been caused by these events. The wet microburst is found in the  middle of an active thunderstorm or intense rain shower and  avoiding the strong downdraft is relatively easy. The dry  microburst, however, is somewhat more insidious because it occurs  with little or no warning in the clear air beneath virga. Dry  microbllrsts are common  in  and near the Rockies and other moun› tainous areas of the western U.S. in the summer. The formation of  the dry micro burst is likely when thunderstorms with bases above  about 3,000 to 5,000 feet AGL exist and the temperature/ dew point  spread on the surface is more than about 40 degrees. If dust is  blowing underneath one of these high based thunderstorms, stay  clear until the event passes (usually only a few minutes).  Mountain valleys are often conducive to the formation of tempera› ture inversions and valley fog at night. This should play a part in  your arrival and departure planning. Since the inversion breaks  and the fog dissipates by late morning, you may have to delay  slightly at some airports.  There are numerous ways that  density altitude affects the air› plane. For example, a normally  aspirated engine will lose 3% of  its power per thousand feet of  density altitude increase. Next,  as density altitude increases, the  wings have less dense  air  with  which to create lift. Since a pro› peller is an airfoil, it, too, will  be less efficient.  All of these factors affect the overall performance of the airplane.  At higher density altitudes, takeoff and landing distances are  increased, rate of climb and actual service ceiling are decreased,  true airspeed is higher for a given indicated airspeetl, and turning  radius is larger at high altitude at a given indicated airspeed.  To help regain some of the lost takeoff and landing performance at  high density altitudes. you should reduce the weight at \rvhich you  Effects on Performance  Density Altitude  Effects on the Airplane  Temperature Inversions  ~....  .  ....  7!  :l~  1’.  "’  z  !;.  ~~  ~  ~  ?~  ;,!  7 