Rain gage catch variation due to airflow disturbances around a standard rain gage

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Abstract
Although the spatial position has an important influence on the performance of a rain gage, the physical presence of the gage itself must be considered a hindrance to its operation. An obstruction to airflow in the vicinity of the gage, owing to the presence of the gage, results in disturbances in the local precipitation pattern in which the rainfall measurement is made. The airflow pattern data obtained in a study of a wind tunnel model and aerodynamic drag characteristics of water drops are used to perform a computer simulation on the movements of the drops as they approach the gage. These drop paths are analyzed to establish the effectiveness of the gage in measuring precipitation. If we use drop sizes of 1–5 mm in diameter, we find that the gage catches from 103 to 11% of the drops as the free stream velocity increases from 10 to 50 feet per second. In general, the decrease in catch is less pronounced as the drop size is increased.

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