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Thunderstorm Weather
LIGHTNING

Aerographers Mate, Module 05-Basic Meteorology
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turbulence and generally heavy rainfall, this particular altitude appears to be the most hazardous for aircraft. SURFACE WIND.—A significant hazard associated   with   thunderstorm   activity   is   the   rapid change in surface wind direction and speed immediately before storm passage. The strong winds at the  surface  accompanying  thunderstorm  passage  are the result of the horizontal spreading out of downdraft current  from  within  the  storm  as  they  approach  the surface of Earth. The  total  wind  speed  is  a  result  of  the  downdraft divergence plus the forward velocity of the storm cell. Thus,  the  speeds  at  the  leading  edge,  as  the  storm approaches, are greater than those at the trailing edge. The  initial  wind  surge,  as  observed  at  the  surface,  is known as the first gust. The speed of the first gust is normally the highest recorded during storm passage, and the direction may vary  as  much  as  180°  from  the  previously  prevailing surface wind. First-gust speeds increase to an average of  about  16  knots  over  prevailing  speeds,  although gusts of over 78   knots (90 mph) have been recorded. The average change of wind direction associated with the first gust is about 40°. In  addition  to  the  first  gust,  other  strong,  violent, and extremely dangerous downdraft winds are associated   with   the   thunderstorm.   These   winds   are referred to as downbursts. Downbursts are subdivided into macrobursts and microbursts. Macrobursts.—Macrobursts are larger scale downbursts. Macrobursts can cause widespread damage  similar  to  tornadoes.  These  damaging  winds can last 5 to 20 minutes and reach speeds of 130 knots (150 mph) or more. Microbursts.—Microbursts are smaller scale downbursts. A microburst can last 2 to 5 minutes and can  also  reach  wind  speeds  in  excess  of  130  knots. Microbursts produce dangerous tailwinds or crosswinds and wind shear for aircraft and are difficult to observe or forecast. Downbursts  are  not  the  same  as  first  gusts.  First gusts  occur  in  all  convective  cells  containing  showers and are predictable and expected. Downbursts, however,   do   not   occur   in   all   convective   cells   and thunderstorms. Classifications All thunderstorms are similar in physical makeup, but for purposes of identification, they may be divided into  two  general  groups,  frontal  thunderstorms  and air-mass thunderstorms. FRONTAL.—Frontal thunderstorms are commonly associated with both warm and cold fronts. The  warm-front  thunderstorm  is  caused  when  warm, moist, unstable air is forced aloft over a colder, denser shelf  of  retreating  air.  Warm-front  thunderstorms  are generally scattered; they are usually difficult to identify because they are obscured by other clouds. The   cold-front   thunderstorm   is   caused   by   the forward motion of a wedge of cold air, into a body of warm,    moist    unstable    air.    Cold-front    storms    are normally  positioned  aloft  along  the  frontal  surface  in what appears to be a continuous line. Under  special  atmospheric  conditions,  a  line  of thunderstorms develops ahead of a cold front. This line of   thunderstorms   is   the   prefrontal   squall   line.   Its distance ahead of the front ranges from 50 to 300 miles. Prefrontal    thunderstorms    are    usually    intense    and appear  menacing.  Bases  of  the  clouds  are  very  low. Tornadoes sometimes occur when this type of activity is present. AIR MASS.—Air-mass thunderstorms are subdivided  into  several  types.  In  this  text,  however, only  two  basic  types  are  discussed,  the  convective thunderstorm and the Orographic thunderstorm. Convective.—Convective thunderstorms may occur over land or water almost anywhere in the world. Their  formation  is  caused  by  solar  heating  of  various areas of the land or sea, which, in turn, provides heat to the    air    in    transit.    The    land    type    of    convective thunderstorm   normally   forms   during   the   afternoon hours after Earth has gained maximum heating from the Sun.   If   the   circulation   is   such   that   cool,   moist, convective,  unstable  air  is  passing  over  the  land  area, heating  from  below  causes  convective  currents  and results  in  towering  cumulus  or  thunderstorm  activity. Dissipation  usually  begins  during  the  early  evening hours. Storms that occur over bodies of water form in the  same  manner,  but  at  different  hours.  Sea  storms usually  form  during  the  evening  after  the  Sun  has  set and dissipate during the late morning. 5-21







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