surrounding it will tighten and tend to intensify the old masked warm front (fig. 3-7, step 3). Later, as the wave moves rapidly eastward, it will pickup this intensified warm front and begin to occlude (fig. 3-7, step 4). This occlusion deepens as much as 10 to 15 hPa in 12 hours. The resultant rapid deepening and increase in cyclonic circulation results in a portion of the original polar front discontinuity between the new and the old cyclone being washed out (fig. 3-7, step 5). INDICATIONS ALOFT FOR DEEPENING AND FILLING OF SURFACE LOWS There are numerous atmospheric factors aloft that affect  the  central  pressure  of  surface  lows.  The following text discusses a few of these factors. Temperature Advection Changes The role of temperature advection in contributing to the pressure or height changes can be misleading. On the one hand, low-level (usually the 1,000- to 500-hPa stratum) warm air advection is frequently cited as responsible  for  the  surface  pressure  falls  ahead  of moving  surface  lows  (the  converse  for  cold  air advection); on the other hand, warm air advection is frequently associated with rising heights in the upper levels. The pressure change at the SURFACE is equal to the pressure change at some UPPER LEVEL, plus the change in mass of the column of air between the two. That is, if the pressure at some upper level remains UNCHANGED and the intervening column is replaced with warmer air, the mass of the whole atmospheric column   (and   consequently   the   surface   pressure) decreases, and so does the height of the 1,000-hPa surface. As an example, assume that warm air advection is indicated below the 500-hPa surface (5,460 meters) above a certain station. If no change in mass is expected above this level, the height of the 500-hPa level (5,460 meters) will remain unchanged. Suppose the 1,000- to 500-hPa  advection  chart  indicates  that  the  5,400-meter thickness line is now over the station in question and will be replaced by the 5,490-meter thickness line in a given time interval; that is, warm air advection of 90 meters.  The  consequence  is  that  the  1,000-hPa  surface, which is now 60 meters above sea level, will lower 90 meters to 30 meters below sea level, and the surface pressure  will  decrease  a  corresponding  amount,  about 11 hPa (7.5 hPa approximately equals 60 meters). Whenever the surface pressure is less than 1,000 hPa, the 1,000-hPa surface is below the ground and is entirely fictitious. In view of the above description of advective temperature  changes,  the  following  rules  may  apply: . Warm air advection between 1,000 and 500 hPa induces  falling  surface  pressures. . Cold air advection between 1,000 and 500 hPa induces  rising  surface  pressures. Indications of Deepening From Vorticity Cyclogenesis  and  deepening  are  closely  related  to cyclonic flow or cyclonic vorticity aloft If you recall from the discussion of vorticity in chapter 1, vorticity is the measure of the path of motion of a parcel plus the wind shear along the path of motion. Thus, we have the following rules for the relationship of vorticity aloft to the deepening or falling of surface lows: @ Increasing cyclonic (positive) relative vorticity induces  downstream  surface  pressure  falls. Q  Increasing  antic  cyclonic  (negative)  relative vorticity  induces  downstream  surface  pressure  rises. l  A  wave  will  be  unstable  and  deepen  if  the 700-hPa  wind  field  over  it  possesses  cyclonic  relative vorticity. . A wave will be stable if the 700-hPa wind over it possesses  anticyclonic  vorticity. . If there are several waves along a front, the one with  the  most  intense  cyclonic  vorticity  aloft  will develop at the expense of the others. This is usually the one nearest the axis of the trough. Deepening of Lows Relative to Upper Contours The amount of deepening of eastern United States lows moving northeastward into the Maritime Provinces of Canada frequently can be predicted by estimating the number of contours at the 200- or 300-hPa level that would  be  traversed  by  the  surface  low  during  the forecast period. For a close approximation, multiply the 200-hPa current height difference in  tens of meters by 3/4   to   obtain   the   surface   pressure   change   in hectopascals.  For  example:  a  240-meter  height difference at 200 hPa results in a pressure change of 18 hPa at the surface. 24 x  3/4 = 18 hPa pressure change If  FALLING  heights  are  indicated  aloft,  the AMOUNT  of  fall  need  not  be  estimated.  The  deepening of  the  surface  low  and  greater  advective  cooling, associated  with  the  occlusion  process,  appear  to compensate  for  the  upper  height  falls. 3-10