Changes in Stability When  convergence  or  divergence  occurs,  whether on a large or small scale, it may have a very pronounced effect on the stability of the air. For example, when convection is induced by convergence, air is forced to rise  without  the  addition  of  heat.  If  this  air  is unsaturated, it cools first at the dry adiabatic rate; or if saturated at the moist rate. The end result is that the air is cooled, which will increase the instability y of that air column due to a net release of heat. Clouds and weather often  result  from  this  process. Conversely,  if  air  subsides,  and  this  process  is produced by convergence or divergence, the sinking air will  heat  at  the  dry  adiabatic  lapse  rate  due  to compression. The warming at the top of an air column will increase the stability of that air column by reducing the lapse rate. Such warming often dissipates existing clouds  or  prevents  the  formation  of  new  clouds.  If sufficient warming due to the downward motion takes place, a subsidence inversion is produced. Effect on Weather The  most  important  application  of  vertical  motion is the prediction of rainfall probability and rainfall amount. In addition, vertical motion affects practically all  meteorological  properties,  such  as  temperature, humidity, wind distribution, and particularly stability. In  the  following  section  the  distribution  of  large-scale and small-scale vertical motions are considered. Since cold air has a tendency to sink, subsidence is likely to be found to the west of upper tropospheric troughs, and rising air to the east of the troughs. Thus, there  is  a  good  relation  between  upper  air  meridional flow and vertical flow. In   the   neighborhood   of   a   straight   Northern Hemisphere jet stream, convergence is found to the north of the stream behind centers of maximum speed as well as to the south and ahead of such centers. Divergence exists in the other two quadrants. Below the regions  of  divergence  the  air  rises;  below  those  of convergence there is subsidence. These general rules of thumb are not perfect, and only yield a very crude idea about distribution of vertical motion  in  the  horizontal.  Particularly  over  land  in summer, there exists little relation between large-scrale weather patterns and vertical motion. Rather, vertical motion is influenced by local features and shows strong diurnal variations. Large-scale vertical motion is of small magnitude at the ground (zero if the ground is flat). Above ground level, it increases in magnitude to at least 500  hPa  and  decreases  in  the  neighborhood  of  the tropopause. There have been several studies of the relation  between  frontal  precipitation  and  large-scale vertical velocities, computed by various techniques. In all cases, the probability of precipitation is considerably higher in the 6 hours following an updraft than following subsidence.  Clear  skies  are  most  likely  with downdrafts. On the other hand, it is not obvious that large-scale vertical motion is related to showers and thunderstorms caused during the daytime by heating. However, squall lines, which are formed along lines of horizontal  convergence,  show  that  large-scale  vertical motion may also play an important part in convective precipitation. Vertical Velocity Charts Vertical  velocity  charts  are  currently  being transmitted   over   the   facsimile   network   and   are computed by numerical weather prediction methods. The  charts  have  plus  signs  indicating  upward  motion and minus signs indicating downward motion. The figures indicate vertical velocity in centimeters per second  (cm/sec).  With  the  larger  values  of  upward motions  (plus  values)  the  likelihood  of  clouds  and precipitation  increases.  However,  an  evaluation  of  the moisture and vertical velocity should be made to get optimum results. Obviously, upward motion in dry air is not as likely to produce precipitation as upward motion in moist air. Studies  have  shown  that  surface  cyclones  and anticyclones are not independent of developments in the upper atmosphere, rather, they work in tandem with one another. The  relationship  of  the  cyclone  to  the large-scale flow patterns aloft must therefore be a part of  the  daily  forecast  routine. Many forecasters have a tendency to shy away from the subject of vorticity, as they consider it too complex a  subject  to  be  mastered.  By  not  considering  vorticity and its effects, the forecaster is neglecting an important forecasting tool. The principles of vorticity are no more complicated than most of the principles of physics, and can  be  understood  just  as  readily.  In  the  following section we will discuss the definition of vorticity, its evaluation, and its relationships to other meteorological parameters. 1-7