmospheric case the model predicts a moderately weak cloud at 265 nm, the wavelength of the SME measurements. An extreme model (cold and moist with high vertical wind and eddy transport) is needed to account for the brightest cloud observed. We estimate an upper limit for the water vapor to be of the order of 5 parts per million by volume. Higher values would imply the existence of clouds which exceed in brightness every cloud observed by the satellite over the time period 1981–1986. SME observations of greater cloud height in the northern hemisphere, despite their greater brightness, possibly imply an excess (by a factor of 2) of northern hemisphere water vapor. This holds if the other atmospheric variables (and cloud particle numbers) are the same in both north and south. Dependence of model cloud brightness on atmospheric pressure (∼P4.4), water vapor mixing ratio (∼w2.8), thickness of the cloud saturation region (∼d4) and advective wind speed (∼υ) are determined. These scalings are shown to result from a calculated proportionality of the cloud brightness to R6 (R is the maximum particle radius), and from simple considerations of ice layer growth, particle sedimentation, and the mass budget of water. Mie scattering calculations for a wavelength of 550 nm show that SME and OGO-6 data on PMC brightnesses are consistent.

Turbulence data obtained from over 100 aircraft penetrations of isolated, nonprecipitating cumulus clouds are used to derive statistical parameters associated with the transport of ozone to and from the underlying boundary layer. The parameters are determined by the difference in ozone concentration between the cloud layer and boundary layer, ΔO3. The total cloud flux of ozone, defined as the transport across a horizontal plane within a cloud due to mean and turbulent cloud motions, is given by , where is the cloud vertical velocity averaged over an observing period. Cloud turbulence contributes about 30% to the total cloud flux.

The extraction of information on cloud cover from present-day multispectral satellite images poses a challenge to the remote sensing specialist. When approached one pixel at a time, the derived cloud cover parameters are inherently nonunique. More information is needed than is available in the radiances from each channel of an isolated pixel. The required additional information can be obtained for each scene, however, by analyzing the distribution of pixels in the multi-dimensional space of channel radiances. The cluster patterns in this space yield statistical information that points to the most likely solution for that scene. The geostationary and polar orbiting meteorological satellites all have, at a minimum, a solar reflection channel in the visible spectrum and a thermal infrared channel in the 8–12 micron window. With the information from the cluster patterns and application of the equations of radiative transfer, the measurements in those channels will yield cloud cover fraction, optical thickness, and cloud-top temperature for an assumed microphysical model of the cloud layer. Additional channels, such as the 3.7 micron channel on the AVHRR of the polar orbiting meteorological satellites, will will yield information on the microphysical model—e.g., distinguishing small liquid liquid droplets (typical of low level clouds) from large ice particles (typical of cirrus and the tops of cumulonimbus). New channels to be included in future satellite missions will provide information on cloud height, independent of temperature, and on a particle size and thermodynamic phase, independently of each other. A proposed STS mission using lidar will pave the way for the use of active sensors that will provide more precise information on cloud height and probe the structure of thin cirrus and the top layer of of the thicker cloud.

The infrared radiation reaching the earth's surface, the outgoing radiation at 50 mbar, and the tropospheric infrared flux divergence have been calculated for various cloud covers, heights, thicknesses, and amount of cloud overlap. The results are presented in a series of diagrams. For comparable cloud covers their effects on the downward surface flux and flux divergence decrease toward lower latitudes because of the shielding by larger water vapor amounts. Surface flux due to cloud cover in the tropical model is increased by as much as 10%, while during winter in the subarctic the increase may amount to 60% of the clear sky value. Tropospheric flux divergence can either increase or decrease depending on cloud height; for clouds below approximately 500 mbar the divergence increases, and in the polar regions with clouds very near the surface this increase can reach values about 80% larger than the divergence for clear skies. The degree of cloud overlap causes as much as a 3% uncertainty in downward flux in the tropics, but in polar latitudes during winter the uncertainty is 26% for two cloud layers each covering 50% of the sky. The water vapor continuum and temperature-dependent transmission functions should be considered if fluxes and flux divergences are required to an accuracy of about 10%.

