3. Radical reorientation of the trough. This usually happens  where  the  trough  is  initially  NE-SW,  resulting in a N-S and in some cases a NW-SE orientation after sufficient time (36 hours). 4. This situation may actually cut off a low in the lower area of the trough. This usually happens when the high-speed   winds   approaching   the   ridge   are southwesterly and approach the ridge at a comparatively high latitude relative to the trough. This frequently reorients  the  trough  line  towards  a  more  NE-SW direction. Usually, the reorientation of the trough occurs simultaneously with 1 and 2. Sharply  Curved  Ridges Closely   related   to   the   previously   mentioned situation are cases of sharply curved ridges where the gradient in the sharply curved portion (usually the northern   portions   of   a   north-south   ridge)   has momentarily built up to a strength that is incompatible with  the  anticyclonic  curvature.  Such  ridges  often collapse  with  great  rapidity  prior  to  the  development  of such excessive gradients, causing rapid filling of the adjacent downstream trough, and large upper contour falls. The   gradient   wind   relation   implies   that subsequent  trajectories  of  the  high-speed  parcels generated in the strong ridge line gradient must be less anticyclonically curved than the contours in the ridge. It  can  also  be  shown  from  the  gradient  wind equation that the anticyclonic curvature increases as the difference  between  the  actual  wind  and  the  geostrophic wind  increases,  until  the  actual  wind  is  twice  the geostrophic wind, when the trajectory curvature is at a maximum. This fact can be used in determining the trajectory  of  high-speed  parcels  approaching  sharply curved  stationary  ridges  or  sharply  curved  stationary ridges  with  strong  gradients.  By  measuring  the geostrophic wind in the ridge, the maximum trajectory curvature can be obtained from the gradient wind scale. This trajectory curve is the one that an air parcel at the origin point of the scale will follow until it intersects the correction  curve  from  the  geostrophic  speed  to  the displacement curve of twice the geostrophic speed. Actual  Wind  Speeds If actual wind speed observations are available for parcels approaching the ridge, comparison can be made with  the  geostrophic  winds  (pressure  gradient)  in  the ridge. If the actual speeds are more than twice the measured geostrophic wind in the ridge, the anticyclonic curvature of these high-speed parcels will be less than the  maximum  trajectory  curvature  obtained  from  the gradient  wind  scale,  and  even  greater  overshooting  of these  high-speed  parcels  will  occur  across  lower contours. Convergence  in  the  west  side  of  the downstream trough results in lifting of the tropopause with dynamic cooling and upper-level contour rises. Subgradient  Winds Low-speed  winds  approaching  an  area  of  stronger gradient become subject to an unbalanced gradient force toward the left due to the weaker Coriolis force. These subgradient  winds  are  deflected  toward  lower  pressure, crossing contours and producing contour rises in the area  of  cross-contour  flow.  This  cross-contour  flow accelerates the air until it is moving fast enough to be balanced by the stronger pressure gradient. Due to the acceleration of the slower oncoming parcels of air, the contour rises propagate much faster than might be expected on the basis of the slow speed of the air as it initially enters the stronger pressure gradient. The following two rules summarize the discussion of  subgradient  winds: .  High-speed  winds  approaching  low-speed  winds with weak cyclonically curved contour gradients are indicative  of  divergence  and  upper-height  falls downstream to the left of the current. .   Low-speed   winds   approaching   strong, cyclonically curved contour gradients or high-speed winds  approaching  low-speed  winds  with  weak anticyclonically  curved  contour  gradients  are  indicative of  convergence  and  upper  height  rises  downstream  and to the left and right of the current, respectively. IMPORTANCE OF CONVERGENCE AND DIVERGENCE Convergence and divergence have a pronounced effect  upon  the  weather  occurring  in  the  atmosphere. Vertical  motion,  either  upward  or  downward,  is recognized   as   an   important   parameter   in   the atmosphere.  For  instance,  extensive  regions  of precipitation  associated  with  extratropical  cyclones  are regions of large-scale upward motion. Similarly, the nearly  cloud-free  regions  in  large  anticyclones  are regions in which air is subsiding. Vertical motions also affect  temperature,  humidity,  and  other  meteorological elements. 1-6