Dixon White on Desert Flying
High desert flying conditions, like those found in the Owens Valley or
Northern Arizona [and West Texas & south central New Mexico] are best for
beginners when the HIGH is directly overhead or within 200 miles. The isobar
spread should be no less than 1 per 300 miles at surface measured in the 4mb
Weather Channel surface map is perfect for this model. The NWS should be
reporting winds at 9000' MSL under 15 knots and 12000' MSL under 20.
Instability should be mild and can be determined by noting a mild to high
pressure and no more than a -20 TI between surface and 12,000' MSL. Watch
surface temps and be leery of puddle temp increases. Summertime in high
desert areas can bring about TI negatives of -30 as early as 9am. For
example, you may measure 115°F after a 10 minute surface temp measurement at
6000' MSL and then note that the NWS has reported 35°F at 12,000' MSL. This
would give a negative thermal index of -47 WOW!!!!! Fly early and late, quit
flying well before trigger temperature is met in order to avoid turbulence.
Advanced pilots seeking stronger XC type conditions will still want fairly light upper level winds, no more than 20 knots at 9,000' MSL and 25 knots at 12,000' MSL. The Jet Stream should still be far away and isobars at least 200 miles apart within the same model as mentioned above. Slightly pre or post frontal can loosen the atmosphere and make thermals appear sooner and last longer, but without the intensity of mid-day high pressure type thermals. Watch for overdevelopment and land as soon as clouds begin getting taller than they are wide. Be prepared that you may easily exceed 18,000' MSL in big desert thermals and proper clothing and oxygen are important.
- from Dixon White’s Airplay