﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  215 
  

  

  lower 
  atmosphere, 
  hence 
  as 
  soon 
  as 
  any 
  appreciable 
  amount 
  of 
  vapor 
  

   diffuses 
  to 
  a 
  higher 
  level 
  than 
  that 
  of 
  the 
  average 
  cirrus 
  it 
  is 
  frozen 
  

   out 
  in 
  invisibly 
  small 
  amounts 
  and 
  the 
  whole 
  of 
  the 
  upper 
  air 
  

   thereby 
  kept 
  too 
  dry 
  for 
  the 
  formation 
  of 
  even 
  the 
  thinnest 
  clouds. 
  

  

  RELATION 
  OF 
  CLOUD 
  HEIGHTS 
  TO 
  SEASON. 
  

  

  Clouds 
  generally 
  are 
  lower 
  during 
  winter 
  than 
  during 
  summer. 
  

   This 
  is 
  because 
  the 
  relative 
  humidity 
  is 
  higher, 
  or 
  the 
  atmosphere 
  

   more 
  nearly 
  saturated, 
  during 
  the 
  colder 
  season 
  than 
  during 
  the 
  

   warmer. 
  And 
  this 
  condition, 
  in 
  turn, 
  is 
  owing 
  largely 
  to 
  the 
  fact 
  

   that 
  vertical 
  convection, 
  which 
  is 
  the 
  chief 
  cause 
  of 
  rain 
  and 
  hence 
  

   the 
  chief 
  drying 
  agency 
  of 
  the 
  air, 
  is 
  most 
  active 
  during 
  summer, 
  

   when 
  the 
  surface 
  of 
  the 
  earth 
  is 
  strongly 
  heated. 
  

  

  RELATION 
  OF 
  CLOUD 
  HEIGHTS 
  TO 
  LATITUDE. 
  

  

  Just 
  as 
  clouds 
  generally 
  are 
  higher 
  in 
  summer 
  and 
  lower 
  in 
  winter, 
  

   so, 
  too, 
  and 
  for 
  the 
  same 
  reason, 
  clouds 
  commonly 
  are 
  highest 
  in 
  

   equatorial 
  regions 
  and 
  gradually 
  descend 
  with 
  increase 
  of 
  latitude 
  

   to 
  their 
  lowest 
  level 
  in 
  polar 
  regions. 
  

  

  RELATION 
  OF 
  TYPE 
  OF 
  CLOUD 
  TO 
  THE 
  SEASON. 
  

  

  As 
  just 
  explained, 
  vertical 
  convection, 
  which 
  is 
  due 
  largely 
  to 
  

   surface 
  heating, 
  is 
  much 
  stronger 
  during 
  summer 
  than 
  during 
  winter. 
  

   Hence 
  the 
  cumulus 
  or 
  woolpack 
  cloud, 
  a 
  product 
  of 
  local 
  convection, 
  

   is 
  characteristic 
  of 
  warm 
  summer 
  days, 
  while 
  the 
  low 
  stratus 
  or 
  layer 
  

   cloud 
  is 
  equally 
  characteristic 
  of 
  the 
  winter 
  season. 
  

  

  RELATION 
  OF 
  TYPE 
  OF 
  CLOUD 
  TO 
  LATITUDE. 
  

  

  For 
  the 
  same 
  reason 
  that 
  the 
  prevailing 
  type 
  of 
  cloud 
  varies 
  with 
  

   the 
  season, 
  it 
  also 
  varies 
  with 
  latitude. 
  That 
  is, 
  in 
  tropical 
  regions, 
  

   where 
  vertical 
  convection 
  is 
  strong, 
  the 
  cumulus 
  cloud 
  is 
  very 
  com- 
  

   mon, 
  whereas, 
  in 
  the 
  high 
  latitude 
  regions, 
  where 
  convection 
  is 
  

   feeble, 
  it 
  is 
  unusual. 
  Here 
  the 
  prevailing 
  cloud 
  is 
  of 
  the 
  stratus 
  or 
  

   layer 
  type. 
  

  

  CLOUD 
  THICKNESS. 
  

  

  The 
  thickness 
  of 
  clouds 
  varies 
  from 
  all 
  but 
  zero 
  in 
  the 
  case 
  of 
  

   faint 
  cirrus, 
  and 
  vanishing 
  wisps 
  and 
  flecks 
  of 
  any 
  other 
  type, 
  up 
  

   to 
  the 
  10 
  miles 
  or 
  more 
  of 
  the 
  deepest 
  tropical 
  cumulus. 
  Habitu- 
  

   ally, 
  however, 
  the 
  high 
  cirrus, 
  only 
  a 
  few 
  hundred 
  feet 
  thick, 
  is 
  the 
  

   thinnest 
  of 
  clouds; 
  the 
  cumulus, 
  especially 
  when 
  the 
  seat 
  of 
  a 
  hail 
  

   storm, 
  is 
  the 
  thickest 
  — 
  often 
  several 
  miles 
  deep. 
  The 
  common 
  nim- 
  

  

  