An increasing moisture source and gravity waves in the  velocity  field  can  also  be  detected.  Gravity  waves should  coincide  with  the  alternating  increases  and decreases in reflectivity. This may also be apparent in the  Echo  Tops  product  if  the  minimum  reflectivity threshold of 18 dBZe is met. Considerations In the Clear Air mode, cloud detection maybe best using VCP-31 due to a better signal-to-noise ratio. If the minimum reflectivity threshold of 18  dBZ is met, the Echo Tops product can provide an indication of the top of a cloud layer. Layer Composite Reflectivity products may show returns at given layers, indicative of clouds. These products should be supported with the use  of  the  Reflectivity  product  to  determine  the existence and extent of the cloud layers. Stratus clouds and fog will usually not be detected by the radar due to small cloud particle size. The  Velocity  Azimuth  Display  Wind  Profile  product may be useful in determining moisture advection for development of cloud layers, depending on distance from  the  radar. For further discussion of preconvective, convective, multicell  and  supercell  development  as  well  as  severe storm identification, refer to part D of the FMH-11. THE STRUCTURE OF LARGE-SCALE PRECIPITATING WEATHER SYSTEMS The character of precipitation is largely controlled by vertical air motions. Radar observations of the aerial extent,  and  lifetime  of  precipitation  systems  are evidence  of  the  physical  processes  at  work  in  the atmosphere.  Depending  on  the  dominant  mechanism responsible for the vertical air motion, precipitation is usually classified into one of these two types: 1.   Stratiform   –   Widespread,   continuous precipitation produced by large-scale ascent due to frontal  or  topographical  lifting  or  large-scale  horizontal air convergence caused by other means. 2.   Convective   –   Localized,   rapidly   changing, showery   precipitation   produced   by   cumulus-scale convection in unstable air. The distinction between stratiform and convective precipitation is not always clear in practice. Widespread precipitation, for example, is often accompanied by fine-scale  structures,  or  embedded  convective  elements. In  fact,  precipitation  systems  generally  are  composed  of a wide spectrum of scales and intensities. Nevertheless, it is usually possible to classify precipitation patterns by their dominant scale. Stratiform Rain or Snow Stratiform  precipitation  is  most  often  produced  in nimbostratus clouds or dissipating cumulus clouds. Upward air motions are weak and the vertical structure of  the  reflectivity  pattern  is  closely  related  to  the precipitation patterns by their dominant scale. The  presence  of  stratiform  precipitation  facilitates wind measurements with the velocity azimuth display (VAD) to much higher altitudes than possible in clear air (part B of the FMH-11). The wind profiles observed during precipitation may be useful in determining the nature of fronts. A layer of warm air advection (veering of wind with height) is evident from the “S” pattern near the  surface  (at  close  slant  ranges).  Cold  air  advection (backing with height) is present at greater altitudes (far slant  ranges).  Generally,  the  reflectivity  pattern  depicts stratiform precipitation without prominent small-scale features. On  some  occasions,  mesoscale  precipitation  bands form  within  the  stratiform  precipitation  area.  The  band is just ahead of and parallel to the wind shift line marking the location of the cold front. Other bands have different orientations with respect to cold fronts. Bands also occur in the vicinity of warm fronts and in precipitation areas  away  from  frontal  locations,  particularly  in  the warm sector of a large-scale system. Mesoscale Convective Systems Some large-scale precipitating systems begin as combinations of a number of convective elements. The resulting  system,  although  still  feeding  on  unstable  air, takes  on  a  character  much  different  from  typical cumulus-scale   convection. Regions   of   strong convection and heavy showers can become randomly distributed within a larger area of developing stratiform precipitation,  or  the  strong  convection  can  be  limited  to the large system’s leading edge with the rest of the system primarily composed of stratiform rain. When organized  in  a  linear  fashion,  the  convective  cells  are typically distributed along a band about 20 km (11 nmi) wide  and  hundreds  of  kilometers  long.  The  bands  are usually  related  to  low-level  convergence  and  wind shear,  but  the  Earth’s  topography  also  affects  the structure  of  the  rain  areas.  The  characteristics  of precipitation bands have been categorized, but it is not completely  understood  why  precipitation  has  the  strong 12-11