The review traces the development of electronics as applied to meteorological instrumentation. A brief introduction and history is followed by a short survey of meteorological communications, illustrating the need for the rapid availability of accurate current data in forecasting work. The development of the use of electronics in instrumental methods is then traced with reference to the work done within the British Meteorological Office. A Section on upper-air instrumentation commences with the general design principles of radiosondes, and includes examples of the techniques given. A survey of methods used for upper-wind finding follows, and the Section ends with an account of techniques employed in measuring atmospheric ozone. In a Section on ground instrumentation, the methods used for measuring the basic parameters of cloud height, visibility, surface wind and sunshine are surveyed, and examples of the various techniques for the automatic acquisition and recording of data follow next. The role of electronics in the instrumentation of ocean-data buoys used for obtaining meteorological data is then treated, with examples of systems in use. The various satellite systems developed for meteorological purposes are then surveyed. In the conclusion, an attempt is made to forecast the part to be played by electronics in meteorological instrumentation during the next decade or so.

Two independent methods of measuring the transmittance of cirrus clouds are compared. Both used a CO(2) pulsed Doppler lidar at a wavelength of 10.59 microm. The first method used backscatter from the calibration target El Chichon stratospheric cloud that was present over Boulder in 1982 and 1983. The second method used conical lidar scans at different zenith angles when uniform cirrus decks were present. Extinction coefficients measured from both methods average 0.1 km(-1) for tenuous cirrus 1.0 km thick to 0.78 km(-1) for cirrus several kilometers thick. There is a wide standard deviation in extinction values. Extinction-tobackscatter ratios S vary from1000 sr for tenuous clouds to 2600 sr for dense clouds. Mie scattering and extinction calculations for spherical ice particles of 10-50 microm in radius lead to ratios S2000 sr, so long as the ice absorption is entered into the calculations. The backscattering ratio for ice cylinders is 1 order of magnitude lower than for spheres. Backscatter in the IR may, therefore, be reasonably well modeled by some combination of spheres and cylinders. Cloud thickness statistics from lidar returns show that cirrus decks average ~500 m thick. Clouds thinner than 300 m were often overlooked by the unaided surface-based observer. These preliminary results are in rather close agreement with the LOWTRAN 6 cirrus cloud model predictions.

Glowing avalanches from the 1974 eruption of the volcano Fuego, Guatemala, deposited between 0.005 and 0.01 km 3 of nonimbricated, nongraded, nonwelded debris on the southern flanks of the volcano. Avalanches emerged from notches in the crater rim, and traveled some 7 km from the vent at an average speed of 60 km/hr. Each glowing avalanche consisted of two parts, (1) the main mass of moving debris, or underflow, and (2) a superjacent gas and dust cloud. The postdepositional thickness of individual underflows is generally less than 2 m, but successions of glowing avalanches produced deposits as much as 15 m thick in some ravines. The underflows and their superjacent gas and dust clouds followed topographic lows. Where underflows were confined laterally by valley walls, the surface of the underflow was featureless, after deposition. Where underflows were able to spread laterally, they developed a surface morphology of channels, levees, and overbank areas. The coarsest fragments were deposited in the channels. The underflows probably flowed in laminar fashion as high concentration dispersions, similar to normal debris flows. No unequivocal evidence of large-scale gas expansion of the underflows was noted. Gas emission is considered to have occurred during transportation, but it probably played a minor role in fluidization of the underflow. The ability of these glowing avalanches to transport particles as much as 5 m in diameter some 7 km from the crater is considered to be a reflection of the high yield strength of the underflow. Deposits from the 1974 eruption had a mean grain size of −3.00ϕ (8 mm), were extremely poorly sorted (σ 1 = 4.6) strongly fine skewed (SK I = 0.63), and platykurtic (K G = 0.71). All grain size distributions were bimodal with a dominant intermodal low at −1.0ϕ (2 mm). The small percentage of particles in the −1.0ϕ size interval is considered to reflect the inherent mechanical instability of particles of this size. Turbulently flowing gas and dust clouds, as much as 1,000 m high, accompanied each avalanche, and cloud height increased with increasing distance of travel. The fine-grained dust of the superjacent gas and dust clouds was not deposited on top of the underflow. Prevailing winds blew the fine-grained component to the west-southwest.

