192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19;J7 



appear close together. Actually, the warm sector is frequently several 

 hundred miles across so that a day or more may go by after the pas- 

 sage of a warm-front before the cold-front arrives. 



Referring to figure 5, h, the warm air at the front of the warm 

 sector is forced upward over the cold wedge of air ahead of it. The 

 phenomenon is much more gradual than in the case of the cold-front, 

 and frequently several hours' warning of its approach is given by the 

 changes in the nature of clouds as indicated in the diagram. An 

 observer at B would first see a few wisplike cirrus clouds high above 

 him. These would increase in number until they formed a liigh 

 unbroken layer of clouds. As the warm-front approached nearer and 

 nearer, more and more moisture would be available for cloud formation, 

 and the clouds would gradually form at lower and lower levels. The 

 gradual change from scattered cirrus through cirrostratus, alto stratus, 

 ^,nd stratus to nimbus or cumulo nimbus therefore gives ample warning 

 of a coming disturbance and evidence of the nature of the disturbance. 

 Thus, an observer familiar with cloud types can frequently predict 

 the coming of rain with i)ossible thiuidershowers after observing the 

 first two or three transitions of the cloud types mentioned. 



Application of knowledge of warm- and cold-fronts should be of 

 value to those interested in i)redictiug the weather from United 

 States Weather Bureau maps. By remembering that the warm-front 

 line and the cold-front line meet at the center of a low, and that the 

 warm sector lies to the south of the low, the position of the fronts can 

 be approximated on the weather map. Knowledge of their locations 

 and indicated strength will aid considerably in making predictions 

 from the maj^s. 



MAINTENANCE OF THUNDERSTORMS 



Humphreys has suggested a plausible explanation for the manner 

 in which the electrical activity of a thunderstorm is maintained as it 

 travels over the country. Referring to figure 6, from Humphreys' ^ 

 Physics of the Air, two main air currents are indicated. A strong 

 relatively cold column of air, formed as the resiUt of cooling by con- 

 tact with cold rain from high within the cloud and evai)oratiou of this 

 rain, descends from a region near the front of the rain area and presses 

 forward near the ground in front of the advancing storm as shown by 

 the downward arrows. It acts as a huge moving wedge forcing warmer 

 moisture-laden air up toward the cloud as shown by the upward 

 arrows, and thus mamtauiing its supply of moisture. Without this 

 the storm would not be maintained after the original supply of mois- 

 ture had fallen in the form of rain. 



The movement of cool air downward and outward from the rain 

 area of a thunderstorm, as described by Humphreys, is frequently 

 indicated by the shape of the rain hne when a storm is viewed from 



