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Roland List

Abstract

In conformity with experimental results the heat transfer of gyrating spherical hailstones is divided into two parts. One takes place over a normally wet but supercooled equatorial region of limited roughness, whiles the other occurs over a rough, dry polar zone which is at a substantially lower temperature than the equatorial region. The sensitivity of this complex heat transfer is studied by establishing the response of the heat transfer to changes of 15 individual parameters relative to the results of a single hailstone growth experiment.

The propagation of errors in individually setting the icing conditions of a laboratory experiment is somewhat different from the sensitivity study. Both sensitivity and error hierarchies are given and conclusions are made about measurement accuracies. The measurements for the case at hand suggest that treating hailstones as smooth, nonrotating particles underestimates the heat transfer by a factor as high as two. For the treatment of a whole dataset containing many hailstone growth experiments, roughness may no longer be considered as a single-valued quantity since it seems to vary with hailstone latitude and icing conditions.

The tool used, the “spreadsheet,” is ideal to assess the complex heat budget. It serves to indicate potential problems before large experimental series are launched.

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Roland List

Abstract

Hollow spheres filled with ethylenedichloride and snowballs of low mechanical cohesion drenched in water were exposed to blast waves. They did not show any effect that could be attributed to the explosion of 1 kg TNT at distances of 5 m and more from the explosive charge. The same conclusion was indicated by similar experiments in which artificial hailstones consisting of ice-water mixtures, grown in a hail tunnel, were used as test objects.

Therefore no justification is given for attempting to combat hail by any mechanical softening of the hail-stones-in the immediate proximity of exploding rockets.

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Roland List

Abstract

Features and performance of a pressure controlled vertical icing tunnel are described which allow the investigation of the formation of single hailstones in simulated cloud conditions. It is expected that new facts and a better understanding of the physics of hail formation can be found by the use of these quite unique facilities.

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Roland List

Abstract

No abstract available.

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Roland List

Abstract

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Roland List

Abstract

By setting up and solving general equations for the heat balance and the material transfer between a spherical hailstone and its environment it is possible to show how the different variables such as air temperature, air pressure, liquid water content of the air (in the form of drops), hailstone diameter, speed of fall of the hailstone, its surface temperature and growth rate are interdependent. At the same time growth zones can he delimited within which accretion is accompanied by evaporation or sublimation of H20, or where an increase of mass by sublimation exceeds the amount of accretion. From the growth conditions it is possible to classify the resulting ice deposits on a physical basis.

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Roland List

Abstract

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Roland List

Abstract

Liquid water skins on spongy deposits of hailstones that grow while gyrating in a wind tunnel environment, have been routinely observed to be supercooled at the water skin–air interface to as low as −5°C and more. The average water skin thickness (up to 1 mm) in the main growth region is calculated on the basis of the molecular conduction of the latent heat of freezing from the spongy substrate at the base of the water skin to its surface. The heat transfer is gradient-driven and relates directly to the speed of ice accretion on the hailstone. An extrapolation of an equation for the ice growth speed in supercooled bulk water suggests a supercooling of the ice–water interface of the order of −0.3°C.

The physical picture emerging is that of an ice sponge from which a fragile dendrite mesh grows into the water skin with a very homogeneous front and an advance speed that is controlled by the diffusion of heat (heat conduction) away from the ice front. Combining all results, it can be categorically stated that all surface points of growing hailstones have temeperatures below the freezing point of water.

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Roland List

Abstract

No abstract available.

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Roland List

Abstract

An all-encompassing new theory of heat and mass transfer (HMT) and growth equations have been developed for freely falling spherical hailstones with diameters of 0.5–8 cm. The initial six variables are diameter, liquid water content, air and hailstone surface temperatures, net collection efficiency, and ice fraction of spongy deposit. They are replaced by three or four new ones, depending on the three growth categories. Two new variables are products of “old” ones: (i) the square root of the Reynolds number Re and the liquid water content and (ii) net collection efficiency and ice fraction of the spongy deposit. Only the products matter, not the individual parts. [The two variables in (ii) are as important as the two in (i).] Two old variables remain: air and surface temperatures. The HMT can be further compacted for hailstorms with specified pressure–air temperature–height profiles. Further, Re for free-fall reveals unexpected complexities—issues important to solve HMT problems.

The “new hailstone physics” is based on 55 years of in-house studies of all aspects of hailstone growth, followed by 5 years of shaping these puzzle pieces and assembling them into a coherent picture. This was only possible by recognizing the free-fall mode: a special gyration that allows hailstones to grow with a radial symmetry and, thus, homogeneous surface temperature.

Part II will display the surprising solutions to growth and HMT and firmly link the hailstones to mostly spongy growth with shedding that favors a hail-coupled rain mechanism.

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