Henry G. Houghton

ON THE ANNUAL HEAT BALANCE OF THE NORTHERN HEMISPHERE

Abstract The increased knowledge of atmospheric radiation during the last twenty years has made it desirable to re-compute the heat balance of the northern hemisphere. Insofar as possible the computations are based on observational data and, in particular, no assumption is made regarding the planetary albedo nor the albedo of clouds. The solar radiation reaching the surface is derived largely from the North American pyrheliometric network. These data are extended to the hemisphere on the basis of the differences between the cloud amounts along complete latitude circles and the cloud amounts along the North American segments. Absorption of solar radiation in the atmosphere is computed from the original Smithsonian data. Absorption by clouds is taken into account. The outgoing long-wave radiation is computed on the Elsasser radiation chart, with use of monthly mean data from a large number of individual stations. The planetary albedo is found to be 0.34, with a minimum of 0.28 in the subtropics and a maximu...

On the Physics of Clouds and Precipitation

The entire science of meteorology is concerned with the physics of the atmosphere, but the term physical meteorology has been accepted as the designation for only one portion of the science. This division of the field includes topics such as atmospheric optics, atmospheric electricity, solar and long-wave radiation, and the physical processes of condensation and precipitation. The latter is the subject of the present contribution. The discussion will start with a consideration of condensation nuclei and will then proceed in turn to treat the initiation of condensation, the growth of the condensation products, and the formation of precipitation elements. A distinction must be made between condensation in the liquid phase and in the solid phase. There are also differences between the formation of solid and liquid precipitation elements. A brief discussion of the artificial dissipation of fog will also be included.

A PRELIMINARY QUANTITATIVE ANALYSIS OF PRECIPITATION MECHANISMS

Abstract The two principal physical mechanisms for the formation of precipitation are considered to be the Bergeron-Findeisen ice-crystal process and the collision of particles of unlike size in the gravitational field. Quantitative estimates of the growth of precipitation elements by the two processes are made. It is found that the ice-crystal effect is much more rapid than the collision process in the initial stages of growth. When the particles are of a mass comparable to drizzle drops, the two processes are equally effective; for larger particles the accretion process becomes more rapid than the ice-crystal effect. For typical mid-latitude conditions it is concluded that neither process, acting alone, is capable of producing rain drops of the customarily observed size. It appears that there is required some combination of the two processes such as the clumping of ice crystals to form snow flakes, the growth of ice crystals which melt and then grow by accretion, or the growth of graupel by the simultan...

ON THE CHEMICAL COMPOSITION OF FOG AND CLOUD WATER

Abstract The partial chemical analysis of some ninety samples of fog and cloud water is reported. The fog samples were collected at four locations along the northeast coast of North America, and the cloud samples were obtained on Mount Washington, New Hampshire. It is found that the soluble nuclei are predominantly chlorides and sulfates. The chlorides are dominant in air with a recent trajectory over the sea, while the sulfate is always present. In many cases it appears that much of the sulfate is in the form of sulfuric acid.

A THEORY OF ENTRAINMENT IN CONVECTIVE CURRENTS

Abstract It is held that entrainment is a necessary dynamic consequence of the vertical stretching of an accelerated convective column. On this basis, equations are developed for the rate of entrainment, the vertical divergence and the lapse rate, for both unsaturated air and cloud air. It is assumed that a steady state exists, the cross section of the rising column is invariant with height, the entrained air is uniformly mixed with the rising air and the environment is at rest. The equations are integrated numerically over height in a number of selected cases. In unsaturated air, entrainment results in a lapse rate which is always greater than the dry adiabatic; if the environmental lapse-rate is superadiabatic, the lapse rate of the rising air is intermediate between the lapse rate of the environment and the dry adiabatic lapse-rate. In a cloud, entrainment results in a lapse rate intermediate between the environmental lapse-rate and the moist adiabatic lapse-rate. The lapse rate of the rising air incre...

A Study of the Evaporation of Small Water Drops

A theoretical expression for the evaporation of small spherical water drops in still air is developed from the general evaporation equation of Jeffreys. Experimental data for the evaporation of drops ranging from 25 to 2600 microns in diameter are obtained at several temperatures and relative humidities. After making approximate corrections for the cooling of the drops it is found that the results are in general agreement with the theoretical evapooration equation. Residual variations which are functions of the drop size and the difference between the water vapor density at the surface of the drop and at a distance from the drop are probably due to inaccuracies in the method used for computing the drop temperatures. The results of the paper furnish a means for computing the total time required for the complete evaporation of a drop of liquid water into a still atmosphere at any given temperature and relative humidity.

The Scattering Cross Section of Water Drops in Air for Visible Light

Additional values of the scattering cross section of water drops in air have been computed from the theory of Mie. The water drops are assumed to be dielectric spheres of index of refraction 4/3. Values of the scattering area coefficient have been computed for 50 values of α = 2πr/λ, ranging from 7 to 24. The curve of the scattering area coefficient versus α has three major maxima and two minima in this range, with indications that the curve continues to oscillate with decreasing amplitude for increasing α. Numerous minor oscillations were found in the vicinity of the major maxima and minima.

The Size and Size Distribution of Fog Particles

An apparatus for the microscopic measurement of fog particles is described. The particles were collected on a slightly greased glass slide and viewed with the aid of dark‐field illumination. The particle sizes were measured either visually with an eyepiece micrometer or, more usually, by the subsequent measurement of photomicrographs. With the aid of this apparatus a series of fog particle size distribution curves were obtained. These curves are characterized by a single maximum which always occurs at a particle diameter which is an integral multiple of 3.1 microns. The curves approach the axis asymptotically for large particle diameters but have a definite minimum. An explanation for this minimum on the basis of certain properties of the nuclei of condensation is offered.

A STUDY OF NON-GEOSTROPHIC FLOW WITH APPLICATIONS TO THE MECHANISM OF PRESSURE CHANGES

Abstract Surface pressure changes can occur only when an accelerational field exists. The regularity of occurrence, the distribution, and the magnitudes of the accelerational fields found in the atmosphere have been determined from the available data. The most direct method used was to plot maps of the deviation of the observed wind from the geostrophic wind. Charts of the horizontal divergence, as determined from the observed winds, were prepared for several levels. Charts were also drawn of the non-geostrophic temperature changes, which are defined as the difference between the actual 12-hour temperature changes and the temperature changes which would result from geostrophic advection of the temperature field. It is shown that the magnitudes of the divergence and the non-geostrophic temperature changes are consistent with the observed deviations from the geostrophic wind. The errors of each method are investigated and it is concluded that they are not sufficient to affect the order of magnitude of the r...

On the Relation Between Visibility and the Constitution of Clouds and Fog

Following Koschmeiders method of attack an expression for the daytime visibility within a cloud or fog is derived in terms of the size of the suspended drops and the number of drops in a unit volume of air. A comparison of typical data for clouds and fogs shows that clouds contain from 50 to 500 drops per cc. with an average drop diameter of about 20 microns while fogs contain only 1 to 10 drops per cc. with an average drop size of about 50 microns. Simultaneous observations of the visibility and the fog characteristics reveal a good qualitative agreement with the theory. In clouds the visibility tends to be inversely proportional to the liquid water content, indicating a constant average drop diameter. In fogs the visibility is more nearly inversely proportional to the two-thirds power of the liquid water content which suggests that the number of drops in a unit volume tends to remain constant. The value of Fechners constant, which represents the threshold contrast between the object and the background, was found to be greater in a fog than when measured in the laboratory and still larger in clouds. I t is suggested that this is due to the diffuse boundary between the object and the background in the case of a fog or cloud.