Showing papers on "Cloud condensation nuclei published in 1971"

Simultaneous airborne measurements of cloud condensation nuclei and sodium‐containing particles over the ocean

Abstract: Simultaneous airborne measurements have been made over the Pacific Ocean of the concentrations of cloud condensation nuclei (CCN) active at 0.5 per cent supersaturation and the concentrations and size-spectra of sodium-containing particles (SCP) over a wide range of sizes. The concentrations of SCP decreased sharply with increasing height above the ocean and it is likely that most of them originated from the ocean. However, even at altitudes as low as 50 ft above the surface of the ocean, and over surf, the measured concentrations of SCP were only a few per cent of the concentrations of CCN at G.5 per cent supersaturation. Also the average concentrations of CCN did not change significantly with altitude up to at least 10,000 ft. These observations indicate that sea-salt particles were not the major source of the CCN. It is suggested that the production of sulphates in cloud droplets, followed by the release of these particles when the droplets evaporate, is an important mechanism for increasing the efficiency of CCN in the air.

Natural Ice-Forming Nuclei in Severe Convective Storms,

Abstract: There are two populations of aerosol particles in severe storms: normal background aerosol and aerosolized soil particles. Concentration of the latter, which depends on local wind speed and soil conditions, may be orders of magnitude higher than that of the former. Condensation nuclei are derived principally from the first source. Concentration of ice-forming nuclei, which derive from the soil particles, increases during storms up to 100 times the pre-storm (background) value. This concentration increase is less than that of the aerosol population, indicating that only a fraction of soil particles exhibit ice-nucleating properties. The fraction of soil particles active as ice-forming nuclei in a given particle size range increases with particle size; however, the concentration of ice-forming nuclei in air is counteracted by a decrease in the concentration of aerosol particles with size. Supercooled water drops are nucleated by hydrosol soil particles at temperatures as high as −5.3C. The quantity...

Effects of Cloud Condensation Nuclei Due to Fires and Surface Sources During South Florida Droughts

Abstract: Cloud condensation nuclei (CCN) at cloud base strongly affect the droplet concentration at cloud base, which in turn influences the life history of a cloud. There are usually more nuclei over land than over water because of surface sources of nuclei, and vegetation fires produce large numbers of nuclei which may keep much of a cloud's water in small droplets that fail to reach raindrop size. Smoke particles from drought fires are thought to result from the burning of four major vegetation types in South Florida. Samples of these were burned in the laboratory and produced (at 0.75% supersaturation) between 109 and 1010 nuclei per gram burned. A simple calculation, assuming reasonable burn rates for these materials, resulted in 4600 CCN cm−2 when mixed uniformly to cloud base over a large area. The drought between 1 April and 15 May of 1967 over Florida was found to be related predominantly to synoptic-scale dryness and subsiding northerly winds aloft. There was no significant large-scale lag in ra...

Numerical Experiments Pertaining to Warm-Fog CLEARING1

Abstract: An attempt has been made quantitatively to assess the prospects for modifying warm fogs by seeding them with condensation nuclei This has been done by calculating the time-dependent changes in the sizes and concentrations of fog droplets that are predicted by the ordinary equations of diffusion of water vapor to and from the surface of droplets Their size, molality, and ambient water-vapor density are taken into account Initial conditions consist of a homogeneous volume of air of specified height and aerosol content An external cooling rate and seed dosage are specified The effects of various combinations of cloud height and seed properties (such as size, mass density, and rate of injection) on the metamorphosis of the fog-droplet population are examined Usually, the cloud-forming process (in which temperature decreases with time) was allowed to continue after the completion of seeding It is tentatively concluded that the optimum-size seeding material is a function of the fog thickness; ma

A Model for Cloud Droplet Growth by Condensation in an Inhomogeneous Medium

Abstract: A new model for droplet growth by condensation is proposed which can produce a broad droplet size spectrum as well as local fluctuations in droplet number concentration and liquid water content. The model depends neither on mixing nor on the type of condensation nuclei present. It is based on small fluctuations in relative humidity coupled with droplet sedimentation. During the early stages of condensation, small local variations in relative humidity can produce significant variations in droplet sizes. The resulting non-uniformities in droplet sedimentation rate create inhomogeneities in droplet number concentration, which, in turn, cause variations in vapor pressure and differences in droplet growth rates. Numerical experiments show that the initial conditions required for this model to produce a broad droplet size spectrum are sufficiently general to be of common occurrence. Although the model does not include any turbulence effects, it is argued that the process modeled does not require the co...

