Appendix II — GLOSSARY 



BRIGHT BAND. The enhanced radar echo of 

 snow as it melts to rain, as displayed on the 

 RHI scope. Located approximately 1,500 feet 

 below the 0° C isotherm. 



BUYS BALLOT'S LAW. The law describing 

 the relationship of the horizontal wind direction 

 in the atmosphere to the pressure distribution: 

 In the Northern Hemisphere, with your back to 

 the wind, the lowest pressure will be on your 

 left; in the Southern Hemisphere, the relation 

 is reversed. 



CENTER OF ACTION. Any one of the semi- 

 permanent highs and lows that appear on mean 

 charts of sea level pressure. 



CENTRAL PRESSURE. The atmospheric pres- 

 sure at the center of a high or low; the highest 

 pressure in a high, the lowest in a low. 



CHINOOK. The warm dry foehn wind on the 

 eastern side of the Rocky Mountains. 



CLOSED HIGH. A high that may be completely 

 encircled by an isobar or contour line. 



CLOSED LOW. A low that may be completely 

 encircled by an isobar or contour line. 



COALESCENCE. In cloud physics, the merging 

 of two water drops into a single larger drop. 



COLD HIGH. At a given level in the atmos- 

 phere, any high that is generally characterized 

 by colder air near its center than around its 

 periphery. 



COLD LOW. Any low that is generally char- 

 acterized at a given level in the atmosphere 

 by colder air near its center than around its 

 periphery. 



CONDITIONAL INSTABILITY. The state of a 

 column of air in the atmosphere when its lapse 

 rate of temperature is less than the dry adia- 

 batic lapse rate but greater than the saturation 

 adiabatic lapse rate. 



CONDENSATION. The physical process by 

 which a vapor becomes a liquid or solid. 



CONTOUR. The term used in meteorology 

 usually referring to a line of constant height 

 on a constant pressure chart. 



CONVECTION. Atmospheric motions that are 

 predominantly vertical, resulting in vertical 

 transport and mixing of atmospheric properties. 



CUT-OFF HIGH. A warm high which has be- 

 come displaced out of the basic westerly, and 

 lies to the north of this current. 



CUT-OFF LOW. A cold low which has be- 

 come displaced out of the basic westerly, and 

 lies to the south of this current. 



CYCLOGENESIS. Any development or 

 strengthening of cyclonic circulation in the atmos- 

 phere. The initial appearance of a low or trough, 

 as well as the intensification of an existing 

 cyclonic flow. 



CYCLOLYSIS. Any weakening of cyclonic cir- 

 culation in the atmosphere. 



CYCLONIC. A counterclockwise rotation in 

 the Northern Hemisphere and a clockwise ro- 

 tation in the Southern Hemisphere. 



DECIBAR. A unit of pressure used principally 

 in oceanography. 



DEEPENING. A decrease in the central pres- 

 sure system. The term usually applies to a 

 low rather than to a high. 



ENTRAINMENT. The mixing of environmental 

 air into a preexisting, organized air current so 

 the environmental air becomes part of the current. 



EVAPORATION. The physical process by 

 which a liquid or solid is transformed to the 

 gaseous state. 



EXTRATROPICAL CYCLONE. Any cyclonic- 

 scale storm that is not a tropical cyclone, usu- 

 ally referring only to the migratory frontal 

 cyclones of middle and high latitudes. 



FALL WIND. A strong, cold, downslope wind. 



FILLING. An increase in the central pres- 

 sure of a pressure system on a constant-height 

 chart. Commonly applied to a low rather than 

 to a high. 



FOEHN WIND. A warm, dry wind on the lee 

 side of a mountain range, the warmth and dry- 

 ness of the air being due to adiabatic compres- 

 sion upon descending the mountain slopes. 



FRONTOGENESIS. The 

 a front or frontal zone. 



initial formation of 



FRONTOLYSIS. The dissipation of a front 

 or frontal zone. 



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