Using a data set including large-scale fields and collocated satellite observations (the NEPHOS file), we test the validity of the approach proposed for validating general circulation model type cloud schemes with the future International Satellite Cloud Climatology Project (ISCCP) and First ISCCP Regional Experiment (FIRE) data sets. An ensemble of 2866 atmospheric situations extracted from the NEPHOS file is used to test the European Center for Medium Range Weather Forecasts operational cloud cover scheme and to study the sensitivity to cloud cover of the regional radiation budget over the eastern Atlantic Ocean and the North Sea. This scheme, based on a simple function of the relative humidity, is shown to underestimate the cloud cover (by 26%), especially when stratocumulus clouds are present. This underestimation is related to too small values of the analyzed relative humidity in the model's planetary boundary layer. The vertical gradient of potential temperature is shown to be a better predictor for this low-level cloudiness. Differences between the observed and generated radiation budget at the top of the atmosphere are induced by the differences in cloud cover. However, differences in the cloud radiative properties introduce some compensating effects, and the mean bias error is only +7 Wm −2. Particularly, the mean value of the shortwave albedo is too high in the model, and its variation as a function of the cloud cover is much too smooth, as the model does not account for the observed increase in the vertical optical depth of clouds when the cloud cover increases.

Recent work on the vacuum UV absorption spectrum of CO to the description of the photodissociation of interstellar CO and its principal isotopic varieties is discussed. The effects of line broadening, self-shielding, shielding by H and H2, and isotope-selective shielding are examined as functions of depth into interstellar clouds. The photodissociation rates of the isotopic species are larger than that of (C-12)O inside the clouds by up to one to two orders of magnitude. A simple approximation to the attenuation by line absorption is given in tabular form. Computed abundances of CO and related species C and C+ are presented for a variety of interstellar clouds ranging from diffuse clouds to dense photodissociation regions. Several series of models of translucent clouds are presented which illustrate how the CO abundance increases rapidly with total cloud thickness. The variations of the isotopic abundances with depth and their sensitivity to temperature and total cloud thickness are explored in detail.

Measurements of the fraction of the solar radiation passing through stratus cloud reflected by the sea surface led to the conclusion that the radiation is not completely diffuse. The average albedo was 10.5 per cent with values varying with cloud thickness and solar elevation. The indication is that for completely diffuse radiation Schmidt's theoretical value of 17 per cent would apply.

A hand-held spectrograph has been constructed for use by an astronaut to test the feasibility of measuring cloud-top altitude from a satellite using the reflected solar radiation from the cloud in the oxygen “A” band at 7600 A. This instrument has been used from an aircraft for preliminary measurements. The transmission of oxygen at 7630 A with 5-A bandpass as function of zenith distance of the sun and altitude (pressure), calculated by Wark and Mercer, were checked experimentally by us using the solar beam reflected from a white diffusing card. The instrument was used to photograph cloud spectra from an aircraft flying above the clouds. The cloud altitudes were computed using the transmission curves and found to be about 140 mb (1.4 km) lower than the actual cloud-top altitudes observed during descent of the aircraft. This apparent error is too large to ascribe to probable experimental errors. It is apparent that the absorption by oxygen inside the cloud contributes significantly to this type of measurement. A method based on the two streams theory of Schuster and the appropriate absorption coefficients of oxygen were used to compute the fractional absorption inside the cloud at the wavelength of interest for cloud models representing low, medium, and high cloud situations. The computed fractional absorption for low cloud accounts for the apparent error in the observations taken on stratocumulus clouds. Before an instrument can be used on a satellite for routine measurements of cloud altitudes, several effects must be investigated further, such as the fractional absorption inside the cloud and its variation with cloud thickness and type, and the effect of presence of lower deck of cloud and the effect of various ground albedos, especially in the case of thin clouds.

