Most commercial jets fly between 8 and 12 km (26,000 and 39,000 feet, or roughly 350 hPa to 200 hPa).[/ex]
30,000 ft = 9144 m. You can use
this chart to do the conversion.
3rd column;
is temperature in c. It's roughly about the same as f at that temp.
If the atmosphere were warmer than the temperature indicated by the 100% line, a contrail could not form even if the relative humidity of the atmosphere were 100 percent
4th column;
is dewpoint. This is the point at which vapor will condense into water.
5th column;
Relative Humidity. Very important. According to NASA;
Both the temperature and humidity of the atmosphere help to determine whether a contrail can form. As the relative humidity in the atmosphere increases, the atmosphere is able to supply more moisture into the jet aircraft exhaust plume, and a contrail is more likely to form. The temperature of the atmosphere does not have to be as cold to form contrails at 60% relative humidity compared to 0% relative humidity.
...
Surprisingly, at cold temperatures ice clouds (including contrails) can form and persist at humidities lower than 100%. The red line (dash-double dot line) in the Appleman chart shows at what humidities contrails can persist (usually between 60% and 70% relative humidity). Thus, if the air is moist enough, and colder than (temperature profile is to the left of the red line), then the Appleman chart indicates that persistent contrails can form.
(bold added)
So, what it boils down to, is that on a clear day, it's very, very unlikely that a contrail would form, let alone persist.
And I've already shown that NASA said it is impossible for clouds to form from contrails.
Last but not least.
the USAF found that the forecasts using the Appleman method were correct about 60 to 80 percent of the time. Looking more closely at the data, they found that when no contrails were forecast, the forecast was correct 98 percent of the time! However, when contrails were forecast to occur, the forecast was correct only 25 to 35 percent of the time, and often failed to predict the occurrence of contrails.
Again, the Nasa link is
Appleman Chart - Student
And the satellite sounding site is;
Atmospheric Soundings
Thanks.
