=  NOW 30 WPM  =  TEXT IS FROM DECEMBER 2020  QST  PAGE 80 =

SUDDENLY RISES, AND THE MOISTURE CONTENT DECREASES SIGNIFICANTLY.  RADIO
SIGNALS GOING THROUGH THE INVERSION ARE BENT AND REFRACTED BACK TOWARDS THE
EARTH.  SOME INVERSIONS BEGIN AT THE GROUND WITH FOG.  THE INVERSIONS THAT
ARE MOST USEFUL FOR VHF/UHF DX TAKE PLACE 1,500 6,000 FEET HIGH.  WHEN A
STRONG INVERSION OCCURS, THE TEMPERATURE RISES, AND THE HUMIDITY OF THE AIR
DECREASES IN A LAYER.  THIS CAN CAUSE SUPER REFRACTION OR DUCTING.  THE
ABILITY OF AIR TO REFRACT RADIO SIGNALS IS DEFINED AS N, WHICH INVOLVES
TEMPERATURE, MOISTURE CONTENT, AND AIR PRESSURE.  COOL, MOIST AIR HAS A
HIGH N VALUE.  WHAT WE ARE INTERESTED IN IS THE CHANGE IN N, OR DN.
TROPOSPHERIC PROPAGATION IS STRONGEST WHERE THE INDEX OF REFRACTION, OR DN,
CHANGES SHARPLY WITH ALTITUDE.  NOT ALL RADIO SIGNALS ARE REFRACTED TO THE
SAME DEGREE.  IT TAKES A HIGHER DN GREATER THAN 19 TO REFRACT 2 METER
SIGNALS THAN 70 CENTIMETER SIGNALS.  THE INVERSION LAYER DEPTH ALSO
DETERMINES THE FREQUENCY OF SIGNALS THAT MAY BE REFRACTED.  UHF SIGNALS CAN
BE REFRACTED BY SHALLOW LAYERS, LOWER FREQUENCIES NEED A DEEPER LAYER.  FOR
50 MHZ SIGNALS TO REFRACT BY TROPO, THE LAYER NEEDS TO BE OVER 1,000 FEET
=  END OF 30 WPM TEXT  =  QST DE W1AW  <