Accurate crop yield estimates at the earliest possible date are of major importance to producers, handlers, warehousers, railroads, and consumers, whose plans and operations depend upon the knowledge of present and future stocks of their commodity. Past attempts at crop prediction have been largely of two types. The Crop Reporting Board of the U. S. Department of Agriculture has prepared its reports from questionnaires sent to large numbers of correspondents. The average of many individual estimates, corrected on the basis of past experience, has been, and probably still is, the most reliable prediction method available. It is obvious, however, that supplementary, objective measurements would greatly increase the value of present estimates. The second method used in the past has attempted to supply objective predictions from Weather Bureau data, and many reports have been published on the correlations between various climatic factors, particularly seasonal rainfall and temperatures, and crop yields (5). Statistical studies of published weather and crop records have not proved to be a promising basis for crop yield predictions. Consequently a CropWeather-Yield Project was set up by the Agricultural Marketing Service, cooperating with various State Agricultural Experiment Stations, to determine more specifically the physiological effects of climate on plant development. This project was intended "To improve the regular forecasts by discovering and applying relationships between yield and weather factors, and to include in an objective basis for forecasting the relationship of structural counts, measurements, and phenological records with yields, either separately or with weather factors." The work with corn was started in 1938 and centered in the Iowa Agricultural Experiment Station with the Statistics, Botany and Plant Pathology, Agronomy and other Sections cooperating. Detailed studies of crop development and microas well as macroclimate were made on the station plots at Ames in 1938,1939 and 1940. The results of these studies have been summarized by Bair (2, 3). Less detailed studies were made at other corn-belt Stations in 1939 and 1940, a part of the results of which have been published by Keller (7).