Vertical eddies are generally found on the lee side of  mountains,  but  with  low  wind  speeds,  stationary eddies  or  rotating  pockets  of  air  are  produced  and remain  on  both  the  windward  and  leeward  sides  of obstructions.  (See  figure  3-29.)    When  wind  speeds exceed about 20 knots, the flow may be broken up into irregular eddies that are carried along with a wind some distance   downstream   from   the   obstruction.   These eddies may cause extreme and irregular variations in the   wind   and   may   disturb   aircraft   landing   areas sufficiently to be a hazard. A   similar   and   much   disturbed   wind   condition occurs  when  the  wind  blows  over  large  obstructions such   as   mountain   ridges.   In   such   cases   the   wind blowing  up  the  slope  on  the  windward  side  is  usually relatively smooth. On the leeward side the wind spills rapidly  down  the  slope,  setting  up  strong  downdrafts and causing the air to be very turbulent. This condition is  illustrated  in  figure  3-30.  These  downdrafts  can  be very  violent.  Aircraft  caught  in  these  eddies  could  be forced to collide with the mountain peaks. This effect is also noticeable in the case of hills and bluffs, but is not as pronounced. Turbulence Turbulence is the irregular motion of the atmosphere  caused  by  the  air  flowing  over  an  uneven surface or by two currents of air flowing past each other in different directions or at different speeds. The main source of turbulence is the friction along the surface of Earth. This is called mechanical turbulence. Turbulence is  also  caused  by  irregular  temperature  distribution. The   warmer   air   rises   and   the   colder   air   descends, causing an irregular vertical motion of air; this is called thermal turbulence. Mechanical turbulence is intensified in unstable air and  is  weakened  in  stable  air.  These  influences  cause fluctuations  in  the  wind  with  periods  ranging  from  a few   minutes   to   more   than   an   hour.   If   these   wind variations are strong, they are called wind squalls and are usually associated with convective clouds. They are an   indication   of   approaching   towering   cumulus   or cumulonimbus clouds. Gustiness and turbulence are more or less synonymous.  Gustiness  is  an  irregularity  in  the  wind speed that creates eddy currents disrupting the smooth airflow.   Thus,   the   term   gust   is   usually   used   in conjunction  with  sudden  intermittent  increases  in  the wind speed near the surface levels. Turbulence, on the other  hand,  is  used  with  reference  to  levels  above  the surface.    Gustiness    can    be    measured;    turbulence, however, unless encountered by aircraft equipped with a gust probe or an accelerometer, is usually estimated. Large-Scale Vertical Waves (Mountain Waves) Mountain    waves    occur    on    the    lee    side    of topographical barriers and occur when the wind-flow is strong,  25  knots  or  more,  and  the  flow  is  roughly perpendicular  to the mountain range. The structure of the barrier and the strength of the wind determines the 3-27 AG5f0329 LOW WIND SPEED (BELOW 20 MPH) HIGH WIND SPEED (ABOVE 20 MPH) Figure 3-29.—Eddy currents formed when wind flows over uneven ground or obstructions. AG5f0330 WINDWARD LEEWARD WIND Figure 3-30.—Effect of windflow over mountains.