Cloud Condensation Nuclei and their Possible Influence on Precipitation

Abstract: Measurements of cloud condensation nuclei (CCN) conducted both from the ground and from an instrumented aircraft during two winter seasons, utilizing a thermal diffusion chamber, suggest that. CCN spectra prove to always have, in both rainy and dry weather, continental-like characteristics. Their concentrations were shown to depend on wind direction and found to be surprisingly high, even in air masses with a partial maritime trajectory. The existence of local sources appears to have a limited and a generally minor effect on the characteristics of CCN spectra. On the average, CCN concentrations tend to decrease by a factor of 2 during the first three days of a rainy spell. Consequently, cloud droplet concentrations, computed for cumulus clouds assumed to have base updraft velocities of 250 cm sec−1, seem to show a similar trend but are always within the range of 180–950 cm−2. Since these variations do not seem to correlate with rainfall, they are not dominant in their effect on the relative effic...

A Comprehensive Report on Nineteen Condensation Nuclei. Part 2

Abstract: : The document contains tabulations of the growth times of condensation nuclei under various assumed supersaturations. The growth time of each nucleus at a given supersaturation is obtained from a newly developed growth rate equation which utilizes a single parameter to denote the hygroscopicity of the nucleus and which includes an additional term to reflect the inefficiency of the condensation process. This volume and its predecessor serve as a valuable reference for the experimental cloud physicist in his study of the growth behaviors of condensation nuclei and for the field cloud physicist in his selection of the proper artificial nuclei for warm fog or cloud modification.

Some experimental results elaborating the equilibrium conditions of small ions in the atmosphere over the pacific ocean

Abstract: ii This paper presents some results of measurements of small ion density and concentration of condensation nuclei over the Pacific Ocean. The results obtained relatively near shore seem capable of being explained in terms of the simplified equilibrium equation which takes into account only the attachment of small ions to condensation nuclei. At greater distances, both the processes of recombination of small ions and attachment of small ions to condensation nuclei to be considered. The difference in the value of the attachment coefficient estimated from the near and distant measurements seems to be consistent qualitatively with the theory of coagula­ tion which predicts an increase in the particle radius with time.

Considerations in the Measurement of Pollution Effects on the Number Concentration of Cloud Condensation Nuclei

Abstract: Pollution effects on cloud condensation nucleus (CCN) concentration vary widely, sometimes even decreasing the CCN. The interpretation of these pollution effects may be invalid if the nuclei are measured by expansion-type Aitken nucleus counters operated at low expansion ratios to provide a readout (with natural nuclei) equal to that of a thermal-gradient diffusion cloud chamber (TGDCC) instrument operated at 1% supersaturation. Calculations indicate that the expansion chamber operated at a ΔP of 1.4 inch Hg produces a supersaturation of 26%, allowing for vapor depletion effects during the first few hundredths of a second after expansion. The maximum supersaturation in clouds is typically 0.1–1%; therefore, 26% causes a, large positive error. Further, vapor depletion in the instrument during droplet growth limits the growth rate and size of droplets at supersaturations which are low compared to those for which the instrument is designed. Therefore, use of the standard calibration curve contribute...

Experiments pertaining to warm-fog clearing

Abstract: An attempt has been made quantitatively to assess the prospects for modifying warm fogs by seeding them with condensation nuclei. This has been done by calculating the time-dependent changes in the sizes and concentrations of fog droplets that are predicted by the ordinary equations of diffusion of water vapor to and from the surface of droplets. Their size, molality, and ambient water-vapor density are taken into account. Initial conditions consist of a homogeneous volume of air of specified height and aerosol content. An external cooling rate and seed dosage are specified. The effects of various combinations of cloud height and seed properties (such as size, mass density, and rate of injection) on the metamorphosis of the fog-droplet population are examined. Usually, the cloud-forming process (in which temperature decreases with time) was allowed to continue after the completion of seeding. It is tentatively concluded that the optimum-size seeding material is a function of the fog thickness; material smaller than about 4 pm in diameter should be excluded, and particles 20 to 50 pm in diameter are most suitable for seeding moderately thick (-100 m) clouds.