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Figure 3-11.Development cycle of a geostrophic wind.
Variations

Aerographers Mate, Module 05-Basic Meteorology
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decreases, the wind speed increases. If the density and the latitude remain constant and the pressure gradient decreases,  the  wind  speed  decreases.  If  the  pressure gradient   and   the   latitude   remain   constant   and   the density  decreases,  the  wind  speed  increases.  If  the density  increases,  the  wind  speed  decreases.     True geostrophic wind is seldom observed in nature, but the conditions  are  closely  approximated  on  upper-level charts. Cyclostrophic Wind In   some   atmospheric   conditions,   the   radius   of rotation  becomes  so  small  that  the  centrifugal  force becomes quite strong in comparison with the Coriolis force. This is particularly true in low latitudes where the Coriolis force is quite small to begin with. In this case, the  pressure  gradient  force  is  nearly  balanced  by  the centrifugal force alone. When this occurs, the wind is said to be cyclostrophic. By definition, a cyclostrophic wind   exists   when   the   pressure   gradient   force   is balanced by the centrifugal force alone. This  exact  situation  rarely  exists,  but  is  so  nearly reached in some situations that the small Coriolis effect is  neglected  and  the  flow  is  said  to  be  cyclostrophic. Winds in a hurricane or typhoon and the winds around a tornado are considered cyclostrophic. Movement of Wind around Anticyclones The movement of gradient winds around anticyclones  is  affected  in  a  certain  manner  by  the pressure  gradient  force,  the  centrifugal  force,  and  the Coriolis  force.  The  pressure  gradient  force  acts  from high   to   low   pressure,   and   the   Coriolis   force   acts opposite  to  the  pressure  gradient  force  and  at  right angles to the direction of movement of the parcel of air. The centrifugal force acts at right angles to the path of motion  and  outward  from  the  center  about  which  the parcel  is  moving.  (See  fig.  3-12.)  In  the  case  of  a high-pressure  center,  the  pressure  gradient  force  and the  centrifugal  force  balance  the  Coriolis  force.  This phenomenon   may   be   expressed   in   the   following manner: PG + CF = D Movement of Wind around Cyclones As in the case of anticyclones, the pressure gradient force, the centrifugal force, and the Coriolis force affect gradient winds around cyclones, but the balance of the forces   is   different.   (See   fig.   3-12.)   In   a   cyclonic situation  the  Coriolis  force  and  the  centrifugal  force balance the pressure gradient force. This balance may be expressed in the following manner: 3-12 1004 1024 1008 1016 1020 1012 PG = PRESSURE GRADIENT CF = CENTRIFUGAL FORCE ANTICYCLONE CYCLONE D = CORIOLIS FORCE W = DIRECTION OF PARCEL OF AIR AG5f0312 H L W D G W PG CF + D CF + Figure 3-12.—Forces acting on pressure systems.







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