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Aerographers Mate, Module 05-Basic Meteorology
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Both  types  of  convective  thunderstorms  occur  in Florida.    The    anticyclonic    circulation    around    the Bermuda high advects moist air over the land surface of Florida in its easterly flow. Thunderstorms off the East Coast of Florida at night occur when this easterly flow passes  over  the  warm  axis  of  the  Florida  current.  In those areas where the air is cooler than the water below it,  the  air  is  heated  and  convective  currents  (lifting) begin. Any nocturnal cooling of the easterly flow aloft aids in establishing the unstable lapse rate necessary for thunderstorm  development.  After  sunrise,  the  air  is heated  and  becomes  warmer  than  the  water,  thereby destroying  the  balance  necessary  to  sustain  or  build similar storms. As the day progresses, the land surface becomes considerably warmer than the air. Convective currents again result, and Florida’s common afternoon thunderstorms   are   observed.   After   sunset   the   land cools, convective currents cease, and the thunderstorms dissipate. The apparent movement of the storms to sea at night, and to shore during the day, is in reality the reformation  of  storms  in  their  respective  areas.  As  a general  rule,  convective  thunderstorms  are  scattered and easily recognized. They build to great heights, and visibility is generally excellent in the surrounding area. Orographic.—Orographic thunderstorms form in mountainous regions, particularly adjacent to individual peaks. A good example of this type of storm occurs in the northern Rocky Mountain region. When the circulation of the air is from the west, moist air from the Pacific Ocean is transported to the mountains where it is forced aloft by the upslope of the terrain. If the air is conditionally   unstable,   this   upslope   motion   causes thunderstorm  activity  on  the  windward  side  of  the mountains.  This  activity  may  form  a  long,  unbroken line of storms similar to a cold front. The storms persist as  long  as  the  circulation  causes  an  upslope  motion. They  tend  to  be  more  frequent  during  afternoon  and early  evening  when  convective  lifting  coincides  with the mechanical lifting of the terrain. LIGHTNING Lightning   is   obviously   the   most   spectacular   of electrometeors and is directly related to the thunderstorm  even  though  classified  independently.  It is  the  bright  flash  of  light  accompanying  a  sudden electrical discharge. Most lightning has its beginning in clouds;  however,  it  generates  from  high  structures  on the    ground    and    mountains,    although    much    less frequently. The   thunderstorm   changes   the   normal   electric field,  in  which  the  ground  is  negatively  charged  with respect to the air above it. Because the upper portion of the thunderstorm cloud is positive and the lower part is negative, the negative charge induces a positive charge on the ground. The distribution of the electric charges in a typical thunderstorm is shown in figure 5-17. The 5-22 AG5f0517 DIRECTION OF MOVEMENT 15  F 32  F SMALL CENTER OF POSITIVE CHARGE AREA OF NEGATIVE RAIN POSITIVE CHARGE CENTER NEGATIVE CHARGE CENTER SURFACE Figure 5-17.—Location of electric charges inside a typical thunderstorm cell.







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