The Great Depression Aids Climatological Study

The U.S. Weather Bureau had made a good start on compiling useful climatological data from stations throughout the United States through the end of World War I. Funding reductions in the immediate postwar years, however, led to the steady deterioration of climatological services after 1920. Lack of station reports was not the problem. Over 5,000 unpaid cooperative observers submitted reports of maximum and minimum temperatures, air pressure readings, and precipitation totals by mail to bureau headquarters at the end of each month. These reports, combined with the detailed observational data collected by official Weather Bureau "first-order" stations, provided valuable information about weather conditions. But, data must be analyzed to be useful, and all too often the data arrived only to be filed away. Observational data that could have yielded scientific insight into climatological problems languished in boxes. The relatively limited averages available in weekly crop bulletins, pamphlets issued by state agricultural offices, or supplements to the Weather Bureau's in-house journal Monthly Weather Review were not sufficiently detailed for wide-scale use by land planners, water resource managers, building construction companies, aircraft designers, or public health workers. The bureau was sitting on a wealth of information and had no way to make it usable by the broader public.


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The U.S. Weather Bureau used punch cards like this one to record thousands of climatology records during the Great Depression.

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The U.S. Weather Bureau used punch cards like this one to record thousands of climatology records during the Great Depression.

long as there was something to which they could cling. For example, sea salt particles, fine dust, or sulfuric acid droplets formed from combustion were all suitable hygroscopic particles because they were from 100 to 1,000 times larger than the individual water molecules. These droplets needed to have diameters of at least 0.04 inch (1 mm) to fall from the cloud. That meant a million of the tiny droplets had to join before they could fall out. Once one droplet attached itself to a cloud condensation nucleus then others would join it. It would take days or weeks for the droplet to grow large enough to fall. Clearly that was not the way rain formed.

There were two possible ways that a drop could form faster. The first way, called drop capture, assumed that larger drops fall faster than

By the 1920s, Great Britain, France, Germany, and the Netherlands had all adopted systems using punched cards to record and sort clima-tological data. At the time, that was really the only way to process large volumes of information. The bureau did not take steps to add punch card machines and techniques to its central office in Washington, D.C., until the Science Advisory Board recommended that it do so in 1934. Punch card machines would not solve the data analysis problem if there were no people to punch the data onto the cards. Since all government agencies had seen severe budget reductions as a result of the Great Depression, the Weather Bureau had fewer people available to compile climatological records than in the past.

That problem was solved later the same year when the Civil Works Administration—a depression-era government agency that created work for over 4 million unemployed men and women desperate for jobs—appropriated enough money to the Weather Bureau to allow the hiring of several hundred people to tackle the backlog. As a result, more than 50 years of marine-weather data (ship and coastal reports), eight years of fire-weather service data, and 35 years of routine climatological data were punched onto cards and sorted by machine. In 1936, the Weather Bureau received another appropriation of funds from the Works Progress Administration (WPA)—another make-work organization—that it used to compile, record, and sort both surface and upper-air (radiosonde and pilot balloon) observational data from approximately 400 airways stations.

Once the data were on cards, they could be analyzed and examined for possible occurrences of weather cycles that could be used in longrange weather forecasting. As the drought that produced the dust bowl continued, the ability to predict the return of a wetter pattern accurately was foremost in the minds of the Weather Bureau's meteorologists. In addition to U.S. data, observational records from other countries were entered onto hemispheric weather maps, which were reanalyzed to show large-scale weather patterns that influenced U.S. weather. Furthermore, the compilation of marine observations provided information for tropical areas that were the spawning grounds for tropical storms and hurricanes.

Although depression-era cuts had significantly hampered the Weather Bureau's research work and forecasting program, later infusions of funds to pay clerical workers to record the data on punch cards would provide a valuable boost to its climatological services. These studies would be critical for advances in atmospheric knowledge in the years ahead.

smaller ones and thus pick up all the water molecules they hit along the way. The second way, due to vapor transfer, had been proposed by Alfred Wegener in 1911 as part of his hoarfrost studies. According to Wegener, if ice crystals were present in the vicinity of supercooled water, molecules from the latter would attach to the crystals until they became heavy enough to fall out. Although Wegener's frost work attracted some attention, its extension to precipitation processes was ignored at the time.