Assuming there is no roll or pitch of ships from which cloud-height measurements are made, three equations for the cloud height have been derived. These are equations of the utmost generality, from which all solutions of possible interest are readily had an special cases by using progressively simpler assumptions. After satisfactorily demonstrating that errors from the ship's roll and pitch can be calculated without regard to the method used in estimating cloud height, such errors are calculated by use of direction cosines.

A theory is developed for the spectral irradiance of the overcast sky, and also of the top of the overcast, in terms of incident radiation, droplet size and concentration, and cloud thickness. The absorption of light by cloud water is allowed for.Some samples of highly polluted cloud water have been spectrophotometered, and calculations made which show that the absorption produces very little effect in the visible spectrum, and only a moderate effect in the ultraviolet, even for very impure cloud water. On the other hand, the color of the ground has a considerable effect on the spectral irradiance of the overcast sky, and thus on its color. The color of the top of the overcast is, however, not greatly affected by that of the ground.

A “mushroom cloud” model describes the depositional features of volcanic ash (and resultant bentonite beds) from highly explosive volcanoes. The model is represented as an expanding disk-shaped cloud from which deposition occurs. The model contains an original particle-sized distribution, allows areal expansion of the distribution in a turbulent atmosphere, and aggregation of particles due to water and static electricity in the volcanic cloud. Evaluation of the model yields elongate tongue-like deposits whose lateral and downwind spread is a function mainly of wind velocity and cloud height, but its dimensions are affected significantly by particle aggregation. Histograms of particle sizes from several positions on an ash bed show change of modal sizes and skewness in hypothetical ash beds. The hypothetical distributions show close affinities to particle sizes in real ash beds.

A method of relating the frequency of six tenths or more sky cover to the frequency of various ceiling heights is presented to show how the latter may be estimated in areas where only total sky cover has been observed and recorded. Tests show that the method has significant value above 1000 ft.

The angular scattering diagram of a Nevada desert atmosphere has been measured under clear sky and cloudy conditions for radiation of 0.40-, 0.45-, 0.50-, and 0.55-μ wavelengths, and attenuation coefficients for scattering and absorption have been determined for radiation of 0.40-, 0.50-, 0.70-, and 0.83-μ wavelengths. The ratio R of scattered-in to direct radiation received from a 4-π source by a flat receiver facing the source was also determined for the latter wavelengths, both with and without cloud cover. Source-receiver distances D of from 1.07 to 17.15 miles were used. Plots of R vs D are presented which show that under conditions of little or no cloud cover, R for each of the four wavelengths increases monotonically with D out to a distance of 17.15 miles, whereas for the case of full overcast at 4000 ft above ground, R increases with D for distances up to about 10 miles and then decreases with increasing distance at least for the next few miles. For lower cloud heights, the maximum in R appears to move to smaller distances, approximately in proportion to cloud height.

IT would be interesting if any of the readers of NATURE could give some information respecting the usual height of cirrus clouds. Mr. Clement Ley, in his work, “The Laws of the Winds,” states—“The time occupied by these clouds in passing from the zenith to 45°, or the contrary, furnishes us with a standard of measurement which is both convenient for simultaneous observations, and also possesses this obvious advantage, that whenever the altitude of the cloud station is at all determinable, none but the simplest of calculations is required in deducing the actual from the apparent velocity.” Granted; but it would have been advantageous had he shown by an example what he means. For, he goes on to say, “The ordinary range of the actual rapidity of this current is about twice as great as that of the rapidity of the surface winds, for while the latter, at stations most fully exposed to their violence, rarely attain, in Europe, a velocity of 60 or 70 miles an hour, the most elevated clouds not uncommonly traverse a distance of 120 miles an hour, and occasionally much more.” Coupling this with the next statement—“I have only once or twice observed an actually motionless cirrus cloud, and it is on rare occasions that an hour is occupied in passing from the zenith to 45°,” let me ask, what would be the vertical height of such a cloud?