Two‐way relationship between the snow or ice limit and 1,000–500 mb thicknesses in the overlying atmosphere

Statistical studies of 1,000–500 mb thickness, (i) occurring with frozen or unfrozen forms of precipitation and (ii) prevailing at various distances from the snow or ice limit over wide regions with snow or ice cover, over bare ground and over open sea are used to discover (a) the likelihood that precipitation will take the form of rain or snow in given circumstances and (b) the behaviour of the thickness when well‐established and extensive snow or ice fields are encountered. The results indicate an important influence produced by snow fields of the order of 1,000 mi and more in longitudinal extent upon the general circulation of the atmosphere. Choice of 1,000–500 mb thickness as an indicator rests upon its adequacy and upon its general use and availability in forecasting offices; some preliminary argument is devoted to showing that the choice is reasonable on these grounds and that the lack of refinement involved in considering thicknesses of so deep a layer is not serious.