One meteorologist who remembered reading Wegener's work and subsequently applied it to the precipitation problem was Tor Bergeron. Bergeron had been fascinated by clouds since he was a boy. In February 1922, just before leaving Sweden to work with the Bjerkneses at the Bergen School, he had spent some time at a moun-

tain resort. While walking along a path cut through the woods, he made an interesting observation. If the air temperature was below freezing, then the supercooled stratus layer that shrouded the hillside did not fill the path—a clear tunnel appeared between the trees, as shown in the illustration below. When the temperature was above freezing, the "tunnel" disappeared as the cloud reached the ground and became fog. Mulling this over, Bergeron thought that when the temperature was below freezing, the ice on the tree limbs pulled the moisture away from the cloud, thus dissipating the cloud below the

Tor Bergeron noticed that supercooled stratus did not drop to the ground and become fog when the temperature was below freezing but did become fog when the temperature rose above freezing.



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tree line. When there was no ice on the trees, then the cloud filled in the space to the ground.

Once Bergeron was in Bergen he was too busy making weather forecasts to spend much time on ice crystals and clouds, but whenever he had a chance he collected additional information about how ice crystals affected the development of both cumuliform (puffy) and stratiform (flat) clouds. In his doctoral dissertation, completed in 1927, Bergeron gave a detailed account of his ideas on ice crystals. Published in the Norwegian journal Geofysiske publikasjoner (Geophysics publications) in 1928 as "Die dreidimensional verknüpfende Wetteranalyse" (Three-dimensionally combining synoptic analysis), his dissertation received limited attention in the United States and England.

Selected to represent Norway at the International Union of Geodesy and Geophysics (IUGG) meeting in Lisbon, Portugal, in 1933, Bergeron used this opportunity to present a detailed paper on his ice crystal theory. He argued that if there are a few ice crystals within a supercooled cloud, the ice crystals will grow at the expense of the supercooled droplets until they are large enough to fall out. Bergeron thought, therefore, that all raindrops originally started as snowflakes (even in the summer) and either continued as snow if the air temperature were cold all the way to the surface or fell as rain if the air temperature were above freezing.

Bergeron's paper attracted much attention in the meteorological world, becoming the topic of discussion at major meetings and much cited in the academic literature. Although there was widespread agreement from those who worked in the middle and high latitudes, those working in tropical areas vehemently disagreed that ice crystals were a major factor in rain production. The German meteorologist Walter Findeisen (1909-45) provided additional measurements and calculations in the late 1930s that helped to refine Bergeron's theory. The ice crystal process of rain formation became known as the Bergeron-Findeisen process and was widely (although not completely) accepted as the dominant precipitation mechanism until additional observations from aviators flying in tropical regions during World War II caused meteorologists to look for another method for forming raindrops.

Even at their tops, tropical clouds are "warm"—their temperatures are higher than 23°F (-5°C). Bergeron's vapor transfer mechanism, which worked so well in supercooled clouds, did not work in these warm clouds. Since aviators reported heavy rains in the Tropics, there had to be another raindrop-creating mechanism at work. Meteorologists began considering other possible causes of drop development, which led to the discovery of the collision-coalescence mechanism of rain creation. In collision-coalescence, approximately one in 1 million droplets has to be larger than the others in its vicinity. This difference allows it to fall faster and pick up additional droplets as it falls. The larger it grows, the faster it falls, and the more droplets it


Supercooled water droplets

Ice crystals

The Bergeron process shows accumulates. Once large enough, the droplet falls out. Later work in how a few ice crystals in minute cloud physics confirmed that midlatitude summer rains are formed by superco°led water droplets can collision-coalescence, while winter rains are formed by the Bergeron-

lead to snow formation.

J Findeisen mechanism. Research on precipitation mechanisms would continue throughout the 20th century as weather radar became more sophisticated, providing detailed information on cloud formation and behavior. Additional research focused on air pollutants and the influence of topography on cloud physics.

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