CHARACTERISTIC AIR MASS PROPERTIES 173 
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Fic. 3. The surface fronts, topography of the frontal surfaces (dashed 
lines) and of the main tropopause (solid lines), Feb. 17, 1935, 07h. Elevation 
contours for each dyn. km. The tropopause is lowest (7.5 km) just behind 
and south of the center of the surface low - a typical occurrence. The contours 
are not all open to the north so this low tropopause could not be explained as 
due solely to advection from the north. 
The variation in the altitude of each 100 mb surface aloft as this cyclone passed over Uccle, 
Belgium, on 16-17 Feb., is shown in Fig. 4 (from Van Mieghem, Ciel et Terre, 1940, no. 1). 
4000mb 
Se ee oa Ai eee 
EE 
vj} 
Fic. 5. Schematic streamlines in (a) a young wave-cyclone and (b) an 
occluded vortex cyclone, on the 500 and 1000 mb surfaces (it is customary in 
German weather services to draw charts of the topography of these surfaces). 
The shaded area is the region of frontal clouds and rain. PL= polar air; 
TL = tropical air; dashed lines are the fronts. 
