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- Author or Editor: J. E. McDonald x
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Abstract
Condensation in vertically moving planetary atmospheric gases containing small quantities of any condensable vapor will occur in updrafts only if the latent heat of the viper is sufficiently large, otherwise the condensation will require adiabatic compression and will occur only in downdrafts, as first noted by Brunt. Several methods are presented for viewing and deriving a quantitative criterion for the intermediate case in which the vapor behaves in neutral fashion in either ascending or descending currents. Put in its most general form, relating the molar entropy of vaporization S of the condensable vapor and the molar specific heat C p , of the carrier gas, the null criterion requires S= C p . The peculiar nature of meteorological processes in a planetary atmosphere in which latent heat considerations imply downdraft condensation is examined in some detail, and some implications of the null condensation criterion for studies of planetary atmospheres and their evolution are discussed.
Abstract
Condensation in vertically moving planetary atmospheric gases containing small quantities of any condensable vapor will occur in updrafts only if the latent heat of the viper is sufficiently large, otherwise the condensation will require adiabatic compression and will occur only in downdrafts, as first noted by Brunt. Several methods are presented for viewing and deriving a quantitative criterion for the intermediate case in which the vapor behaves in neutral fashion in either ascending or descending currents. Put in its most general form, relating the molar entropy of vaporization S of the condensable vapor and the molar specific heat C p , of the carrier gas, the null criterion requires S= C p . The peculiar nature of meteorological processes in a planetary atmosphere in which latent heat considerations imply downdraft condensation is examined in some detail, and some implications of the null condensation criterion for studies of planetary atmospheres and their evolution are discussed.
Abstract
The theory of heterogeneous nucleation on partially-wettable insoluble spherical particles as developed by Fletcher is used to obtain relations readily applicable in the meteorologically important limit of low supersaturation ratio S. Taking 1.03 as the probable absolute maximum value of S in natural clouds, it is found that activation of a particle demands that the contact angle θ be less than 12°. Data from surface chemistry indicate that this θ-limit separates airborne particulates into a small class of active nucleating substances and a remaining broad class of nonactive cases. However, roughness effects (Wenzel's rule) may shift some from the latter to the former class, a trend possibly offset under special conditions (e.g., over industrial areas) by adsorption of non-wetting layers. A simple physical basis for understanding nucleation enhancement in terms of reduced size of the critical embryo is suggested, and a rule for predicting the approximate size-limits for activation of fully wettable particles of irregular shape is deduced. The silicate dusts comprise one important class of wettable atmospheric particulates, but it is concluded that typical clouds characterized by average values of maximum S (≐1.001) can activate only silicates of size larger than a few microns. The most favorable conditions (maritime cumuli, strong updrafts) may lead to activation of dusts with maximum diameters just under 0.1 μ, contributing to the global precipitation-scavenging of the silicate dust component of the atmospheric aerosol, a contribution especially important for those in the gap between the particle sizes scavenged by Brownian diffusion on cloud droplets and sizes scavenged by aerodynamic impaction by raindrops.
Abstract
The theory of heterogeneous nucleation on partially-wettable insoluble spherical particles as developed by Fletcher is used to obtain relations readily applicable in the meteorologically important limit of low supersaturation ratio S. Taking 1.03 as the probable absolute maximum value of S in natural clouds, it is found that activation of a particle demands that the contact angle θ be less than 12°. Data from surface chemistry indicate that this θ-limit separates airborne particulates into a small class of active nucleating substances and a remaining broad class of nonactive cases. However, roughness effects (Wenzel's rule) may shift some from the latter to the former class, a trend possibly offset under special conditions (e.g., over industrial areas) by adsorption of non-wetting layers. A simple physical basis for understanding nucleation enhancement in terms of reduced size of the critical embryo is suggested, and a rule for predicting the approximate size-limits for activation of fully wettable particles of irregular shape is deduced. The silicate dusts comprise one important class of wettable atmospheric particulates, but it is concluded that typical clouds characterized by average values of maximum S (≐1.001) can activate only silicates of size larger than a few microns. The most favorable conditions (maritime cumuli, strong updrafts) may lead to activation of dusts with maximum diameters just under 0.1 μ, contributing to the global precipitation-scavenging of the silicate dust component of the atmospheric aerosol, a contribution especially important for those in the gap between the particle sizes scavenged by Brownian diffusion on cloud droplets and sizes scavenged by aerodynamic impaction by raindrops.
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The work of the nineteenth-century American meteorologist, James P. Espy, is discussed in relationship to the early development of understanding of the thermodynamics of clouds and the dynamics of convection. Espy was the first to recognize the important role of release of latent heat of condensation in sustaining cloud and storm circulations.
The work of the nineteenth-century American meteorologist, James P. Espy, is discussed in relationship to the early development of understanding of the thermodynamics of clouds and the dynamics of convection. Espy was the first to recognize the important role of release of latent heat of condensation in sustaining cloud and storm circulations.
A study of the history of the early developments in the theory of the saturated adiabatic process reveals that the first quantitative formulation of both the dry- and the saturated-adiabatic cooling rates were given by the British physicist, William Thomson (later Lord Kelvin), in a paper read in 1862, although the first published analysis was presented in 1864 by a Swiss meteorologist, Reye. Reye's paper presents both the differential form and the exact integrated form of the reversible saturated adiabatic relationship, which thus attained its modern form at the moment of its first appearance. Reye also gave a very clear, though brief, discussion of the concepts of atmospheric stability of both dry and saturated layers. The papers of Hann (1874) and Hertz (1884), frequently misquoted as the original treatments of the saturated adiabatic theory, are thus shown to be secondary papers in the actual history of this important theory.
A study of the history of the early developments in the theory of the saturated adiabatic process reveals that the first quantitative formulation of both the dry- and the saturated-adiabatic cooling rates were given by the British physicist, William Thomson (later Lord Kelvin), in a paper read in 1862, although the first published analysis was presented in 1864 by a Swiss meteorologist, Reye. Reye's paper presents both the differential form and the exact integrated form of the reversible saturated adiabatic relationship, which thus attained its modern form at the moment of its first appearance. Reye also gave a very clear, though brief, discussion of the concepts of atmospheric stability of both dry and saturated layers. The papers of Hann (1874) and Hertz (1884), frequently misquoted as the original treatments of the saturated adiabatic theory, are thus shown to be secondary papers in the actual history of this important theory.
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Mean upper-air pressure heights, temperatures, and mixing ratios are presented for 48 radiosonde stations well distributed in longitude around the globe. Of the 48 stations whose means are given, a majority were selected from arid zones, equatorial regions, and from the Southern Hemisphere. This geographical distribution, plus the fact that humidity means are given, constitute the chief value of the data presented.
Abstract
Mean upper-air pressure heights, temperatures, and mixing ratios are presented for 48 radiosonde stations well distributed in longitude around the globe. Of the 48 stations whose means are given, a majority were selected from arid zones, equatorial regions, and from the Southern Hemisphere. This geographical distribution, plus the fact that humidity means are given, constitute the chief value of the data presented.
