A few European scientists briefly looked into the nature of raindrops in the 1800s. The most prominent was Philipp Lenard, a German physicist. He was a brilliant experimental physicist who received the Nobel Prize in physics in 1905 for his work with cathode rays. He studied or taught at many of the major universities in Germany and Eastern Europe during the late nineteenth and early twentieth centuries.
Lenard began studying at raindrops in 1898. At the same time as Lenard began his work on raindrops, investigations of raindrop size were also taking place across the Atlantic on the farm of Wilson A. Bentley, a farmer/scientist best known for his photography of snow crystals. It also appears Lenard was not aware that E.J. Lowe (1892) and J. Wiesner (1895) had made the first measurements of raindrop size a few years previous. Nor was he aware of the work being undertaken by Bentley.
Lenard published the results of his extensive investigation in June 1904 (four months before Bentley would published his findings) in a paper titled Uber Regen in the German journal Meteorologische Zeitschrift. It presents his work on the shape, size and stability of raindrops during their descent from clouds.
Faced with the problem of how to measure raindrop sizes during a rainstorm, Lenard chose to use blotter paper dusted with a water-soluble dye as a drop collector. When raindrops fell on the impregnated blotter, they produced coloured wet spots which could then be measured. Lenard was concerned that the size of the wet spot on the blotter paper might not reflect the true size of the drop that made it. He thus undertook to establish whether a relationship between the spot size and the drop diameter existed. By dropping known size drops onto the blotter paper and measuring their splash print, Lenard was able to develop a calibration curve for the method.
Lenard partitioned his raindrop data into 0.5 mm (0.02 inch) diameter intervals, reporting it as the number of raindrops of a particular size range falling on an area of one square metre in one second. He used the technique to collect only ten field samples of drop size distribution, and therefore, could not draw many general conclusions relating drop size to rain event conditions. Since Lenard found no drops with diameters less than 0.5 mm (0.02 inch), he did conclude that the updrafts in the clouds must be of sufficient strength to prevent such small drops from falling out. We also know that he only recorded one drop in the 4.75 to 5.25 mm diameter range.
Much of Lenard's work focused on the behaviour of raindrops as they fell from the clouds. To do this, he constructed an innovative vertical wind tunnel in which he could vary the upward speed of the airflow to simulate atmospheric updrafts. And by adjusting the airflow rate, he could briefly balance a drop in the air stream. This balancing act simulated the aerodynamic forces acting on a drop falling freely through a still air column. And a balancing act it was. The turbulence levels in the airflow of his wind tunnel were so high that drops could not be held steady for more than a few seconds.
Using the wind tunnel to observe drop behaviour in an airstream, Lenard could see the actual shape a raindrop took while falling. [A drop's shape is the same whether it is falling through still air or holding its position in an updraft.] By suspending drops of known size, he determined that small drops up to about 2 mm (0.08 inches) in diameter "fell" as spheres. Larger drops, however, deformed while falling acquiring a shape with a flat bottom and rounded top similar to that of a hamburger bun. Thus, Lenard was the first to report that raindrops were not the stereotypical teardrop shape but were spherical when small and shaped much like a hamburger bun when larger.
Drops, however, became unstable at diameters greater than 5.5 mm (0.21 inches), Lenard found. They lasted less than a few seconds before breaking apart in the airflow, torn asunder by the aerodynamic forces acting on the drop. This observation combined with the lack of drops larger than this size in his rainfall measurements led Lenard to conclude that the maximum drop size possible in nature was just larger than 5 mm.
Lenard also used his wind tunnel to determine the fall velocity of drops by increasing the flow rate until the drop became suspended. This flow speed was also the drop's fall velocity. He found that the fall speed increased with drop diameter until a size of 4.5 mm (0.18 inch). For larger drops, however, the fall speed did not increase beyond 8 metres per second (26 ft/sec). He attributed this to the changes in drop shape caused by the air flow as the drop size increased. The change in shape thus increased the air resistance of the drop and slowed its fall rate.
Although Lenard's paper reported many major insights into the shape, size and stability of raindrops, his work, like that of Bentley, was virtually ignored by contemporaries and only years later was he eventually credited for his contribution when other researchers re-discovered his findings. And like Bentley, there was no encore to Lenard's 1904 paper. Neither ever published on this topic again. In part it was due to a lack of interest in the atmospheric sciences in the sub- processes of rain.
But perhaps had Lenard not shifted his interest to other problems in physics, his prestige in the academic world may have forced others to look further into the subject of raindrops. Lenard would only publish one more paper related to atmospheric phenomena in his lifetime. Published in 1915, it dealt with the electrification produced by the splashing and breaking of water drops during free fall.
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