U M> \ K rOUKCASTING 



81 



formation of ducts depends principally on the tem- 

 perature difference between the air and the ground 



or sea surface anil on the humidity of the air. Data 

 on sea temperature, which is usually fairly constant, 

 are collected while over land it is necessary to obtain 

 data on the diurnal variation of the soil temperature. 

 Wind velocities may be gathered from the weather 

 map, and the trajectory of the air previous to and 

 during the forecast period can then be determined. 

 If the relative humidity of the air is known, it is 

 possible from the theories at hand to draw estimated 

 curves of the temperature and moisture variation 

 in the lowest layers. From these an estimated M 

 curve is obtained. The success of this method depends 

 to a large degree upon the familiarity of the forecaster 

 with local conditions. 

 The forecasting of advective ducts over the ocean 



is the main problem in which radio forecasting 

 requires other tools than those used for ordinary 

 weather forecasting; but most other problems are 

 closely similar to those presented by conventional 

 practice, among which are the forecasting of subsi- 

 dence from upper air meteorological data, the 

 forecasting of nocturnal temperature inversions in 

 dry climates, and the forecasting of standard 

 propagation conditions. 



In order to facilitate weather forecasting in the 

 Pacific, where data have been very scanty during 

 the war, a system has been worked out whereby 

 localities in the Pacific area are compared to those 

 of closely similar climatic and meteorological charac- 

 ter in the Atlantic. A rough estimate of propagation 

 conditions to be expected may be derived there- 

 from. 2S - 215 



