If  the  velocity  of  fall  of  the  ball  through  the  super-cooled
region  has  been  at  all  great,  it  arrives  in  the  region  where  the
temperature  is  just  32°  F.  with  its  own  temperature  lower  than
32°  F.  Consequently  any  further  falling  means  that  water  is
directly  deposited  upon  it,  which  water  covers  the  surface  -of
the  ball  with  a  uniform  layer  of  liquid  which  quickly  freezes  as
a  hard,  transparent  covering  with  little  or  no  air  imprisoned  in
it.  Finally  the  ball  falls  out  of  the  base  of  the  cloud  as  a
hailstone.
Hailstones  cut  open  to  reveal  their  structure  show  the  soft
core  of  white  ice  and  the  hard  transparent  layer  of  clear  ice
which  are  in  conformity  with  the  theory  of  formation  which
has  been  outlined.
But  there  is  a  further  addition  to  be  made  to  the  life  story
of  large  hailstones.  An  ascending  current  of  air  is  not  steady
but  composed  of  increases  and  decreases,  of  gusts  and  com-
parative lulls.  A  hailstone  which  has  reached  the  lower  part
of  the  cloud  may  meet  one  of  the  gusts  and  be  blown  upwards
and  thus  be  forced  to  go  through  the  whole  process  again,  a
layer  of  white,  soft  ice  being  formed  around  the  previous
transparent  layer,  and  another  transparent  layer  being  added
around  that.  Further  repetitions  of  the  carrying  up  add
additional  layers.  A  hailstone,  if  cut  open  when  it  finally  falls
after  such  a  protracted  history,  would  show  several  concentric
layers  of  white  and  clear  ice.  Such  hailstones  have  frequently
been  found.
Two  conditions,  then,  seem  essential  for  the  production  of
large  hailstones : —

(a)  the  clouds  must  possess  great  vertical  structure  so  that
the  three  regions — the  water  region,  the  super-cooled
region,  and  the  snow  region — are  well  developed.
