The fact that stratus clouds first form at the top of the friction layer is adduced. A method is described for finding this height. Stability conditions in the friction layer are inferred from the result and some aspects of various types of stratus formation are discussed. A new method for obtaining the “roughness length” at a place is described.

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* A preliminary version of this paper was awarded Second Prize for 1949 by the Air Transport Association in its annual contest for airline employees performing the best original research having a practical application towards improving weather analysis and forecasting in relation to aircraft dispatching.—Ed.