. Compendium of meteorology. Meteorology. METEOROLOGICAL ANALYSIS IN MIDDLE LATITUDES 721 casting has been under consideration by many investi- gators during the past few years [5, 17J. The three factors whicli hu,\-e been found to be most important for precipitation forecasts are the orientation and curva- ture of the flow patterns at the altostratus le\'el (about 700 mb), the location of areas of pressure fall at sea level, and the extension of warm-air advection near the 850-mb \c\e\. Since the detection of ad\'ection between 700 mb and the surface can be accomplished at a glance all three
. Compendium of meteorology. Meteorology. METEOROLOGICAL ANALYSIS IN MIDDLE LATITUDES 721 casting has been under consideration by many investi- gators during the past few years [5, 17J. The three factors whicli hu,\-e been found to be most important for precipitation forecasts are the orientation and curva- ture of the flow patterns at the altostratus le\'el (about 700 mb), the location of areas of pressure fall at sea level, and the extension of warm-air advection near the 850-mb \c\e\. Since the detection of ad\'ection between 700 mb and the surface can be accomplished at a glance all three of these charts the relation between the two sets of isopleths and the weather and cloud data throws into full relief the following information: the principal activating mechanisms of precipitation and cloudiness, the ar^al extention of warm and cold air advection, the movement and spatial distribution of pressure-fall areas on the surface map with respect to the steering flow aloft, the relationship between moisture sources and the orientation of the upper flow, the likelihood of vertical. Fig. 2—Combined sea-level and 700-mb analysis for the United States at 0630Z, October 24, 1940. when the charts of these two levels are superimposed, and since the 700-mb contours represent the air flow at the level where most of the altostratus rain clouds are centered, the 700-mb chart superimposed on the sea- level chart is considered the most useful combination in giving a comprehensive \'iew of the weather prospects in stations where issuance of 24- and 48-hr forecasts to the general public is the prime function. The authors feel that this is such an important subject that some examples of the combined analysis of surface and upper- level charts are presented here in detail. These examples include three (•onsecuti^'e sea-level charts with the corresponding 700-mb contours super- imposed upon them, constructed for 24-hr intervals. On displacement of an air mass above a front, and the
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