during the mature stage.    In  subtropical  and  tropical thunderstorms, hail seldom reaches the ground. It is generally believed that these thunderstorms contain less hail aloft than do midlatitude storms. Rain Thunderstorms  contain  considerable  quantities  of moisture that may or may not be falling to the ground as rain. These water droplets may be suspended in, or moving  with,  the  updrafts.  Rain  is  encountered  below the freezing level in almost all penetrations of fully developed thunderstorms. Above the freezing level, however, there is a sharp decline in the frequency of rain. There seems to be a definite correlation between turbulence   and   precipitation.   The   intensity   of turbulence,  in  most  cases,  varies  directly  with  the intensity  of  precipitation.    This  relationship  indicates that most rain or snow in thunderstorms is held aloft by updrafts. Icing Where the air temperatures are at or below freezing, icing   should   be   expected   in   flights   through thunderstorms. In general, icing is associated with temperatures  from  0°  to  –20°C.  Most  severe  icing occurs   from   0°C   to   –10°C.   The   heaviest   icing conditions  usually  occur  in  that  region  above  the freezing level where the cloud droplets have not yet turned to ice crystals. When the thunderstorm is in the cumulus stage, severe icing may occur at any point above the freezing level. However, because of the formation of ice crystals at high levels and the removal of liquid water by precipitation, icing conditions are usually  somewhat  less  in  the  mature  and  dissipating stage. THUNDERSTORM  ELECTRICITY  AND LIGHTNING The thunderstorm changes the normal electrical field, in which the earth is negative with respect to the air  above  it,  by  making  the  upper  portion  of  the thunderstorm  cloud  positive  and  the  lower  portion negative. This negative charge then induces a positive charge  on  the  ground.  The  distribution  of  the  electric charges in a typical thunderstorm is shown in figure 5-1. The  lightning  first  occurs  between  the  upper  positive charge area and the negative charge area immediately below it. Lightning discharges are considered to occur most frequently in the area roughly bounded by the 0°C and the -9°C temperature levels. However, this does not mean that all discharges are confined to this region, for as the thunderstorm develops, lightning discharges may occur in other areas, and from cloud to cloud, as well as cloud to ground. Lightning can do considerable damage  to  aircraft,  especially  to  radio  equipment. THUNDERSTORMS IN RELATION TO THE WIND FIELD During all stages of a cell, air is being brought into the  cloud  through  the  sides  of  the  cloud.  This  process is known as  entrainment.  A cell entrains air at a rate of 100 percent per 500 hPa; that is, it doubles its mass in an ascent of 500 hPa. The factor of entrainment is important in establishing a lapse rate within the cloud that is greater than the moist adiabatic rate, and in maintaining  the  downdraft. When there is a marked increase with height in the horizontal wind speed, the mature stage of the cell may be  prolonged.  In  addition,  the  increasing  speed  of  the wind  with  height  produces  considerable  tilt  to  the updraft of the cell, and in fact, to the visible cloud itself. Thus, the falling precipitation passes through only a small section of the rising air; it falls thereafter through the relatively still air next to the updraft, perhaps even outside the cell boundary. Therefore, since the drag of the falling water is not imposed on the rising air currents within the thunderstorm cell, the updraft can continue until its source of energy is exhausted. Tilting of the thunderstorm   explains   why   hail   is   sometimes encountered in a cloudless area just ahead of the storm. RADAR  DETECTION Radar, either surface or airborne, is the best aid in detecting thunderstorms and charting their movements. A  thunderstorm’s  size,  direction  of  movement,  shape and height, as well as other significant features, can be determined   from   a   radar   presentation.   Radar interpretation is mentioned in chapter 12 of this manual, and for a more detailed discussion, refer to the Federal Meteorological   Handbook   No.   7,   Part  B,  and  the Federal  Meteorological  Handbook  No.  11,  Part B. THUNDERSTORM  FLIGHT  HAZARDS Thunderstorms are often accompanied by extreme fluctuations   in   ceiling   and   visibility.   Every thunderstorm  has  turbulence,  sustained  updrafts  and downdrafts,   precipitation,   and   lightning.   Icing conditions,  though  quite  localized,  are  quite  common  in thunderstorms,  and  many  contain  hail.  The  flight 5-2