THERMALS.—Thermals are vertical convective currents that result from local heating. They stop short of  the  condensation  level.  Thermal  convection  is  the usual result of strong heating of the lower atmosphere by  the  ground  surface.  A  superadiabatic  lapse  rate immediately  above  the  ground  is  necessary  to  the development   of   strong   thermals.   They   form   most readily over areas of bare rock or sand and in particular over sand dunes or bare rocky hills. In the presence of a moderate or fresh breeze, especially in a hilly terrain, it is  impossible  to  distinguish  between  turbulent  and thermal convection currents. Pure thermal convection normally occurs on clear summer days with very light prevailing   wind.   In   the   eastern   United   States,   dry thermals are usually of only moderate intensity, seldom reaching an elevation in excess of 5,000 feet above the surface. The high moisture content of the air masses in this section in summer reduces the intensity of surface heating to some extent. This moisture content usually keeps the condensation level of the surface air near or even below a height of 5,000 feet above the ground. In the dry southwestern part of the country, where ground heating  during  clear  summer  days  is  extreme,  dry thermal convection may extend to a height of 10,000 feet   or   more.   Under   these   conditions,   extremely turbulent   air   conditions   can   occur   locally   up   to whatever   heights   the   thermals   extend,   frequently without a cloud in the sky. One variation of the dry thermal is seen in the dust or sand whirls, sometimes called dust devils. They are formed  over  heated  surfaces  when  the  winds  are  very light.  Dust  whirls  are  seldom  more  than  two  or  three hundred  feet  high  and  they  last  only  a  few  minutes  at most.  Over  the  desert  on  clear  hot  days  as  many  as  a dozen columns of whirling sand may be visible at once. The   large   desert   sand   whirls   can   become   several hundred feet in diameter, extend to heights of 4,000 feet or higher, and in some cases last for an hour or more. They have been observed to rotate both anticyclonically and cyclonically, the same as tornadoes. An almost identical phenomenon is observed over water in the form of the waterspout. Waterspouts occur frequently in groups and form in relatively cool humid air over a warm water surface when the wind is light. The  waterspout  is  visible  due  to  the  condensed  water vapor,   or   cloud   formation,   within   the   vortex.   The condensation   is   the   result   of   dynamic   cooling   by expansion  within  the  vortex.  In  this  respect  it  differs from the sand whirl, which is always dry. Both the sand whirl   and   the   waterspout   represent   simple   thermal convection  of  an  extreme  type.  They  are  not  to  be confused with the more violent tornado. When   dry   thermal   convection   extends   to   an elevation where the dry thermals reach the condensation level, then cumulus convection takes the place  of  the  dry  convection.  A  cumulus  cloud,  whose base is at the condensation level of the rising air, tops each individual thermal current. Beneath every building cumulus  cloud  a  vigorous  rising  current  or  updraft  is observed.  Thus  the  local  thermal  convection  pattern becomes  visible  in  the  cumulus  cloud  pattern.  The cumulus  clouds  form  first  over  the  hills  where  the strongest thermals develop.   Under stable atmospheric conditions, little convective cloud development occurs. However,   under   unstable   conditions   these   thermals may develop cumulonimbus clouds. INDUCED OR DYNAMIC TERTIARY CIRCULATIONS There are four types of induced or dynamic tertiary circulations.  They  are  eddies,  turbulence,  large-scale vertical waves, and Foehn winds. Eddies An  eddy  is  a  circulation  that  develops  when  the wind flows over or adjacent to rough terrain, buildings, mountains  or  other  obstructions.  They  generally  form on   the   lee   (downwind   or   sheltered)   side   of   these obstructions. The size of the eddy is directly proportional to the size of the obstruction and speed of the   wind.   Eddies   may   have   horizontal   or   vertical circulations that can be either cyclonic or anticyclonic. Horizontal eddies form in sheltered areas downwind of rough coastlines or mountain chains. An example  of  a  horizontal  eddy  is  the  weak  cyclonic circulation that develops in the channel off the coast of Santa Barbara, California. The winds frequently blow parallel to the northern California coastline during the winter   fog   and   stratus   season.   The   Santa   Barbara channel often remains fog-free because the waters are protected  from  winds  that  transport  the  fog  inland. However,   when   the   winds   are   sufficiently   strong, friction along the tough coastal range produces a weak cyclonic  eddy  over  the  channel.  This  cyclonic  flow, though weak, is sufficient to advect fog into the region. 3-26